Setting the lamp umzch. Setting up a Lanzar power amplifier - a circuit diagram of a power amplifier, a description of the circuit diagram, recommendations for assembly and adjustment

Due to the increased popularity of tube sound, many rushed to design tube amplifiers. But, although LUs are less whimsical in terms of modes and element base, they still need to be configured after assembly, taking into account some features.

Attention! Voltages in anode circuits can be life threatening. De-energize the device before intervention, discharge smoothing capacitors, carry out work with tools with reliable electrical insulation and, if it is necessary to work under voltage, ensure the presence of persons capable of giving you first aid in case of electric shock.

As in any other U., checking and tuning should be carried out from the “tail” to the “head”. Let's start with a 1-stroke circuit (Fig. 1).

Surely everyone collected something similar at the dawn of their hobby.

Setting the output stage.

So let's start with the output stage. We remove C7 from the circuit and consider the cascade on VL2.

1. A hum is heard at a frequency of 50 Hz.

1-1. BP problem.

The capacitance of the capacitors in the smoothing filter or the inductance of the inductor is small. Usually electrolytic capacitors are used there, which eventually lose their capacity - they “dry out”. Start with the capacitor closest to the rectifier. It is also possible that the rectifier circuit itself is not suitable for the current drawn. I recommend bridge rectifiers - their capacitors are almost 2 times smaller than in other circuits.

1-2. There is a pickup on the grid circuit.

You can reduce R9 a little, but the smaller the change, the better, since in such a circuit this will lead to a decrease in the input impedance of the stage and a deterioration in the frequency response.

If possible, it is better to shield all signal paths. In particular, from C7 to the control grid VL2.

Another possible cause may be excessive resistance R10. But it should be selected with extreme caution, since its selection affects the mode of the DC stage and can lead to an increase in non-linear distortion.

1-3. Small capacitance C8. Needs to be replaced or upgraded. However, keep in mind that excess capacitance will lead to RF losses.

2. Noise is heard.

Here you should determine the tone of the noise "brown (pink)" or "white". I have attached the samples in the archive.

2-1. In the case of low-pitched noise you need to check the capacitors in the anode and cathode circuits (as well as other reactive elements, if any). This is the so-called. local feedbacks (hereinafter referred to as OS. OOS - negative feedback - antiphase signal with respect to the working one, POS - positive feedback - common mode signal), which limit the gain, but at the same time suppress noise, non-linear distortions and self-excitation. They may not correspond to the declared parameters, be absent or have a missing contact (poorly soldered). Also, a mistake by the developer of the circuit itself is not ruled out (usually such elements are marked with “*”, i.e. the element needs to be selected).

2-2. High-pitched ("white") noise appears as a result of a lamp malfunction or the same missing contact. Do not rush to change the lamp immediately. Most likely it is an oxidized panel. It is better to wash it with something neutral, or replace it. Processing with abrasive tools can lead to the opposite results. The physics of this process is quite clear: when the pins are in loose contact with the socket, spark discharges take place, and the ozone that is formed in this case oxidizes both surfaces even more actively. You can determine the source of the problem by clicking on the lamp with your finger. A rustling sound is a malfunction of the panel, a ringing sound is a malfunction of the lamp. If this method fails, temporarily replace the lamp and try again.

2-3. Excessive resistance of the anode-cathode circuit can also be the cause of any noise. Start picking up R10 (to start in a small range, otherwise damage the lamp and transformer). If the selection of this resistor does not give tangible results, I do not envy you - the problem is in the anode circuit mode for direct current. This means that the transformer does not meet the required parameters of the cascade. You will either have to pick up another transformer, or rewind the existing one. God forbid you survive this!

3. Nonlinear distortions. This is a type of distortion that can be observed as geometric changes in the shape of a waveform on an oscillogram. By ear, they are determined by different signs: at low frequencies, wheezing increases noticeably, at high frequencies, “whistles” become “hissing”. As usual, such distortions are the result of overloading - excessive gain, excessive input signal level, operating point shift, etc. Let's deal with the most characteristic sources.

3-1. Shortage/excess of anode voltage. All this leads to a shift in the operating point, therefore, some half-waves are suppressed by the direct current lamp mode. The situation is similar to paragraphs 2-3. You should work in the same way, but before that you should check the supply voltage of the U. in silent mode and in the presence of a signal (if lowering the input signal level allows you to remove distortion, then the output stage is working). Actually, in this case it is inappropriate to talk about the device as a class "A" amplifier.

3-2. The weakening of the heat. The CVC of the lamp, in this case, is also far from ideal. This can be easily verified by giving a signal to a poorly heated lamp. Actually, this is not such a serious problem. It all comes down to the ready time of the U. This can happen with a transistor U., only there the time depends on the capacitance (charging time) of the smoothing capacitors.

3-3. Too much input voltage. You can put a resistor between the decoupling capacitor C7 and the control grid VL2. The additional resistor and R9 form a divider that will lower the signal. This will change the frequency response, but the rise in bass can be solved by selecting C7 (decrease). By the way, R9 also has a certain effect on the DC mode, so by selecting it, you can also achieve the desired results.

Setting up prestages. Now let's put C7 back in place and remove C2. Thus, a ready-made U., covered by the OS, is obtained. By and large, the 2nd stage is needed only to compensate for losses in the fine-tuning circuits. Those. with an input signal voltage of 1.5-2V, the 1st stage can be completely excluded. In fairness, it should be noted that each stage inevitably introduces distortion and noise, and at the output it all adds up. In reality, everyone decides for himself how many stages are needed to provide the desired gain. What was said above is also true for triodes. Here the task is even somewhat simplified, since the anode is loaded not on the transformer, but on the usual active load - a resistor, part of which, if necessary, can be replaced with a trimmer. I would not advise you to get carried away with this, since variable resistors can also be a source of noise (including white noise, which many, due to inexperience, attribute to the sins of the lamp). So, we will not discuss the mode of the VL1-2 cascade and move on to U. as a whole. As can be seen from the diagram, a very important circuit has been put into operation - the loop of the general environmental protection. As we know, the OS phase depends on which output of the secondary winding the loop is connected to. Since the difference is 180g, the OS can become positive. If, when turned on, the noise or background has sharply increased, then U. has become a generator. Before you conjure over the triode, transfer the OS circuit to another output of the secondary winding (remaining, respectively, switch to the common one). The loop consists of R8R11R12. The resistor in the cathode circuit VL1-2 is the load of this divider. As a rule, the OS does not have a significant effect on the direct current cathode mode, but for this the condition R11+R12>>R8 must be satisfied. With the help of OOS, noise and distortion can be significantly reduced, but without fanaticism, since this effect is achieved by reducing the gain until the signal is completely obstructed.

Now consider 2-stroke amplifiers. In fact, the preamplifier in such circuits is no different, but instead of the output stage there is a phase inverter that decomposes the signal into half-waves and amplifies each separately. It is quite clear that the direct current mode in such stages is shifted to “-”, which allows you to maximize the positive half-wave and ignore the negative one, which is shifted by a phase inverter by 180g and amplified by the second shoulder. In circuitry, this is implemented in 2 ways. Figure 2 shows a method where the triode is both an inverter, as preliminary stages and a cathode follower.

Such a cascade, while seeming simple, is quite difficult to set up. First of all, this is due to the fact that the inverter and the repeater have different output resistances and, accordingly, different load capacity. To drive such a cascade into mode, it is necessary not only to achieve its symmetry with respect to the power poles, but also to carefully select the constant voltage on the grid (respectively, the anode voltage of the left triode L2) so that the amplitudes of the separated signals are equal in absolute value (reminiscent of the operation of Maxwell's pendulum), but the phase inverter itself did not leave the linear mode. Judge for yourself the consequences of FI imbalance. My subjective opinion is God bless her, with simplicity, for the sake of getting rid of such difficulties and an extra lamp, it’s not a pity. Another option is when the FI consists of 2 conventional cascades with a common cathode (Fig. 3).

The left triode L1 rotates the phase by 180 degrees. and transmits to the second triode and the lower anti-phase pentode. The right triode turns the phase another 180 degrees (returns to its original state) and transmits to the common-mode pentode. In addition to the described operations with single-cycle cascades, we just have to choose the input divider of the right triode in such a way that the amplitudes of the anode signals are equal.

By the lamps, perhaps, everything. In the next article, we will consider semiconductor UMZCH. We will discuss questions at.

Sincerely, Pavel A. Ulitin. Chistopol (Tatarstan).

The article uses illustrations from the book R. Svorenya "Amplifiers and radio nodes" (1965)

What I currently have:

1. Amplifier itself:

2. Naturally, the power supply of the final amplifier:

When setting up the PA, I use a device that provides a safe connection of the PA transformer to the network (through a lamp). It is made in a separate box with its own cord and socket and, if necessary, connects to any device. The diagram is shown below in the figure. This device requires a relay with a 220 AC winding and two groups of make contacts, one momentary pushbutton (S2), one latching pushbutton or switch (S1). When S1 is closed, the transformer is connected to the network through the lamp, if all PA modes are normal, when the S2 button is pressed, the relay closes the lamp through one group of contacts and connects the transformer directly to the network, and the second group of contacts, duplicating the S2 button, constantly connects the relay to the network. The device is in this state until S1 opens, or the voltage decreases below the holding voltage of the relay contacts (including short circuit). The next time S1 is turned on, the transformer is again connected to the network through the lamp, and so on ...

Noise immunity of various ways of shielding signal wires

3. We also have assembled AC protection against DC voltage:

Implemented in defense:
speaker connection delay
protection against constant output, against short circuit
airflow control and shutdown of the speakers when the radiators overheat

