Tube preamplifier. Homemade tube amplifier

This module preamplifier with an input switch was developed by the Frenchman JL. Vandersleyen to work with audiophile power amplifiers of any level. It is implemented on pentode 6Zh32P(similar to EF86), allows you to connect up to four signal sources and provides amplification in 16 dB. A small switchable low-frequency correction allows you to compensate for the influence of the listening room.

The appearance of the structure is shown in the figure:

(click to enlarge)

Amplifier Specifications:

Bandwidth (at 1dB flatness) 10 Hz - 100 kHz
Bandwidth (at 0.1dB ripple) 20 Hz - 50 kHz
Active equalization (see description) + 3 dB at 50 Hz
rise time<2 мксек
distortion<0,1% при амплитуде сигнала 1 В в полосе 100 Гц — 10 кГц (на частоте 1 кГц типичное значение 0,03%)
Maximum output signal ~30 V with distortion up to 2% (THD)
Feedback depth - 18 dB
Signal to noise ratio > 90 dB
Input impedance 50 kOhm
The output impedance of the amplifier itself is 5 kOhm
Circuit output impedance - 100K logarithmic potentiometer
Channel separation > 50 dB
Inputs - RCA
Power: 6V - 400mA / 320VDC - 7mA
Dimensions 135 x 100 x 30 mm

Due to its rather compact dimensions, the unit can be built into the chassis of a finished amplifier or used as a stand-alone device (with an external power supply).

Figure 1 shows how the amplification stage works.

Part of the output signal is fed back - to the input, in antiphase, for tight control of the gain of the circuit. Thus, 18dB of negative feedback reduces the overall gain from +34dB to +16dB while reducing the stage's own distortion.
Due to the decrease in the influence of the RC feedback circuit (C11, R31) at low frequencies, the gain of the circuit in this range increases. The specified values ​​of 220 kΩ and 3.3 nF provide a 3 dB gain gain for frequencies below 100 Hz. (See below)

The preamplifier is implemented on pentode 6Zh32P, which was developed specifically for use in the input stages of tape recorders and is characterized by low microphone effect and high linearity.

The lamp characteristic has excellent linearity at a bias voltage of -3 V, and an anode voltage of 50 V DC, the voltage on the second grid is 180 V, on the third - 0 V (the characteristic is highlighted in red):

(click to enlarge)

circuit diagram

The preamplifier circuit is shown in the figure:

(click to enlarge)

One of the four inputs is selected by the S1 switch. The diagram does not indicate the values ​​​​of the resistors R1, R5, R9, R13, they are selected based on the required input sensitivity.
The input impedance of the amplifier is 50 kOhm. The relatively low input impedance of the lamp is further reduced by negative feedback. Therefore, the input resistance of the circuit is determined mainly by the value of the resistor R19.

Self-amplification of the lamp 50, due to feedback is reduced to 6.5.
Own distortion of the lamp due to the OOS decreased to 0.03% with a signal amplitude of 1V at the output.

note that own lamp noise, does not decrease due to feedback, but with the selected modes they turn out to be very low: the signal-to-noise ratio exceeds 90 dB.

An RC circuit has been added to the feedback circuit to compensate for the loss of low frequency gain that typically results from insufficient listening space. As stated at the beginning of the article, the boost is 3dB for frequencies below 100Hz.

If you do not need such a function, elements C11-C12, D1, K1-K2 can be omitted, and resistors R31-R32 can be replaced with jumpers.

Setting the volume control at the output of the preamplifier is optimal for minimizing
signal to noise ratio. At the same time, the risk of introducing the cascade into the limiting mode is excluded, since in order to obtain the maximum signal amplitude at the output of 30 V, an input signal with an amplitude of 4.6 V is needed! (a rare source is able to give out)

Power supply for the preamplifier.

The filament voltage of the lamps is applied to the contacts on the printed circuit board. Thanks to this, it is possible to switch the filaments in parallel, then a voltage of 6-6.3 V is required at a current consumption of 400 mA. Or you can connect the filaments of both lamps in series, then you need a voltage of 12V with a current of 200mA ...