Adjustment:
Suppose everything is assembled from serviceable and tested transistors and diodes by the tester. Initially, put the trimmers in the following positions: R6 - in the middle, R12, R13 - in the top according to the diagram.
Do not solder the VD7 zener diode at first. On the protection PCB, the Zobel circuits necessary for the stability of the amplifier are separated, if they are already on the UMZCH boards, then they do not need to be soldered, and the coils can be replaced with jumpers. Otherwise, the coils are wound on a mandrel with a diameter of 10 mm, for example, the tail of a drill with a wire with a diameter of 1 mm. The length of the resulting winding should be such that the coil fits into the holes allotted for it on the board. After winding, I recommend impregnating the wire with varnish or glue, for example, epoxy or BF - for rigidity.
The wires going from the protection to the outputs of the amplifier, while connected to the common wire, disconnecting from its outputs, of course. It is necessary to connect the ground protection polygon marked on the PCB with the mark “Main GND” with the “Mecca” of the UMZCH, otherwise the protection will not work correctly. And, of course, the GND pads next to the coils.
Having turned on the protection with the speakers connected, we begin to reduce the resistance R6 until the relay clicks. Having unscrewed one or two more turns of the trimmer, we turn off the protection from the network, turn on two speakers in parallel on any of the channels and check whether the relays will work. If they don’t work, then everything works as intended, with a load of 2 Ohms, the amplifiers will not connect to it, in order to avoid damage.
Next, we disconnect the wires “From UMZCH LC” and “From UMZCH PC” from the ground, turn everything on again and check whether the protection will work if a constant voltage of about two or three volts is applied to these wires. The relays should turn off the speakers - there will be a click.
You can enter the indication "Protection" if you connect a chain of a red LED and a 10 kΩ resistor between ground and VT6 collector. This LED will indicate a fault.
Next, set up the temperature control. We put the thermistors in a waterproof tube (attention! They should not get wet during the test!).
It often happens that a radio amateur does not have thermistors indicated in the diagram. Two of the same ones available, with a resistance of 4.7 kOhm, will do, but in this case, the resistance R15 should be equal to twice the resistance of the series-connected thermistors. Thermistors must have a negative resistance coefficient (reduce it with heating), the thermistors work the other way around and they have no place here. Boil a glass of water. We give it 10-15 minutes to cool in calm air and lower the thermistors into it. We twist R13 until the LED goes out "Overheat" - Overheat, which should have been lit initially.
When the water cools down to 50 degrees (this can be accelerated, how exactly is a big secret) - turn R12 so that the “Blow” LED or FAN On goes out.
We solder the VD7 zener diode in place.
If there are no glitches from the sealing of this zener diode, then everything is fine, but it was such that without it the transistor part works flawlessly, but with it it does not want to connect the relay to any. In this case, we change it to any one with a stabilization voltage from 3.3 V to 10V. The reason is the leakage of the zener diode.
When the thermistors are heated to 90 * C, the “Overheat” LED should light up - Overheating and the relay will disconnect the speakers from the amplifier. With some cooling of the radiators, everything will be connected back, but this mode of operation of the device should at least alert the owner. With a working fan and a tunnel not clogged with dust, thermal operation should not be observed at all.
If everything is fine, solder the wires to the output of the amplifier and enjoy.
Airflow (its intensity) is adjusted by selecting resistors R24 and R25. The first determines the performance of the cooler with the airflow turned on (maximum), the second determines the performance of the cooler when the radiators are only slightly warm. R25 can be excluded altogether, but then the fan will operate in ON-OFF mode.
If the relays have windings for 24V, then they must be connected in parallel, if for 12, then in series.
Parts replacement. As an op amp, you can use almost any dual cheap op amp in SOIK8 (from 4558 to ORA2132, although I hope it will not reach the latter), for example, TL072, NE5532, NJM4580, etc.
Transistors - 2n5551 are changed to BC546-BC548, or to our KT3102. We will replace BD139 with 2SC4793, 2SC2383, or with a similar current and voltage, it is possible to put at least KT815.
The field worker changes to a similar one used, the choice is huge. A field radiator is not required.
Diodes 1N4148 are changed to 1N4004 - 1N4007 or to KD522. In the rectifier, you can put 1N4004 - 1N4007 or use a diode bridge with a current of 1 A.
If the blower control and overheating protection of the UMZCH are not needed, then the right side of the circuit is not soldered - the op-amp, thermistors, field, etc., except for the diode bridge and the filter capacitor. If you already have a 22..25V power supply in the amplifier, then you can use it, not forgetting about the protection current consumption of about 0.35A when the blower is turned on.

Recommendations for assembling and configuring UMZCH:
Before assembling the printed circuit board, you should perform relatively simple operations with the board, namely, look through the light to see if there are short circuits between the tracks that are hardly noticeable under normal lighting. Factory production does not exclude manufacturing defects, unfortunately. Soldering is recommended to be carried out with solder POS-61 or similar with a melting point not higher than 200 * C.

First you need to decide on the applied OS. The use of op-amps from Analog Devices is highly not recommended - in this UMZCH their sound character is somewhat different from what the author intended, and an excessively high speed can lead to irremovable self-excitation of the amplifier. The replacement of ORA134 with ORA132, ORA627 is welcomed. they have less distortion at high frequencies. The same applies to the op amp DA1 - it is recommended to use OPA2132, OPA2134 (in order of preference). It is acceptable to use OPA604, OPA2604, but there will be slightly more distortion. Of course, you can experiment with the type of op-amp, but at your own peril and risk. UMZCH will also work with KR544UD1, KR574UD1, but the zero offset level at the output will increase and the harmonics will grow. The sound is ... I think no comments are needed.

From the very beginning of the installation, it is recommended to select transistors in pairs. This is not a necessary measure as the amplifier will work with a spread of 20-30%, but if you set a goal to get the maximum quality, then pay attention to this. Of particular note is the selection of T5, T6 - they are best used with maximum H21e - this will reduce the load on the op-amp and improve its output spectrum. T9, T10 should also have as close gain as possible. For latch transistors selection is optional. Output transistors - if they are from the same batch, you can not select them, because. the culture of production in the West is somewhat higher than we are used to, and the spread is within 5-10%.

Further, instead of the terminals of the resistors R30, R31, it is recommended to solder pieces of wire a couple of centimeters long, since it will be necessary to select their resistances. An initial value of 82 ohms will give a UN quiescent current of about 20..25 mA, but statistically it turned out from 75 to 100 ohms, this strongly depends on the specific transistors.
As already noted in the topic on the amplifier, you should not use transistor optocouplers. Therefore, it is worth focusing on AOD101A-G. Imported diode optocouplers were not tested due to unavailability, this is temporary. The best results are obtained on AOD101A of one batch for both channels.

In addition to transistors, it is worth picking up UNA complementary resistors in pairs. The spread should not exceed 1%. You need to carefully select R36=R39, R34=R35, R40=R41. For reference, I note that with a spread of more than 0.5%, it is better not to switch to the option without environmental protection, because. there will be an increase in even harmonics. It was the impossibility to get the exact details that stopped the author's experiments in the non-OOS direction at the time. The introduction of balancing into the current feedback circuit does not completely solve the problem.

Resistors R46, R47 can be soldered at 1 kOhm, but if there is a desire to more accurately adjust the current shunt, then it is better to do the same as with R30, R31 - solder the wires for soldering.
As it turned out in the course of repeating the circuit, under some set of circumstances, excitation in the EA tracking circuit is possible. This manifested itself in the form of an uncontrolled drift of the quiescent current, and especially in the form of oscillations with a frequency of about 500 kHz on collectors T15, T18.
The necessary adjustments were originally included in this version, but it's still worth checking with an oscilloscope.

Diodes VD14, VD15 are placed on the radiator for temperature compensation of the quiescent current. This can be done by soldering the wires to the leads of the diodes and gluing them to the heatsink with Moment glue or similar.

Before turning it on for the first time, it is necessary to thoroughly wash the board from traces of flux, look for the absence of short circuits with solder, make sure that the common wires are connected to the midpoint of the power supply capacitors. It is also strongly recommended to use the Zobel circuit and the coil at the output of the UMZCH, they are not shown in the diagram, because. the author considers their application as a rule of good form. The ratings of this circuit are common - these are a 10 Ohm 2 W resistor connected in series and a K73-17 capacitor or similar with a capacity of 0.1 μF. The coil is wound with a varnished wire with a diameter of 1 mm on an MLT-2 resistor, the number of turns is 12 ... 15 (before filling). On the protection board, this circuit is completely wired.

All transistors VK and T9, T10 in UN are mounted on a radiator. Powerful VK transistors are installed through mica spacers and KPT-8 type paste is used to improve thermal contact. It is not recommended to use near-computer pastes - there is a high probability of a fake, and tests confirm that KPT-8 is often the best choice, and also very inexpensive. In order not to fly into a fake, use KPT-8 in metal tubes, like toothpaste. We haven't gotten there yet, fortunately.

For transistors in an insulated package, the use of a mica gasket is optional and even undesirable, because. worsens the conditions of thermal contact.
Be sure to turn on a 100-150W light bulb in series with the primary winding of the network transformer - this will save you from many troubles.

Short the D2 optocoupler LED pins (1 and 2) and turn on. If everything is assembled correctly, then the current consumed by the amplifier should not exceed 40 mA (the output stage will operate in mode B). The DC bias voltage at the UMZCH output should not exceed 10 mV. Turn on the LED. The current consumed by the amplifier should increase to 140 ... 180 mA. If it increases more, then check (it is recommended to do this with a pointer voltmeter) collectors T15, T18. If everything works correctly, there should be voltages that differ from the supply voltages by about 10-20 V. In the case when this deviation is less than 5 V, and the quiescent current is too large, try changing the diodes VD14, VD15 to others, it is very desirable that they were from the same party. The UMZCH quiescent current, if it does not fit in the range from 70 to 150 mA, can also be set by selecting resistors R57, R58. Possible replacement for diodes VD14, VD15: 1N4148, 1N4001-1N4007, KD522. Or, reduce the current flowing through them by simultaneously increasing R57, R58. In my thoughts there was the possibility of implementing a bias of such a plan: instead of VD14, VD15, use transitions of BE transistors from the same batches as T15, T18, but then you will have to significantly increase R57, R58 - until the resulting current mirrors are fully tuned. In this case, the newly introduced transistors must be in thermal contact with the radiator, as well as the diodes, instead of which they are placed.

Next, you need to set the quiescent current UNA. Leave the amplifier on and after 20-30 minutes check the voltage drop across resistors R42, R43. 200 ... 250 mV should fall there, which means a quiescent current of 20-25 mA. If it is greater, then it is necessary to reduce the resistances R30, R31, if less, then increase accordingly. It may happen that the quiescent current of the UNA will be asymmetrical - in one arm 5-6mA, in the other 50mA. In this case, unsolder the transistors from the latch and continue without them for now. The effect did not find a logical explanation, but disappeared when the transistors were replaced. In general, it makes no sense to use transistors with a large H21e in a latch. A gain of 50 is enough.

After setting the UNA, we again check the quiescent current of the VC. It should be measured by the voltage drop across resistors R79, R82. A current of 100 mA corresponds to a voltage drop of 33 mV. Of these 100 mA, about 20 mA is consumed by the pre-terminal stage and up to 10 mA can go to control the optocoupler, therefore, in the case when, for example, 33 mV drops across these resistors, the quiescent current will be 70 ... 75 mA. You can refine it by measuring the voltage drop across the resistors in the emitters of the output transistors and subsequent summation. The quiescent current of the output transistors from 80 to 130 mA can be considered normal, while the declared parameters are fully preserved.

Based on the results of measuring the voltages on the collectors T15, T18, we can conclude that the control current through the optocoupler is sufficient. If T15, T18 are almost saturated (the voltages on their collectors differ from the supply voltages by less than 10 V), then you need to reduce the values \u200b\u200bof R51, R56 by about one and a half times and re-measure. The voltage situation should change, and the quiescent current should remain the same. The optimal case is when the voltages on the collectors T15, T18 are equal to about half of the supply voltages, but a deviation from the supply by 10-15V is quite enough, this is the reserve that is needed to control the optocoupler on a music signal and a real load. Resistors R51, R56 can heat up to 40-50 * C, this is normal.

Instantaneous power in the most difficult case - with an output voltage close to zero - does not exceed 125-130 W per transistor (according to technical conditions, up to 150 W is allowed) and it acts almost instantly, which should not lead to any consequences.

Latch actuation can be determined subjectively by a sharp decrease in output power and a characteristic “dirty” sound, in other words, there will be a highly distorted sound in the speakers.

4. Preamplifier and its PSU

High quality PU material:

Serves for tone correction and loudness when adjusting the volume. Can be used to connect headphones.

The well-proven TB Matyushkina was used as a timbre block. It has 4-stage bass control and smooth treble control, and its frequency response is well suited to auditory perception, in any case, the classic bridge TB (which can also be used) is rated lower by listeners. The relay allows, if necessary, to turn off any frequency correction in the path, the output signal level is adjusted by a tuning resistor according to gain equality at a frequency of 1000 Hz in the TB mode and when bypassed.

Design characteristics:

Kg in the frequency range from 20 Hz to 20 kHz - less than 0.001% (typical value is about 0.0005%)

Rated input voltage, V 0.775

Overload capacity in the TB bypass mode is at least 20 dB.

The minimum load resistance at which the operation of the output stage in mode A is guaranteed is at the maximum range of the output voltage “from peak to peak” 58V 1.5 kOhm.

When using the PU only with CD players, it is permissible to reduce the buffer supply voltage to +\-15V, because the output voltage range of such signal sources is obviously limited from above, this will not affect the parameters.

A complete set of boards consists of two PU channels, RT Matyushkin (one board for both channels) and a power supply. Printed circuit boards designed by Vladimir Lepekhin.

Measurement results:

A transistor UMZCH with a differential stage (DC) at the input is traditionally built according to three cascade schemes: DC input voltage amplifier; voltage amplifier; output two-stroke current amplifier. In this case, it is the output stage that makes the greatest contribution to the distortion spectrum. These are, first of all, "step" type distortions, switching distortions aggravated by the presence of resistances in the emitter (source) circuits, as well as thermal distortions, which until recently were not given due attention. All these distortions, being phase-shifted in negative feedback circuits, contribute to the formation of a wide range of harmonics (up to 11th). This is what determines the characteristic transistor sound in a number of unsuccessful developments.

For all cascades, a huge set of circuit solutions has been accumulated to date, from simple asymmetric cascades to complex fully symmetrical ones. However, the search for solutions continues. The art of circuitry lies in the fact that simple solutions achieve a good result. One of such successful solutions is published in . The authors note that the mode of operation of the most common output stages with a common collector is set by the voltage at the emitter junctions, which strongly depends on both the collector current and temperature. If in low-power emitter followers it is possible to stabilize the emitter base voltage by stabilizing the collector current, then in high-power class AB output stages this is almost impossible to do.

Thermal stabilization circuits with a temperature-sensitive element (most often a transistor), even when the latter is installed on the case of one of the output transistors, are inertial and can track only the average change in the temperature of the crystal, but not instantaneous, which leads to additional modulation of the output signal. In some cases, thermal stabilization circuits are a source of soft excitation or sub-excitation, which also gives the sound a certain coloration. For a fundamental solution to this problem, the authors proposed to implement the output stage according to the OE scheme (the idea is not new, see for example). As a result, unlike the traditional three-stage construction (each stage with its own cutoff frequency and its own spectrum of harmonics), we got only a two-stage amplifier. Its simplified scheme is shown in Fig.1.

The first stage is made according to the traditional DC scheme with a load in the form of a current mirror. Symmetric pickup of the signal from the DC using a current mirror (counter dynamic load) makes it possible to obtain twice as much amplification while simultaneously reducing noise. The output impedance of the cascade with such a signal pickup is quite high, which determines its operation in the current generator mode. In this case, the current in the load circuit (the base of the transistor VT8 and the emitter of the transistor VT7) depends little on the input resistance and is determined mainly by the internal resistance of the current source. The emitter currents of transistors VT8, VT9 are basic for transistors VT10, VT11. The current generator I2 and the level shift circuit on transistors VT5 VT7 set and stabilize the initial current of transistors VT8 VT11, regardless of their temperature.

Let us consider in more detail the operation of the current control circuit of the output transistors. Transitions base emitter transistors VT5 VT8 form two parallel circuits between the output of the current source I2 and the base of the transistor VT10. This is nothing more than a complex scale current reflector. The principle of operation of the simplest current reflector is based on the fact that a specific value of the collector (emitter) current corresponds to a well-defined voltage drop at its base-emitter junction and vice versa, i.e. if this voltage is applied to the base-emitter junction of another transistor with the same parameters, then its collector current will be equal to the collector current of the first transistor. The right circuit (VT7, VT8) consists of base-emitter junctions with different collector (emitter) currents. For the "current reflector" principle to work, the left circuit must be mirrored with respect to the right, i.e. contain identical elements. In order for the collector current of transistor VT6 (aka current generator current I2) to match the collector current of transistor VT8, the voltage drop at the base-emitter junction of transistor VT5, in turn, must be equal to the voltage drop at the base-emitter junction of transistor VT7.

To do this, in a real circuit (Fig. 2), the VT5 transistor is replaced by a composite transistor according to the Shiklai scheme. Based on the foregoing, the following conditions are met:

static current transfer coefficients of transistors VT7, VT8, VT11 (VT12) must be equal;
static current transfer coefficients of transistors VT9 and VT10 should also be equal to each other, and even better if all 6 transistors (VT7 VT12) have the same characteristics, which is difficult to do with a limited number of transistors available;
as transistors VT8, VT9, it is necessary to select transistors with a minimum base-emitter voltage (taking into account the spread of parameters), since these transistors operate at a reduced emitter-collector voltage;
products of static current transfer coefficients of transistors VT11, VT13 and VT12, VT14 should also be close.

Thus, if we want to set the collector current of transistors VT13, VT14 to 100 mA and have output transistors with h21e=25, then the current generator current on transistor VT6 should be: Ik(VT6)/h21e=100/25=4 mA, which and determines the resistance of the resistor R11 about 150 ohms (0.6 V / 0.004 A = 150 ohms).

Since the output stage is controlled by the output current of the DC, the total emitter bias current is chosen to be large enough, about 6 mA (determined by resistor R6), it also determines the maximum possible output current of the DC. From here you can calculate the maximum output current of the amplifier. For example, if the current gain product of the output transistors is 1000, then the maximum output current of the amplifier will be close to 6 A. For the declared maximum output current of 15 A, the current gain of the output stage must be at least 2500, which is quite realistic. Moreover, in order to increase the load capacity of the DC, the total emitter bias current can be increased to 10 mA by reducing the resistance of the resistor R6 to 62 ohms.

The following amplifier specifications:

Output power in the band up to 40 kHz at a load of 8 ohms - 40 watts.
Pulse power at a load of 2 ohms - 200 watts.
The amplitude value of the undistorted output current is 15 A.
Harmonic coefficient at a frequency of 1 kHz (1 W and 30 W, Fig. 3) - 0.01%
Output voltage slew rate - 6 V/μs
Damping coefficient, not less than - 250

The plot of the harmonic coefficient at an output power of 1 W (curve a) and at an output power of 30 W (curve b) at a load of 8 ohms is shown in Fig. 3. The comments on the circuit state that the amplifier has high stability, there are no "switching distortions", as well as higher-order harmonics.

Before assembling a prototype amplifier, the circuit was mocked up virtually and studied using the Multisim 2001 program. Since the output transistors indicated in the circuit were not found in the program database, they were replaced by the closest analogues of domestic transistors KT818, KT819. Studies of the circuit (Fig. 4) gave results somewhat different from those given in. The load capacity of the amplifier turned out to be lower than the declared one, and the harmonic coefficient was more than an order of magnitude worse. The phase safety factor of only 25° was also insufficient. The slope of the frequency response in the region of 0 dB is close to 12 dB / oct., which also indicates the lack of stability of the amplifier.

For the purpose of experimental verification, an amplifier layout was assembled and installed in the guitar amp of the rock group "Aphasia". To increase the stability of the amplifier, the correction capacitance is increased to 2.2 nF. Field tests of the amplifier in comparison with other amplifiers confirmed its merits and the amplifier was highly appreciated by musicians.

Technical parameters of the amplifier

Bandwidth at 3dB-15Hz-190kHz
Harmonic distortion at 1 kHz (25 W, 8 ohms) - 0.366%
Unity gain frequency - 3.5 MHz
Phase Margin - 25°

Strictly speaking, the above arguments about the current control of the output stage are valid for an amplifier with an open feedback loop. With a closed CNF, in accordance with its depth, not only the output impedance of the amplifier as a whole, but also of all its cascades decreases, i.e. they essentially begin to work as voltage generators.

Therefore, in order to obtain the declared technical characteristics, the amplifier was modified to the form of Fig. 5, and the result of its study is shown in Fig. 6. As can be seen from the figure, only two transistors have been added to the circuit, which form a push-pull hybrid class A repeater. The introduction of a buffer stage with a high load capacity made it possible to more effectively use the voltage amplifying properties of the DC and significantly increase the load capacity of the amplifier as a whole. An increase in gain with a broken OOS had a positive effect on a decrease in the harmonic distortion coefficient.

Increasing the correction capacitance from 1 nF to 2.2 nF, although it narrowed the bandwidth from above to 100 kHz, but increased the phase margin by 30 ° and ensured a frequency response slope in the unity gain region of 6 dB / oct., which guarantees good stability of the amplifier.

A meander-type signal with a frequency of 1 kHz (calibration signal from an oscilloscope) was applied to the amplifier input as a test signal. The output signal of the amplifier had neither a rollover nor spikes at the signal fronts, i.e. fully consistent with the input.

Specifications of the modified amplifier

3 dB bandwidth - 8 Hz - 100 kHz
Unity gain frequency - 2.5 MHz Phase margin - 55°
Gain - 30 dB
Harmonic coefficient at a frequency of 1 kHz (25 W, 8 Ohm) - 0.007%
Harmonic coefficient at a frequency of 1 kHz (50 W, 4 Ohm) - 0.017%
Harmonic coefficient at Ku=20 dB - 0.01%

For the purpose of full-scale testing of the modified amplifier, two samples were made in the dimensions of the Lorta 50U 202S amplifier board (aka Amfiton 001) and installed in the indicated amplifier. At the same time, the volume control was finalized in accordance with.

As a result of refinement, the owner of the amplifier completely abandoned the tone control, and full-scale tests showed its clear advantage over the previous amplifier. The sound of the instruments has become more pure and natural, apparent sound sources (SIS) have become more clearly formed, they have become, as it were, more "tangible". The undistorted output power of the amplifier has also increased noticeably. The thermal stability of the amplifier exceeded all expectations. After a two-hour test of the amplifier at an output power close to the maximum, the side heat sinks turned out to be practically cold, while with the previous amplifiers, even in the absence of a signal, the amplifier, being left on, warmed up quite strongly.

Construction and details
The board (with elements in the light) of the amplifier intended for installation in the Lorta amplifier is shown in Fig. 7. The board provides places for installing a diode bridge and resistor R43 from the old circuit, as well as places for installing current equalizing base and emitter resistors for paired output transistors. At the bottom of the board, places are reserved for installing elements of an active current source (AIT) in the form of a current reflector, consisting of a current-setting resistor with a resistance of 75 kOhm from the PA output, two KT3102B transistors and two 200 Ohm resistors to actively turn off the lower arm of the amplifier (in the prototype was not installed). Capacitors C4, C6 type K73 17. The capacitance of capacitor C2 can be painlessly increased to 1 nF, while the cutoff frequency of the input low-pass filter will be 160 kHz.

Transistors VT13, VT14 are equipped with small aluminum flags 2 mm thick. Transistors VT8 and VT12 for better thermal stabilization of the amplifier are installed on both sides of the common flag, and the transistor VT8 through a mica gasket or an elastic heat-conducting insulator of the "Nomakon Gs" type TU RB 14576608.003 96. As for the parameters of the transistors, they are discussed in detail above. As transistors VT1, VT5, you can use transistors KT503E, and instead of transistors VT2, VT3, transistors of the KT3107 type with any letter index. It is desirable that the static current gains of the transistors be equal in pairs with a spread of no more than 5%, and the gains of transistors VT2, VT4 be slightly greater than or equal to the gains of transistors VT1, VT5.