According to the anode voltage, the amplifier consumes 7 mA. If you recalculate the value of the resistor R33, you can power the amplifier with a voltage of 300 to 320 VDC.

To enable low-frequency equalization, +24 VDC is required to drive two 12-volt relays.

Preamp design

Printed circuit boards

All elements of the circuit, including input connectors, relays, switch, volume control, are mounted on printed circuit boards. (Fig. 5). All connections are made on connectors, with the exception of filament circuits, which are soldered directly to the board.

Main board

The circuit board has no features; all circuit elements are mounted on it. First, 7 1.3 mm contacts are soldered (see photo of the design), then thirteen jumpers. The rest of the other elements are installed in the order of the circuit numbers, the potentiometer and the nut switch are mounted last.
The common (ground) wire is connected between the two double RCA input jacks.

View of the board from the side of the conductors:

(click to enlarge)

Location of elements on the board:

(click to enlarge)

Lamp board

The board is soldered into the main board of the amplifier using 5 mm contacts at an angle of 90 degrees.
The drawing of the board is shown in the figure below:

The location of the elements on the lamp board is shown in the figure:

Inclusion

To test the amplifier, you will need a 6 or 12 V power supply for the filament circuits and 320 V for the anode voltage.
When first turned on, it is desirable to supply high voltage from a regulated source.
The control voltage values ​​are indicated in the diagram.
When a signal with an amplitude of 300 mV is applied to the input, the output should be a signal with an amplitude of about 2 V.

To check the low-frequency correction, a + 24V source is required.
With the correction enabled, the signal rise frequency 60Hz should be 3 dB.

Measurement results

The measurement results are shown in the waveforms below.

The response of the amplifier to a pulsed signal shows its good stability and fast rise time:

(click to enlarge)

The cutoff frequency is about 140kHz with a -1dB rolloff.
The level of distortion at a signal level of 1 V is less than 0.03%.
The spectral distribution of harmonics and noise is presented on the spectrograms:

(click to enlarge)

Note that the spectrum is dominated by second harmonic. However, its level is lower -70 d B, which eliminates the “velvety” color (typical of tube amplifiers, the so-called warm sound) of the signal.
The task of any amplifier is to amplify the signal without making any changes to it.
This amp does just that!

The total noise level of the amplifier before the volume control is -90 dB.

The graph shows the frequency response with the low-frequency correction circuit turned on:

(click to enlarge)

Pay attention to the low influence of the correction on the frequency response and phase response of the amplifier. The Backsandel timbre block (rather classical circuit) has a much greater influence on the output signal.

Construction details.

Resistors:
R1, R2, R5, R6, R9. R10, R13, R14: selected according to the required sensitivity of the inputs (or jumpers)
R3, R4, R7, R8, R11, R12, R15, R16, R17, R18: 470 kΩ / 0.5 W / 1%
R19, ​​R20: 47kΩ/1/0.5W/1%
R21, R22: 150 kΩ / 2 W / 5%
R23, R24: 100 kΩ / 2 W / 5%
R25, R26: 47 kΩ / 2 W / 5%
R27, R28: 1.2kΩ/1/0.5W/1%
R29, R30: 360 kΩ / 0.5 W / 1%
R31, R32: 220 kΩ / 0.5 W / 1%
R33 1 kΩ / 2 W / 5%

Capacitors

C1, C2: 1uF/50V/5mm,
C3, C4: 1uF / 250V / 5mm,
C5, C6: 0.1uF/50V/5mm
C7, C8: 100uF/ 6.3V/ 3.5mm,
C9, C10: 470nF / 400V / 15mm C11,
C12: 3.3nF / 100V / 5mm
C13: 10uF/400V/5mm

Miscellaneous:

Lamp: V1, V2 - 6Zh32P (EF86)
Diodes: D1-1N4007
Variable resistor: P1- 100 kOhm (Log/ALPS)
Relay: K1, K2 - SIL / Meder SIL12-1A72-71L
Rotary switch: S1 - 5P/2C / Lorlin PT6422
Toggle switch: S2 - NKK B12AH
Connectors: RCA (dual) - 2 pcs., RCA (single) - 1 pc.