As transistors VT3, VT6, you can use transistors of the types KT815G, KT6117A, KT503E, KT605. Transistors VT8, VT12 can be replaced by transistors of the KT626V type. In this case, the transistor VT12 is attached to the box, the atransistor VT8 to the transistor VT12. Under the head of the screw on the side of the transistor VT8, place a texto-lithic washer. As a transistor VT10 from domestic field-effect transistors, a transistor of the type KP302A, 2P302A, KP307B (V), 2P307B (V) is best suited. It is advisable to select transistors with an initial drain current of 7-12 mA and a cut-off voltage in the range of (0.8-1.2) V. Resistor R15 of type SP3 38b. Transistors VT15, VT16 can be replaced, respectively, KT837 and KT805, as well as KT864 and KT865 with higher frequency characteristics. The board was developed for the installation of paired output transistors (KT805, KT837). For this purpose, the board provides places for installing both basic (2.2-4.3 Ohm) and emitter (0.2-0.4 Ohm) current equalizing resistors. In the case of installing single output transistors, instead of current equalizing resistors, solder the jumpers or immediately solder the wires of the output transistors to the appropriate places on the board. The "native" output transistors were left in the experimental sample, only they had to be swapped.

In the amplifier, it is desirable to increase the power capacitances (in the original amplifier, 2.2200 uF.50 V in each arm). At a minimum, it is desirable to add another 2200 uF to each arm, and even better, replace it with a 10000 uF capacitor. 50 V. At 50 V, foreign capacitors are relatively cheap.

Establishment
Before connecting the output transistors, it is necessary to temporarily solder any medium power diodes (for example, KD105, KD106) in place of the base-emitter junctions of the output transistors, apply power to the board and, without connecting the load, make sure that the amplifier works out the midpoint. Apply a signal to the input of the amplifier and check with an oscilloscope that it is amplified without distortion and excitation at idle. This indicates the correct installation and serviceability of all elements of the amplifier. Only after that you can solder the output transistors and proceed to setting their quiescent current.

To set the quiescent current, it is necessary to set the slider of the resistor R15 to the lower position according to the diagram, remove the fuse in one of the arms of the amplifier and turn on the ammeter instead. The current consumption is set under the trimmer resistor R15 in the range of 110-130 mA (taking into account the DC current of about 6 mA and the buffer follower current of about 3-5 mA). Then the sensitivity of the amplifiers is checked and, if necessary, the OS resistors are adjusted.

After that, you can proceed to various studies, if, of course, the equipment of the radio amateur laboratory allows. For this purpose, you can use the direct input of the amplifier by removing the jumper plug from it on the back of the amplifier.

Literature

Digest UMZCH//Radiohobby. 2000. No. 1. S.8 10.
Petrov A. Superlinear EP with high load capacity//Radioamator. 2002. No. 4. C.16.3.
Dorofeev M. Mode B in AF power amplifiers//Radio. 1991. No. 3. S.53 56.
Petrov A. Refinement of the volume control of the amplifier "Lorta 50U 202S"//Radioamator. 2000. No. 3. p.10

- The neighbor got tired of knocking on the battery. He turned the music up louder so that he could not be heard.
(From audiophile folklore).

The epigraph is ironic, but the audiophile is not necessarily “sick in the head” with the physiognomy of Josh Ernest at a briefing on relations with the Russian Federation, who is “rushing” because the neighbors are “happy”. Someone wants to listen to serious music at home as in the hall. The quality of the equipment for this is necessary, which for fans of the decibel of loudness as such simply does not fit where sane people have a mind, but for the latter, this mind comes from the prices of suitable amplifiers (UMZCH, audio frequency power amplifier). And someone along the way has a desire to join useful and exciting areas of activity - the technique of sound reproduction and electronics in general. Which in the digital age are inextricably linked and can become a highly profitable and prestigious profession. The first step in this matter, optimal in all respects, is to make an amplifier with your own hands: it is UMZCH that allows, with initial training based on school physics, on the same table, to go from the simplest structures for half an evening (which, nevertheless, “sing” well) to the most complex units, through which a good rock band will play with pleasure. The purpose of this publication is to cover the first stages of this path for beginners and, perhaps, to tell something new to experienced ones.

Protozoa

So, for starters, let's try to make a sound amplifier that just works. In order to thoroughly delve into sound engineering, you will have to gradually master quite a lot of theoretical material and do not forget to enrich your knowledge base as you progress. But any “smartness” is easier to digest when you see and feel how it works “in hardware”. In this article, further, too, it will not do without theory - in what you need to know at first and what can be explained without formulas and graphs. In the meantime, it will be enough to be able to use the multitester.

Note: if you have not soldered electronics yet, please note that its components must not be overheated! Soldering iron - up to 40 W (better than 25 W), the maximum allowable soldering time without interruption is 10 s. The soldered lead for the heat sink is held 0.5-3 cm from the place of soldering from the side of the device case with medical tweezers. Acid and other active fluxes must not be used! Solder - POS-61.

On the left in fig.- the simplest UMZCH, "which just works." It can be assembled on both germanium and silicon transistors.

On this crumb, it is convenient to master the basics of setting up the UMZCH with direct connections between the cascades, which give the clearest sound:

Before the first power-up, the load (speaker) is turned off;
Instead of R1, we solder a chain of a constant resistor of 33 kOhm and a variable (potentiometer) of 270 kOhm, i.e. first note. four times smaller, and the second approx. twice the face value against the original according to the scheme;
We supply power and, by rotating the potentiometer slider, at the point marked with a cross, set the specified collector current VT1;
We remove the power, solder the temporary resistors and measure their total resistance;
As R1, we set the nominal resistor from the standard row closest to the measured one;
We replace R3 with a constant 470 Ohm chain + 3.3 kOhm potentiometer;
The same as according to paragraphs. 3-5, incl. a set the voltage equal to half the supply voltage.

Point a, from where the signal is taken to the load, is the so-called. middle point of the amplifier. In UMZCH with unipolar power, half of its value is set in it, and in UMZCH with bipolar power - zero relative to the common wire. This is called adjusting the balance of the amplifier. In unipolar UMZCH with capacitive load decoupling, it is not necessary to turn it off during setup, but it is better to get used to doing it reflexively: an unbalanced 2-polar amplifier with a connected load can burn its own powerful and expensive output transistors, or even “new, good” and very expensive powerful speaker.

Note: components that require selection when setting up a device in a layout are indicated on the diagrams either with an asterisk (*) or an apostrophe dash (‘).

In the center in the same Fig.- a simple UMZCH on transistors, which already develops power up to 4-6 W at a load of 4 ohms. Although it works, like the previous one, in the so-called. class AB1, not intended for Hi-Fi sound, but if you replace a pair of such class D amplifier (see below) in cheap Chinese computer speakers, their sound improves markedly. Here we learn another trick: powerful output transistors must be placed on radiators. Components that require additional cooling are circled in the diagrams with a dotted line; however, not always; sometimes - with an indication of the required dissipating area of the heat sink. Adjustment of this UMZCH - balancing with R2.

On the right in fig.- not yet a 350 W monster (as was shown at the beginning of the article), but already quite a solid beast: a simple 100 W transistor amplifier. You can listen to music through it, but not Hi-Fi, the work class is AB2. However, for scoring a picnic area or an outdoor meeting, a school assembly or a small trading floor, it is quite suitable. An amateur rock band, having such an UMZCH for an instrument, can perform successfully.

In this UMZCH, 2 more tricks appear: firstly, in very powerful amplifiers, the buildup cascade of a powerful output also needs to be cooled, so VT3 is put on a radiator from 100 sq. see. For output VT4 and VT5, radiators from 400 square meters are needed. see Secondly, UMZCH with bipolar power supply are not balanced at all without load. Either one or the other output transistor goes into cutoff, and the conjugated one goes into saturation. Then, at full supply voltage, current surges during balancing can destroy the output transistors. Therefore, for balancing (R6, did you guess?), the amplifier is powered from +/-24 V, and instead of the load, a 100 ... 200 Ohm wire resistor is included. By the way, the squiggles in some of the resistors in the diagram are Roman numerals, denoting their required heat dissipation power.

Note: a power source for this UMZCH needs a power of 600 watts or more. Smoothing filter capacitors - from 6800 uF to 160 V. In parallel with the electrolytic capacitors of the IP, ceramic ones of 0.01 uF are turned on to prevent self-excitation at ultrasonic frequencies, which can instantly burn out the output transistors.

On the field workers

On the trail. rice. - another option for a fairly powerful UMZCH (30 W, and with a supply voltage of 35 V - 60 W) on powerful field-effect transistors:

The sound from it already draws on the requirements for entry-level Hi-Fi (if, of course, the UMZCH works on the corresponding acoustic systems, speakers). Powerful field workers do not require much power for buildup, so there is no pre-power cascade. Even powerful field-effect transistors do not burn the speakers under any malfunctions - they themselves burn out faster. Also unpleasant, but still cheaper than changing an expensive bass speaker head (GG). Balancing and generally adjustment to this UMZCH are not required. It has only one drawback, like a design for beginners: powerful field-effect transistors are much more expensive than bipolar ones for an amplifier with the same parameters. IP requirements are the same as before. occasion, but its power is needed from 450 watts. Radiators - from 200 sq. cm.

Note: no need to build powerful UMZCH on field-effect transistors for switching power supplies, for example. computer. When trying to “drive” them into the active mode necessary for the UMZCH, they either simply burn out, or they give a weak sound, but “none” in quality. The same applies to powerful high-voltage bipolar transistors, for example. from the horizontal scanning of old TVs.

Right up

If you have already taken the first steps, then it will be quite natural to want to build UMZCH class Hi-Fi, without going too deep into the theoretical jungle. To do this, you will have to expand the instrument park - you need an oscilloscope, an audio frequency generator (GZCH) and an AC millivoltmeter with the ability to measure the DC component. It is better to take the UMZCH E. Gumeli, described in detail in Radio No. 1 for 1989, as a prototype for repetition. To build it, you will need a few inexpensive affordable components, but the quality meets very high requirements: power up to 60 W, bandwidth 20-20,000 Hz, frequency response unevenness 2 dB, non-linear distortion factor (THD) 0.01%, self-noise level -86 dB. However, setting up the Gumeli amplifier is quite difficult; if you can handle it, you can take on any other. However, some of the circumstances now known greatly simplify the establishment of this UMZCH, see below. Bearing this in mind and the fact that not everyone succeeds in getting into the Radio archives, it would be appropriate to repeat the main points.

Schemes of a simple high-quality UMZCH

UMZCH Gumeli schemes and specifications for them are given in the illustration. Radiators of output transistors - from 250 sq. see for UMZCH according to fig. 1 and from 150 sq. see for variant according to fig. 3 (numbering is original). The transistors of the pre-output stage (KT814/KT815) are mounted on radiators bent from aluminum plates 75x35 mm 3 mm thick. It is not worth replacing KT814 / KT815 with KT626 / KT961, the sound does not noticeably improve, but it is seriously difficult to establish.

This UMZCH is very critical to the power supply, installation topology and general, therefore, it must be adjusted in a structurally finished form and only with a standard power source. When trying to power from a stabilized IP, the output transistors burn out immediately. Therefore, in fig. drawings of original printed circuit boards and instructions for setting up are given. It can be added to them that, firstly, if “excitation” is noticeable at the first start, they fight with it by changing the inductance L1. Secondly, the leads of the parts installed on the boards must be no longer than 10 mm. Thirdly, it is highly undesirable to change the installation topology, but, if it is very necessary, there must be a frame screen on the side of the conductors (ground loop, highlighted in color in the figure), and the power supply paths must pass outside it.

Note: breaks in the tracks to which the bases of powerful transistors are connected - technological ones, for establishing, after which they are sealed with drops of solder.

The establishment of this UMZCH is greatly simplified, and the risk of encountering "excitation" in the process of use is reduced to zero if:

Minimize interconnect wiring by placing boards on high-power transistor heatsinks.
Completely abandon the connectors inside, performing the entire installation only by soldering. Then you will not need R12, R13 in a powerful version or R10 R11 in a less powerful one (they are dotted on the diagrams).
Use the minimum length of oxygen-free copper audio wires for indoor wiring.

When these conditions are met, there are no problems with excitation, and the establishment of UMZCH is reduced to a routine procedure, described in Fig.

Wires for sound

Audio wires are not idle fiction. The need for their use at the present time is undeniable. In copper with an admixture of oxygen, the thinnest oxide film is formed on the faces of metal crystallites. Metal oxides are semiconductors and if the current in the wire is weak without a constant component, its shape is distorted. In theory, distortions on myriads of crystallites should compensate each other, but very little (it seems, due to quantum uncertainties) remains. Enough to be noticed by discerning listeners against the background of the purest sound of modern UMZCH.

Manufacturers and traders without a twinge of conscience slip ordinary electrical copper instead of oxygen-free copper - it is impossible to distinguish one from the other by eye. However, there is a scope where a fake does not go unambiguously: a twisted-pair cable for computer networks. Put a grid with long segments on the left, it will either not start at all, or it will constantly fail. Dispersion of impulses, you know.

The author, when there was still talk about audio wires, realized that, in principle, this was not empty chatter, especially since oxygen-free wires by that time had long been used in special-purpose equipment, with which he was well acquainted with the type of activity. Then I took it and replaced the regular cord of my TDS-7 headphones with a home-made one from a “vitukha” with flexible stranded wires. The sound, by ear, has steadily improved for analog tracks through, i.e. on the way from the studio microphone to the disc, never digitized. Recordings on vinyl made using DMM technology (Direct Meta lMastering, direct metal deposition) sounded especially bright. After that, the interblock editing of all home audio was converted to "vitushny". Then completely random people began to notice the improvement in sound, they were indifferent to music and not forewarned in advance.

How to make interconnect wires from twisted pair, see next. video.

Video: do-it-yourself twisted-pair interconnect wires

Unfortunately, the flexible "vituha" soon disappeared from sale - it did not hold well in crimped connectors. However, for the information of readers, flexible “military” wire MGTF and MGTFE (shielded) is made only from oxygen-free copper. Forgery is impossible, because. on ordinary copper, fluoroplastic tape insulation spreads rather quickly. MGTF is now widely available and is much cheaper than branded, guaranteed audio wires. It has one drawback: it cannot be done colored, but this can be corrected with tags. There are also oxygen-free winding wires, see below.

Theoretical interlude

As you can see, already at the very beginning of mastering sound engineering, we had to deal with the concept of Hi-Fi (High Fidelity), high fidelity of sound reproduction. Hi-Fi comes in different levels, which are ranked next. main parameters:

Band of reproducible frequencies.
Dynamic range - the ratio in decibels (dB) of the maximum (peak) output power to the level of self-noise.
Self-noise level in dB.
Nonlinear distortion factor (THD) at rated (long-term) output power. SOI at peak power is assumed to be 1% or 2% depending on the measurement technique.
Irregularities in the amplitude-frequency characteristic (AFC) in the reproducible frequency band. For speakers - separately at low (LF, 20-300 Hz), medium (MF, 300-5000 Hz) and high (HF, 5000-20,000 Hz) audio frequencies.

Note: the ratio of the absolute levels of any values of I in (dB) is defined as P(dB) = 20lg(I1/I2). If I1

You need to know all the subtleties and nuances of Hi-Fi when designing and building speakers, and as for a home-made Hi-Fi UMZCH for the home, before moving on to these, you need to clearly understand the requirements for their power required for scoring a given room, dynamic range (dynamics), self-noise level and SOI. To achieve a frequency band of 20-20,000 Hz from the UMZCH with a blockage at the edges of 3 dB and a frequency response unevenness at the midrange of 2 dB on a modern element base is not very difficult.

Volume

The power of the UMZCH is not an end in itself, it should provide the optimal volume of sound reproduction in a given room. It can be determined by curves of equal loudness, see fig. Natural noise in residential premises is quieter than 20 dB; 20 dB is the wilderness in complete calm. The volume level of 20 dB relative to the threshold of hearing is the threshold of intelligibility - you can still make out the whisper, but the music is perceived only as a fact of its presence. An experienced musician can tell which instrument is playing, but not exactly what.

40 dB - the normal noise of a well-insulated city apartment in a quiet area or a country house - represents the threshold of intelligibility. Music from the threshold of intelligibility to the threshold of intelligibility can be listened to with a deep frequency response correction, primarily in bass. To do this, the MUTE function is introduced into modern UMZCH (mute, mutation, not mutation!), Which includes resp. corrective circuits in UMZCH.

90 dB is the volume level of a symphony orchestra in a very good concert hall. 110 dB can give out an expanded orchestra in a hall with unique acoustics, of which there are no more than 10 in the world, this is the threshold of perception: louder sounds are perceived even as distinguishable in meaning with an effort of will, but already annoying noise. The loudness zone in residential premises of 20-110 dB is the zone of full audibility, and 40-90 dB is the zone of the best audibility, in which unprepared and inexperienced listeners fully perceive the meaning of the sound. If, of course, he is in it.

Power

Calculating the power of the equipment for a given volume in the listening area is perhaps the main and most difficult task of electroacoustics. For yourself, in conditions, it is better to go from acoustic systems (AS): calculate their power using a simplified method, and take the nominal (long-term) power of the UMZCH equal to the peak (musical) speakers. In this case, the UMZCH will not noticeably add its distortions to those speakers, they are already the main source of non-linearity in the audio path. But the UMZCH should not be made too powerful: in this case, the level of its own noise may be above the threshold of audibility, because. it is considered from the voltage level of the output signal at maximum power. If we consider it very simply, then for a room of an ordinary apartment or house and speakers with normal characteristic sensitivity (sound output), we can take a trace. UMZCH optimal power values:

Up to 8 sq. m - 15-20 W.
8-12 sq. m - 20-30 W.
12-26 sq. m - 30-50 W.
26-50 sq. m - 50-60 W.
50-70 sq. m - 60-100 watts.
70-100 sq. m - 100-150 watts.
100-120 sq. m - 150-200 watts.
Over 120 sq. m - is determined by calculation according to acoustic measurements on site.

Dynamics

The dynamic range of UMZCH is determined by equal loudness curves and threshold values for different degrees of perception:

Symphonic music and jazz with symphonic accompaniment - 90 dB (110 dB - 20 dB) ideal, 70 dB (90 dB - 20 dB) acceptable. Sound with dynamics of 80-85 dB in a city apartment will not be distinguished from ideal by any expert.
Other serious musical genres - 75 dB is excellent, 80 dB is over the roof.
Pops of any kind and movie soundtracks - 66 dB for the eyes is enough, because. these opuses are already compressed in levels up to 66 dB and even up to 40 dB during recording, so that you can listen to anything.

The dynamic range of the UMZCH, correctly selected for a given room, is considered equal to its own noise level, taken with a + sign, this is the so-called. signal-to-noise ratio.

SOI

Nonlinear distortions (NI) UMZCH are components of the spectrum of the output signal, which were not in the input. Theoretically, it is best to “push” the NI under the level of its own noise, but technically this is very difficult to implement. In practice, they take into account the so-called. masking effect: at volume levels below approx. 30 dB the range of frequencies perceived by the human ear narrows, as does the ability to distinguish sounds by frequency. Musicians hear notes, but it is difficult to assess the timbre of the sound. In people without a musical ear, the masking effect is already observed at 45-40 dB of volume. Therefore, UMZCH with a THD of 0.1% (-60 dB from a volume level of 110 dB) will be assessed as a Hi-Fi by an ordinary listener, and with a THD of 0.01% (-80 dB) can be considered not distorting the sound.

Lamps

The last statement, perhaps, will cause rejection, up to furious, among adherents of tube circuitry: they say that only tubes give real sound, and not just any, but certain types of octal ones. Calm down, gentlemen - a special tube sound is not fiction. The reason is fundamentally different distortion spectra for electronic tubes and transistors. Which, in turn, are due to the fact that the electron flow in the lamp moves in a vacuum and quantum effects do not appear in it. A transistor is a quantum device, where minor charge carriers (electrons and holes) move in a crystal, which is generally impossible without quantum effects. Therefore, the spectrum of tube distortions is short and clean: only harmonics up to the 3rd - 4th are clearly traced in it, and there are very few combination components (sums and differences of the frequencies of the input signal and their harmonics). Therefore, in the days of vacuum circuitry, SOI was called the harmonic coefficient (KH). In transistors, the distortion spectrum (if they are measurable, the reservation is random, see below) can be traced up to the 15th and higher components, and there are more than enough combination frequencies in it.

At the beginning of solid-state electronics, the designers of transistorized UMZCH took for them the usual "tube" SOI of 1-2%; a sound with a tube distortion spectrum of this magnitude is perceived by ordinary listeners as clean. By the way, the very concept of Hi-Fi did not exist then. It turned out - they sound dull and deaf. In the process of the development of transistor technology, an understanding was developed of what Hi-Fi is and what is needed for it.

At present, the growing pains of transistor technology have been successfully overcome and side frequencies at the output of a good UMZCH are hardly captured by special measurement methods. And lamp circuitry can be considered to have passed into the category of art. Its basis can be any, why can't electronics go there? An analogy with photography would be appropriate here. No one can deny that a modern digital SLR gives an image immeasurably clearer, more detailed, deeper in terms of brightness and color range than a plywood box with an accordion. But someone with the coolest Nikon "clicks pictures" like "this is my fat cat got drunk like a bastard and sleeps with his paws spread", and someone with Smena-8M on a Svemov b / w film takes a picture in front of which people are crowding at a prestigious exhibition.

Note: and once again calm down - not everything is so bad. To date, low-power lamp UMZCHs have at least one application left, and not of the least importance, for which they are technically necessary.

Experimental stand

Many audio lovers, having barely learned how to solder, immediately "go into the lamps." This is by no means deserving of condemnation, on the contrary. Interest in the origins is always justified and useful, and electronics has become such on lamps. The first computers were tube-based, and the on-board electronic equipment of the first spacecraft was also tube-based: there were already transistors at that time, but they could not withstand extraterrestrial radiation. By the way, then, under the strictest secrecy, tube ... microcircuits were also created! Cold cathode microlamps. The only known mention of them in open sources is in the rare book by Mitrofanov and Pickersgil "Modern receiving-amplifying lamps".

But enough of the lyrics, let's get down to business. For those who like to tinker with the lamps in fig. - a diagram of a bench lamp UMZCH, designed specifically for experiments: SA1 switches the operating mode of the output lamp, and SA2 switches the supply voltage. The circuit is well known in the Russian Federation, a slight refinement touched only the output transformer: now you can not only “drive” your native 6P7S in different modes, but also select the screen grid switching ratio for other lamps in ultra-linear mode; for the vast majority of output pentodes and beam tetrodes, it is either 0.22-0.25, or 0.42-0.45. See below for output transformer manufacturing.

Guitarists and rockers

This is the case when you can not do without lamps. As you know, the electric guitar became a full-fledged solo instrument after the pre-amplified signal from the pickup began to pass through a special prefix - fuser - deliberately distorting its spectrum. Without this, the sound of the string was too sharp and short, because. an electromagnetic pickup reacts only to the modes of its mechanical oscillations in the plane of the soundboard of the instrument.

An unpleasant circumstance soon emerged: the sound of an electric guitar with a fuser gains full strength and brightness only at high volumes. This is especially evident for guitars with a humbucker pickup, which gives the most "evil" sound. But what about a beginner, forced to rehearse at home? Do not go to the hall to perform, not knowing exactly how the instrument will sound there. And just rock lovers want to listen to their favorite things in full juice, and rockers are generally decent and non-conflict people. At least those who are interested in rock music, and not outrageous surroundings.

So, it turned out that the fatal sound appears at volume levels acceptable for residential premises, if the UMZCH is tube. The reason is the specific interaction of the signal spectrum from the fuser with a clean and short spectrum of tube harmonics. Here again, an analogy is appropriate: a b / w photo can be much more expressive than a color one, because. leaves only the contour and the light for viewing.

Those who need a tube amplifier not for experiments, but because of technical necessity, have no time to master the intricacies of tube electronics for a long time, they are passionate about others. UMZCH in this case, it is better to do transformerless. More precisely, with a single-ended matching output transformer that operates without constant bias. This approach greatly simplifies and speeds up the manufacture of the most complex and critical assembly of the lamp UMZCH.

“Transformerless” UMZCH tube output stage and preamplifiers for it

On the right in fig. a diagram of a transformerless output stage of a tube UMZCH is given, and on the left are options for a preamplifier for it. Above - with a tone control according to the classic Baksandal scheme, which provides a fairly deep adjustment, but introduces small phase distortions into the signal, which can be significant when operating the UMZCH on a 2-way speaker. Below is a simpler preamplifier with tone control that does not distort the signal.

But let's get back to the end. In a number of foreign sources, this circuit is considered a revelation, however, identical to it, with the exception of the capacity of electrolytic capacitors, is found in the Soviet Radio Amateur's Handbook of 1966. A thick book of 1060 pages. There was no Internet then and databases on disks.

In the same place, on the right in the figure, the shortcomings of this scheme are briefly but clearly described. Improved, from the same source, given on the trail. rice. on right. In it, the screen grid L2 is powered from the midpoint of the anode rectifier (the anode winding of the power transformer is symmetrical), and the screen grid L1 through the load. If, instead of high-impedance speakers, you turn on a matching transformer with a conventional speaker, as in the previous. circuit, the output power is approx. 12 W, because the active resistance of the primary winding of the transformer is much less than 800 ohms. SOI of this final stage with a transformer output - approx. 0.5%

How to make a transformer?

The main enemies of the quality of a powerful signal low-frequency (sound) transformer are the magnetic stray field, the lines of force of which are closed, bypassing the magnetic circuit (core), eddy currents in the magnetic circuit (Foucault currents) and, to a lesser extent, magnetostriction in the core. Because of this phenomenon, a carelessly assembled transformer "sings", buzzes or squeaks. Foucault currents are fought by reducing the thickness of the plates of the magnetic circuit and additionally isolating them with varnish during assembly. For output transformers, the optimal thickness of the plates is 0.15 mm, the maximum allowable is 0.25 mm. Thinner plates should not be taken for the output transformer: the fill factor of the core (the central core of the magnetic circuit) with steel will fall, the cross section of the magnetic circuit will have to be increased to obtain a given power, which will only increase distortion and losses in it.

In the core of an audio transformer operating with a constant bias (eg, anode current of a single-ended output stage), there must be a small (determined by calculation) non-magnetic gap. The presence of a non-magnetic gap, on the one hand, reduces signal distortion from constant bias; on the other hand, in a conventional magnetic circuit it increases the stray field and requires a larger core. Therefore, the non-magnetic gap must be calculated at the optimum and performed as accurately as possible.

For transformers operating with magnetization, the optimal type of core is made of Shp plates (punched), pos. 1 in fig. In them, a non-magnetic gap is formed during the penetration of the core and therefore is stable; its value is indicated in the passport for the plates or measured with a set of probes. The stray field is minimal, because the side branches through which the magnetic flux closes are solid. Shp plates are often used to assemble transformer cores without magnetization, because Shp plates are made of high quality transformer steel. In this case, the core is assembled in an overlap (the plates are placed with a notch in one direction or the other), and its cross section is increased by 10% against the calculated one.

It is better to wind transformers without magnetization on USh cores (reduced height with widened windows), pos. 2. In them, the reduction of the stray field is achieved by reducing the length of the magnetic path. Since USh plates are more accessible than Shp, transformer cores with magnetization are often also made from them. Then the assembly of the core is carried out in a cut: a package of W-plates is assembled, a strip of non-conductive non-magnetic material is laid with a thickness equal to the value of the non-magnetic gap, covered with a yoke from a package of jumpers and pulled together with a clip.

Note:"Audio" signal magnetic circuits of the ShLM type for output transformers of high-quality tube amplifiers are of little use, they have a large stray field.

At pos. 3 is a diagram of the dimensions of the core for calculating the transformer, at pos. 4 winding frame design, and on pos. 5 - patterns of its details. As for the transformer for the "transformerless" output stage, it is better to do it on the SLMme with an overlap, because. the bias is negligible (the bias current is equal to the current of the screen grid). The main task here is to make the windings as compact as possible in order to reduce the stray field; their active resistance will still turn out to be much less than 800 ohms. The more free space left in the windows, the better the transformer turned out. Therefore, the windings wind turn to turn (if there is no winding machine, this is a terrible machine) from the thinnest possible wire, the anode winding laying coefficient for the mechanical calculation of the transformer is taken as 0.6. The winding wire is of the PETV or PEMM brands, they have an oxygen-free core. It is not necessary to take PETV-2 or PEMM-2, they have an increased outer diameter due to double varnishing and the scattering field will be larger. The primary winding is wound first, because. it is its stray field that most affects the sound.

Iron for this transformer must be looked for with holes in the corners of the plates and clamps (see the figure on the right), because. "For complete happiness" the assembly of the magnetic circuit is carried out in the next. order (of course, the windings with leads and outer insulation should already be on the frame):

Prepare half-diluted acrylic varnish or, in the old fashioned way, shellac;
Plates with jumpers are quickly varnished on one side and put into the frame as quickly as possible, without pressing hard. The first plate is placed with the lacquered side inward, the next - with the unvarnished side to the lacquered first, etc.;
When the frame window is full, staples are applied and tightened tightly with bolts;
After 1-3 minutes, when the extrusion of varnish from the gaps apparently stops, the plates are added again until the window is filled;
Repeat paragraphs. 2-4 until the window is tightly packed with steel;
The core is pulled tightly again and dried on a battery or the like. 3-5 days.

The core assembled using this technology has very good plate insulation and steel filling. Losses due to magnetostriction are not detected at all. But keep in mind - for the cores of their permalloy, this technique is not applicable, because. from strong mechanical influences, the magnetic properties of permalloy irreversibly deteriorate!

On microchips

UMZCH on integrated circuits (ICs) is most often done by those who are satisfied with sound quality up to average Hi-Fi, but are more attracted by cheapness, speed, ease of assembly and the complete absence of any adjustment procedures that require special knowledge. Simply, an amplifier on microcircuits is the best option for dummies. The classic of the genre here is UMZCH on the TDA2004 IC, standing on the series, God forbid, for 20 years, on the left in fig. Power - up to 12 W per channel, supply voltage - 3-18 V unipolar. Radiator area - from 200 sq. see for maximum power. The advantage is the ability to work on a very low-resistance, up to 1.6 Ohm, load, which allows you to remove full power when powered from the 12 V on-board network, and 7-8 W - with a 6-volt power supply, for example, on a motorcycle. However, the TDA2004 output in class B is non-complementary (on transistors of the same conductivity), so the sound is definitely not Hi-Fi: THD 1%, dynamics 45 dB.

The more modern TDA7261 gives no better sound, but more powerful, up to 25 W, because. the upper limit of the supply voltage has been increased to 25V. TDA7261 can be run from almost all on-board networks, except for aircraft 27 V. With the help of hinged components (strapping, on the right in the figure), TDA7261 can operate in mutation mode and with the St-By (Stand By, wait) function, which switches the UMZCH to the minimum power consumption mode when there is no input signal for a certain time. Amenities cost money, so for a stereo you will need a pair of TDA7261 with radiators from 250 sq. see for each.

Note: if you are attracted to amplifiers with the St-By function, keep in mind that you should not expect speakers wider than 66 dB from them.

"Super-economical" in terms of power TDA7482, on the left in the figure, working in the so-called. class D. Such UMZCH are sometimes called digital amplifiers, which is not true. For true digitization, level samples are taken from an analog signal at a quantization frequency of at least twice the highest of the reproducible frequencies, the value of each sample is recorded in an error-correcting code and stored for future use. UMZCH class D - pulsed. In them, the analogue is directly converted into a sequence of high-frequency pulse-width modulated (PWM) pulses, which is fed to the speaker through a low-pass filter (LPF).

Class D sound has nothing to do with Hi-Fi: THD of 2% and dynamics of 55 dB for UMZCH class D are considered very good indicators. And TDA7482 here, I must say, the choice is not optimal: other companies specializing in class D produce UMZCH ICs cheaper and require less strapping, for example, the Paxx D-UMZCH series, on the right in Fig.

Of the TDAs, it should be noted the 4-channel TDA7385, see the figure, on which you can assemble a good amplifier for speakers up to medium Hi-Fi inclusive, with frequency separation into 2 bands or for a system with a subwoofer. The filtering of low-frequency and mid-high frequencies in both cases is done at the input on a weak signal, which simplifies the design of the filters and allows for a deeper separation of the bands. And if the acoustics are subwoofer, then 2 channels of the TDA7385 can be allocated for the sub-ULF of the bridge circuit (see below), and the remaining 2 can be used for midrange-high frequencies.

UMZCH for subwoofer

A subwoofer, which can be translated as a "subwoofer" or, literally, "a subwoofer" reproduces frequencies up to 150-200 Hz, in this range, human ears are practically unable to determine the direction to the sound source. In speakers with a subwoofer, the “subwoofer” speaker is placed in a separate acoustic design, this is the subwoofer as such. The subwoofer is placed, in principle, as it is more convenient, and the stereo effect is provided by separate MF-HF channels with their own small-sized speakers, for the acoustic design of which there are no particularly serious requirements. Connoisseurs agree that it is still better to listen to stereo with full channel separation, but subwoofer systems significantly save money or labor on the bass path and make it easier to place acoustics in small rooms, which is why they are popular with consumers with normal hearing and not particularly demanding.

“Leakage” of midrange-high frequencies into the subwoofer, and from it into the air, greatly spoils the stereo, but if you sharply “cut off” the subbass, which, by the way, is very difficult and expensive, then a very unpleasant sound jump effect will occur. Therefore, channel filtering in subwoofer systems is done twice. At the input, MF-HF with bass "tails" are distinguished by electric filters, which do not overload the MF-HF path, but provide a smooth transition to sub-bass. Bass with midrange "tails" are combined and fed to a separate UMZCH for the subwoofer. The midrange is additionally filtered so that the stereo does not deteriorate, it is already acoustic in the subwoofer: the subwoofer is placed, for example, in the partition between the resonator chambers of the subwoofer that do not let the midrange out, see on the right in Fig.

A number of specific requirements are imposed on the UMZCH for a subwoofer, of which the "dummies" consider the greatest possible power to be the main one. This is completely wrong, if, say, the calculation of acoustics for a room gave peak power W for one speaker, then the power of the subwoofer needs 0.8 (2W) or 1.6W. For example, if speakers S-30 are suitable for the room, then a subwoofer is needed 1.6x30 \u003d 48 watts.

It is much more important to ensure the absence of phase and transient distortions: if they go, there will definitely be a sound jump. As for THD, it is acceptable up to 1%. Bass distortions of this level are not audible (see equal loudness curves), and the “tails” of their spectrum in the best audible midrange region will not get out of the subwoofer.

In order to avoid phase and transient distortions, the amplifier for the subwoofer is built according to the so-called. bridge circuit: the outputs of 2 identical UMZCH are turned on in the opposite direction through the speaker; the signals to the inputs are in antiphase. The absence of phase and transient distortion in the bridge circuit is due to the complete electrical symmetry of the output signal paths. The identity of the amplifiers that form the shoulders of the bridge is ensured by the use of paired UMZCH on ICs, made on the same chip; this is perhaps the only case when an amplifier on microcircuits is better than a discrete one.

Note: the power of the bridge UMZCH does not double, as some people think, it is determined by the supply voltage.

An example of a bridge UMZCH circuit for a subwoofer in a room up to 20 sq. m (without input filters) on the TDA2030 IC is given in fig. left. Additional midrange filtering is carried out by the R5C3 and R'5C'3 circuits. Radiator area TDA2030 - from 400 sq. see. Bridge UMZCHs with an open output have an unpleasant feature: when the bridge is unbalanced, a constant component appears in the load current that can disable the speaker, and protection circuits on the subbass often fail, turning off the speaker when not needed. Therefore, it is better to protect the expensive “dubovo” woofer with non-polar batteries of electrolytic capacitors (highlighted in color, and the diagram of one battery is given in the sidebar.

A little about acoustics

The acoustic design of a subwoofer is a special topic, but since a drawing is given here, explanations are also needed. Case material - MDF 24 mm. The resonator tubes are made of sufficiently durable non-ringing plastic, for example, polyethylene. The internal diameter of the pipes is 60 mm, the protrusions inward are 113 mm in the large chamber and 61 in the small one. For a specific speaker head, the subwoofer will have to be reconfigured for the best bass and, at the same time, for the least impact on the stereo effect. To tune the pipes, they take obviously longer lengths and, pushing in and out, achieve the desired sound. The outward protrusions of the pipes do not affect the sound, they are then cut off. The pipe settings are interdependent, so you have to tinker.

Headphone Amplifier

A headphone amplifier is made by hand most often for 2 reasons. The first is for listening "on the go", i.e. outside the home, when the power of the audio output of the player or smartphone is not enough to build up "buttons" or "burdocks". The second is for high-end home headphones. Hi-Fi UMZCH for an ordinary living room is needed with dynamics up to 70-75 dB, but the dynamic range of the best modern stereo headphones exceeds 100 dB. An amplifier with such dynamics is more expensive than some cars, and its power will be from 200 watts per channel, which is too much for an ordinary apartment: listening at a very low power level spoils the sound, see above. Therefore, it makes sense to make a low-power, but with good dynamics, a separate amplifier specifically for headphones: the prices for household UMZCHs with such a makeweight are obviously too high.

The diagram of the simplest headphone amplifier on transistors is given in pos. 1 fig. Sound - except for Chinese "buttons", works in class B. It also does not differ in efficiency - 13-mm lithium batteries last for 3-4 hours at full volume. At pos. 2 - TDA classic for on-the-go headphones. The sound, however, gives quite decent, up to average Hi-Fi, depending on the parameters of the track digitization. Amateur improvements to the TDA7050 strapping are innumerable, but no one has yet achieved the transition of sound to the next level of class: the “mikruha” itself does not allow. TDA7057 (pos. 3) is simply more functional, you can connect the volume control on a regular, not dual, potentiometer.

UMZCH for headphones on the TDA7350 (pos. 4) is already designed to build up good individual acoustics. It is on this IC that headphone amplifiers are assembled in most household UMZCHs of the middle and high class. The UMZCH for headphones on the KA2206B (pos. 5) is already considered professional: its maximum power of 2.3 W is enough to drive such serious isodynamic "burdocks" as TDS-7 and TDS-15.

UMZCH repair technique

UMZCH repair is almost the most frequent of the questions asked on amateur radio forums. And it is also one of the most difficult. Of course, there are “favorite” malfunctions, but in principle, any of several dozen, or even hundreds of components that make up the amplifier can fail. Moreover, there are a great many UMZCH schemes.

Of course, it is not possible to cover all the cases encountered in repair practice, however, if you follow a certain algorithm, then in the vast majority of cases it is possible to restore the device to working capacity in a quite acceptable time. This algorithm was developed by me from the experience of repairing about fifty different UMZCH, from the simplest, for a few watts or tens of watts, to concert "monsters" of 1 ... 2 kW per channel, most of which were sent for repair without circuit diagrams.

The main task of repairing any UMZCH is to localize a failed element, which resulted in the inoperability of both the entire circuit and the failure of other cascades. Since there are only 2 types of defects in electrical engineering:

the presence of contact where it should not be;
lack of contact where it should be,

then the “super task” of repair is to find a broken or torn element. And for this - to find the cascade where it is located. Next - "a matter of technology." As doctors say: "A correct diagnosis is half the cure."

The list of equipment and tools necessary (or at least highly desirable) for repairs:

Screwdrivers, side cutters, pliers, scalpel (knife), tweezers, magnifier - that is, the minimum required set of conventional mounting tools.
Tester (multimeter).
Oscilloscope.
A set of incandescent lamps for various voltages - from 220 V to 12 V (2 pcs each).
Low-frequency sinusoidal voltage generator (highly desirable).
Bipolar regulated power supply for 15 ... 25 (35) V with output current limitation (highly desirable).
Capacitance and Equivalent Series Resistance Meter ( ESR ) capacitors (highly desirable).
And finally, the most important tool is the head on the shoulders (required!).

Consider this algorithm using the example of repairing a hypothetical transistor UMZCH with bipolar transistors in the output stages (Fig. 1), which is not too primitive, but not very complicated either. Such a scheme is the most common "classic of the genre." Functionally, it consists of the following blocks and nodes:

A) bipolar power supply (not shown);

b) transistor differential input stage VT2, VT 5 with current mirror on transistors VT1 and VT 4 in their collector loads and their emitter current stabilizer on VT3;

V) voltage amplifier VT6 and VT 8 in cascode connection, with a load in the form of a current generator on VT7;

G) node of thermal stabilization of the quiescent current on the transistor VT9;

e) node for protecting output transistors from overcurrent on transistors VT 10 and VT 11;

e) current amplifier on complementary triplets of transistors connected according to the Darlington circuit in each arm ( VT 12 VT 14 VT 16 and VT 13 VT 15 VT 17).

Rice. 1.

The first point of any repair is an external examination of the subject and its sniffing (!). This alone allows sometimes at least to assume the essence of the defect. If it smells burnt, it means that something is clearly on fire.

Checking the presence of mains voltage at the input: the mains fuse has blown stupidly, the fastening of the wires of the mains cord in the plug has become loose, a break in the mains cord, etc. The stage is the most banal in nature, but at which the repair ends in about 10% of cases.

We are looking for a circuit for an amplifier. In the instructions, on the Internet, from acquaintances, friends, etc. Unfortunately, more and more often in recent years - unsuccessfully. We didn’t find it - we sigh heavily, sprinkle ashes on our heads and set about drawing a circuit for the board. You can skip this step. If the result is unimportant. But it's better not to miss it. It's dreary, long, disgusting, but - "It is necessary, Fedya, it is necessary ..." ((C) "Operation" Y "...).

We open the subject and make an external examination of its "offal". Use a magnifying glass if needed. You can see the destroyed cases of p / n devices, darkened, charred or destroyed resistors, swollen electrolytic capacitors or electrolyte leaks from them, broken conductors, printed circuit board tracks, etc. If one is found, this is not yet a reason for joy: the destroyed parts may be the result of the failure of some “flea”, which is visually intact.

We check the power supply. We unsolder the wires going from the PSU to the circuit (or disconnect the connector, if any). We take out the mains fuse and solder the lamp for 220 V (60 ... 100 W) to the contacts of its holder. It will limit the current in the primary winding of the transformer, as well as the currents in the secondary windings.

We turn on the amplifier. The lamp should blink (during the charging of the filter capacitors) and go out (a weak glow of the thread is allowed). This means that K.Z. there is no mains transformer on the primary winding, just as there is no obvious short circuit. in its secondary windings. With a tester in alternating voltage mode, we measure the voltage on the primary winding of the transformer and on the lamp. Their sum must be equal to the network. We measure the voltage on the secondary windings. They must be proportional to what is actually measured on the primary (relative to the nominal). You can turn off the lamp, put the fuse back in place and turn on the amplifier directly to the network. We repeat the voltage test on the primary and secondary windings. The ratio (proportion) between them should be the same as when measuring with a lamp.

The lamp burns constantly at full incandescence - which means we have a short circuit. in the primary circuit: we check the integrity of the insulation of the wires coming from the network connector, the power switch, the fuse holder. We solder one of the reasons going to the primary winding of the transformer. The lamp went out - most likely the primary winding (or interturn short circuit) failed.

The lamp burns constantly in an incomplete glow - most likely, a defect in the secondary windings or in the circuits connected to them. Solder one wire from the secondary windings to the rectifier(s). Do not confuse, Kulibin! So that later it would not be excruciatingly painful from the wrong soldering back (mark, for example, using pieces of adhesive masking tape). The lamp went out - it means that everything is in order with the transformer. Lit - again we sigh heavily and either look for a replacement for him, or rewind.

It was determined that the transformer is in order, and the defect is in the rectifiers or filter capacitors. We call the diodes (it is advisable to unsolder under one wire going to their terminals, or solder it if it is an integral bridge) with a tester in ohmmeter mode at the minimum limit. Digital testers in this mode often lie, so it is advisable to use a pointer device. Personally, I have been using a “beeper” dialer for a long time (Fig. 2, 3). Diodes (bridge) are broken or broken - we change. Integers - “call” the filter capacitors. Before measuring, they must be discharged (!!!) through a 2-watt resistor with a resistance of about 100 ohms. Otherwise, you can burn the tester. If the capacitor is intact, when closing, the arrow first deviates to the maximum, and then rather slowly (as the capacitor charges) “creeps” to the left. We change the connection of the probes. The arrow first goes off scale to the right (there is a charge left on the capacitor from the previous measurement) and then creeps to the left again. If there is a capacitance meter and ESR , it is highly recommended to use it. Broken or broken capacitors are changed.

Rice. 2. Fig. 3.

Rectifiers and capacitors are intact, but is there a voltage stabilizer at the output of the power supply? No problem. Between the output of the rectifier(s) and the input(s) of the stabilizer(s), we turn on the lamp(s) (a chain(s) of lamps) for a total voltage close to that indicated on the filter capacitor housing. The lamp caught fire - a defect in the stabilizer (if it is integral), or in the circuit for generating the reference voltage (if it is on discrete elements), or the capacitor at its output is broken. A broken control transistor is determined by ringing its outputs (solder out!).

Is everything in order with the power supply (are the voltages at its output symmetrical and nominal)? Let's move on to the most important thing - the amplifier itself. We select a lamp (or chains of lamps) for a total voltage not lower than the nominal voltage from the PSU output and through it (them) we connect the amplifier board. Moreover, it is desirable to each of the channels separately. Turn on. Both lamps lit up - both arms of the output stages were broken. Only one - one of the shoulders. Although not a fact.

The lamps do not light up or only one of them burns. This means that the output stages are most likely intact. We connect a 10 ... 20 Ohm resistor to the output. Turn on. The lamps should blink (there are usually more power capacitors on the board). We apply a signal from the generator to the input (gain control - to the maximum). Lamps (both!) lit up. This means that the amplifier amplifies something (although it wheezes, phonitis, etc.) and further repair consists in finding an element that brings it out of the mode. More on this below.

For further verification, I personally do not use the standard amplifier power supply, but use a 2-polar stabilized PSU with a current limit of 0.5 A. If there is none, you can also use the amplifier PSU connected, as indicated, through incandescent lamps. You just need to carefully isolate their bases so as not to accidentally cause a short circuit and be careful not to break the flasks. But an external PSU is better. At the same time, the consumed current is also visible. A well-designed UMZCH allows fluctuations in supply voltages within fairly large limits. After all, we don’t need its super-duper parameters when repairing, just working capacity is enough.

So BP is fine. Let's move on to the amplifier board (Fig. 4). First of all, it is necessary to localize the cascade(s) with broken(s)/broken(s) component(s). For this extremely desirable have an oscilloscope. Without it, the efficiency of repair drops significantly. Although with the tester you can also do a lot of things. Almost all measurements are made without load(at idle). Let's say that at the output we have a "skew" of the output voltage from a few volts to the full supply voltage.

To begin with, we turn off the protection unit, for which we unsolder the right terminals of the diodes from the board VD 6 and VD 7 (in my practice it was three the case when the cause of inoperability was the failure of this particular node). We look at the voltage is not output. If it has returned to normal (there may be a residual skew of a few millivolts - this is the norm), we call VD 6, VD 7 and VT 10, VT 11. There may be breaks and breakdowns of passive elements. We found a broken element - we change and restore the connection of the diodes. Zero output? Is there an output signal (when a signal from the generator is applied to the input)? Repair completed.

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Rice. 4.

Has anything changed with the output signal? Leave the diodes disabled and move on.

We solder the right output of the OOS resistor from the board ( R 12 together with the right output C 6), as well as the left conclusions R 23 and R 24, which we connect with a wire jumper (shown in red in Fig. 4) and through an additional resistor (without numbering, about 10 kOhm) we connect to a common wire. We bridge with a wire jumper (red color) collectors VT8 and VT 7, excluding the capacitor C8 and the quiescent current thermal stabilization unit. As a result, the amplifier is separated into two independent nodes (an input stage with a voltage amplifier and a stage of output followers), which must work independently.

Let's see what we have in the end. Is there voltage fluctuation? This means that the transistor (s) of the “skewed” shoulder is broken. Solder, call, replace. At the same time, we also check passive components (resistors). The most common variant of the defect, however, it should be noted that very often it is consequence failure of some element in the previous cascades (including the protection node!). Therefore, the following points are still desirable to perform.

Is there no crossover? So the output stage is presumably intact. Just in case, we send a signal from the generator with an amplitude of 3 ... 5 V to point "B" (connection of resistors R 23 and R 24). The output should be a sinusoid with a well-defined "step", the upper and lower half-waves of which are symmetrical. If they are not symmetrical, it means that one of the shoulder transistors, where it is lower, has “burned out” (lost parameters). We drink, we call. At the same time, we also check passive components (resistors).

Is there no output at all? This means that the power transistors of both arms "through" flew out. It's sad, but you have to solder everything and ring with a subsequent replacement.

Component breaks are not ruled out. Here it is necessary to include the "8th tool". Checking and replacing...

Have you achieved symmetrical repetition at the output (with a step) of the input signal? The output stage has been repaired. And now you need to check the operability of the quiescent current thermal stabilization unit (transistor VT 9). Sometimes there is a violation of the contact of the variable resistor engine R 22 with resistive track. If it is included in the emitter circuit, as shown in the above diagram, nothing bad can happen to the output stage, because. at the connection point of the base VT 9 to divider R 20– R 22 R 21, the voltage simply rises, it opens more and, accordingly, the voltage drop between its collector and emitter decreases. A pronounced “step” will appear in the idle output signal.

However (very often), a tuning resistor is placed between the collector and the VT9 base. Extremely "fool-proof" option! Then, when the engine loses contact with the resistive track, the voltage at the base of VT9 decreases, it closes and, accordingly, the voltage drop between its collector and emitter increases, which leads to a sharp increase in the quiescent current of the output transistors, their overheating and, of course, thermal breakdown. An even more stupid version of this cascade is if the VT9 base is connected only to the variable resistor engine. Then, if contact is lost, anything can be on it, with corresponding consequences for the output stages.

If possible, it is worth rearranging R 22 into the base-emitter circuit. True, in this case, the adjustment of the quiescent current will become expressed non-linear from the angle of rotation of the engine, but IMHO it's not such a big price to pay for reliability. You can just replace the transistor VT 9 on the other, with the reverse type of conductivity, if the layout of the tracks on the board allows. This will not affect the operation of the thermal stabilization unit in any way, because. he is bipolar and does not depend on the type of conductivity of the transistor.

Verification of this cascade is complicated by the fact that, as a rule, connections to collectors VT8 and VT 7 are made by printed conductors. You will have to lift the legs of the resistors and make connections with wires (Fig. 4 shows breaks in the conductors). Between the positive and negative supply voltages and, respectively, the collector and emitter VT 9, resistors of approximately 10 kΩ are turned on (without numbering, shown in red) and the voltage drop across the transistor is measured VT 9 when rotating the trimmer slider R 22. Depending on the number of cascades of repeaters, it should vary within the range of approximately 3 ... 5 V (for “triples, as in the diagram) or 2.5 ... 3.5 V (for “twos”).

So we got to the most interesting, but also the most difficult - a differential cascade with a voltage amplifier. They work only together and it is fundamentally impossible to separate them into separate nodes.

We bridge the right terminal of the OOS resistor R 12 with manifolds VT 8 and VT 7 (dot " A", which is now his "exit"). We get a “stripped down” (without output stages) low-power op-amp, which is fully operational at idle (no load). We apply a signal with an amplitude of 0.01 to 1 V to the input and see what will happen at the point A. If we observe an amplified signal of a form symmetrical with respect to the ground, without distortion, then this cascade is intact.

The signal is sharply reduced in amplitude (low gain) - first of all, check the capacitance of the capacitor (s) C3 (C4, because manufacturers very often put only one polar capacitor for a voltage of 50 V or more to save money, counting that in reverse polarity it will still work, which is not gut). When it dries up or breakdown, the gain decreases sharply. If there is no capacitance meter, we simply check it by replacing it with a known good one.

The signal is skewed - first of all, check the capacitance of capacitors C5 and C9, shunting the power buses of the preamplifier after resistors R17 and R19 (if these RC filters exist at all, because they are often not installed).

The diagram shows two common options for balancing the zero level: resistor R6 or R 7 (there may be, of course, others), if the contact of the engine is broken, the output voltage may also be skewed. Check by rotating the engine (although if the contact is “majorly” broken, this may not work). Then try to bridge their extreme conclusions with the output of the engine with tweezers.

There is no signal at all - we look to see if there is one at all at the input (open R3 or C1, short circuit in R1, R2, C2, etc.). Only first you need to unsolder the VT2 base, because. on it the signal will be very small and look at the right terminal of the resistor R3. Of course, the input circuits can be very different from those shown in the figure - include the "8th tool". Helps.

Naturally, it is not realistic to describe all possible causal variants of defects. Therefore, further I will simply state how to check the nodes and components of this cascade.

Current stabilizers VT 3 and VT 7. Breakdowns or breaks are possible in them. Collectors are soldered from the board and the current between them and the ground is measured. Naturally, you first need to calculate the voltage at their bases and the values \u200b\u200bof the emitter resistors, what it should be. ( N. B .! In my practice, there was a case of self-excitation of the amplifier due to an excessively large resistor value R 10 supplied by the manufacturer. It helped to adjust its value on a fully working amplifier - without the above division into cascades).

Similarly, you can check the transistor VT 8: if you bridge the collector-emitter of the transistor VT 6, it also stupidly turns into a current generator.

differential stage transistors VT2V5T and current mirror VT 1 VT 4 and also VT 6 are checked by their continuity after soldering. It is better to measure the gain (if the tester has such a function). It is desirable to choose with the same gain.

A couple of words "off the record". For some reason, in the vast majority of cases, transistors of more and more power are put into each subsequent cascade. There is one exception to this dependence: on transistors of the voltage amplification stage ( VT8 and VT 7) dissipates 3...4 times more power than on pre-driver VT 12 and VT 23 (!!!). Therefore, if there is such an opportunity, they should be immediately replaced with medium power transistors. A good option would be KT940 / KT9115 or similar imported ones.

Quite common defects in my practice were non-soldered ("cold" soldering to the tracks / "patch" or poor tinning of the leads before soldering) component legs and broken transistor leads (especially in a plastic case) right next to the case, which were very difficult to see visually. Shake the transistors, carefully observing their conclusions. Worst case, unsolder and re-solder.

If all active components have been checked, and the defect persists, you need (again, with a heavy sigh), to remove at least one leg from the board and check the ratings of the passive components with a tester. There are frequent cases of breaks in fixed resistors without any external manifestations. Non-electrolytic capacitors, as a rule, do not break through / break, but anything can happen ...

Again, from the experience of repair: if darkened / charred resistors are visible on the board, and symmetrically in both arms, it is worth recalculating the power allocated to it. In the Zhytomyr amplifier " Dominator "The manufacturer put 0.25 W resistors in one of the cascades, which burned regularly (before I had 3 repairs). When I calculated their required power, I almost fell off my chair: it turned out that 3 (three!) Watts should be dissipated on them ...

Finally, everything worked ... We restore all the "broken" connections. The advice seems to be the most banal, but how many times forgotten !!! We restore in the reverse order and after each connection we check the amplifier for operability. Often, a cascading check, it seems, showed that everything was in order, and after the restoration of connections, the defect “creeped out” again. The last to solder the diodes of the current protection cascade.

Set the quiescent current. Between the PSU and the amplifier board, we turn on (if they were turned off earlier) a “garland” of incandescent lamps for the corresponding total voltage. We connect the load equivalent (4 or 8 ohm resistor) to the UMZCH output. Trimmer engine R 22, we set it to the lower position according to the diagram and apply a signal from a generator with a frequency of 10 ... 20 kHz (!!!) to the input of such an amplitude that the signal at the output is no more than 0.5 ... 1 V. step”, which is difficult to notice on a large signal and low frequency. By rotating the R22 engine, we achieve its elimination. In this case, the filaments of the lamps should glow slightly. You can also control the current with an ammeter by connecting it in parallel with each garland of lamps. Do not be surprised if it differs noticeably (but no more than 1.5 ... 2 times in a larger direction) from what is indicated in the tuning recommendations - after all, it is not “compliance with the recommendations” that matters to us, but the sound quality! As a rule, in the "recommendations" the quiescent current is significantly overestimated, in order to guarantee the achievement of the planned parameters ("for the worst"). We jump the “garlands” with a jumper, increase the output signal level to a level of 0.7 from the maximum (when the amplitude limitation of the output signal begins) and let the amplifier warm up for 20 ... 30 minutes. This mode is the most difficult for the output stage transistors - the maximum power is dissipated on them. If the "step" did not appear (at a low signal level), and the quiescent current increased by no more than 2 times, we consider the setting complete, otherwise we remove the "step" again (as indicated above).

We remove all temporary connections (do not forget !!!), assemble the amplifier completely, close the case and pour a glass, which we drink with a feeling of deep satisfaction with the work done. And that will not work!

Of course, within the framework of this article, the nuances of repairing amplifiers with "exotic" stages, with an op-amp at the input, with output transistors connected with an OE, with "two-story" output stages, and much more ...

Falconist

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