Conclusion

The preamplifier based on a 6Zh32P tube turned out to be absolutely transparent for sound, not introducing tube “warmth” and “velvety”, with a stable gain and low noise level.

A slight low-frequency correction makes it possible to compensate for signal attenuation in the low-frequency region by the listening room, and the compact size of the design allows it to be built into an already finished amplifier.

The article was prepared based on the materials of the magazine Electronique Pratique.

Successful creativity!

- most connoisseurs of quality music, who know how to handle soldering equipment and have some experience in repairing radio equipment, can try to assemble a high-end tube amplifier, which is usually called Hi-End, on their own. Tube devices of this type belong in all respects to a special class of household radio-electronic equipment. Basically, they have an attractive design, while nothing is covered by a casing - everything is in sight.

After all, it is clear that the more you can see the electronic components installed on the chassis, the more authority the device has. Naturally, the parametric values ​​of the tube amplifier are significantly superior to models made on integral or transistor elements. In addition to this, when analyzing the sound of a tube device, all attention is given to a personal assessment of the sound, rather than the image on the oscilloscope screen. In addition, it differs in a small set of used parts.

How to choose a tube amplifier circuit

In the case of choosing a pre-amplifier circuit, there are no particular problems, then when choosing a suitable final stage circuit, difficulties may arise. Tube Audio Power Amplifier may have several options. For example, there are single-cycle and push-pull devices, and also have different modes of operation of the output path, in particular "A" or "AB". The output stage of the single-ended amplification is by and large a model, because it is in the "A" mode.

This mode of operation is characterized by the lowest values ​​of non-linear distortion, but its efficiency is not high. Also, the output power of such a cascade is not very large. Therefore, if it is necessary to sound an internal space of medium size, a push-pull amplifier with an “AB” operating mode will be required. But when a single-cycle device can be made with only two stages, one of which is preliminary and the other is amplifying, then a driver is needed for a push-pull circuit and its correct operation.

But if single tube audio power amplifier may consist of only two stages - a pre-amplifier and a power amplifier, then a push-pull circuit for normal operation requires a driver or a stage that generates two voltages of identical amplitude, shifted in phase by 180. The output stages, regardless of whether it is single-ended or push-pull, assume the presence in the circuit output transformer. Which acts as a matching device for the interelectrode resistance of a radio tube with low acoustic resistance.

True fans of "tube" sound argue that the amplifier circuit should not have any semiconductor devices. Therefore, the power supply rectifier must be implemented on a vacuum diode, which is specially designed for high-voltage rectifiers. If you intend to repeat a working, proven tube amplifier circuit, then you do not need to immediately assemble a difficult push-pull device. To sound a small room and get the perfect sound picture, a single-ended tube amplifier is fully enough. In addition, it is easier to manufacture and configure.

The principle of assembly of tube amplifiers

There are certain rules for the installation of radio-electronic structures, in our case it is tube audio power amplifier. Therefore, before starting the manufacture of the device, it would be desirable to thoroughly study the paramount principles for assembling such systems. The main rule when assembling structures on vacuum radio tubes is the wiring of connecting conductors along the shortest possible path. The most effective method is to refrain from using wires in places where you can do without them. Fixed resistors and capacitors must be installed directly on the lamp sockets. At the same time, special “petals” should be used as auxiliary points. This method of assembling a radio-electronic device is called "hinged mounting".

In practice, when creating tube amplifiers, printed circuit boards are not used. Also, one of the rules says - avoid laying conductors parallel to each other. However, such, at first glance, chaotic wiring is considered the norm and is fully justified. In many cases, when the amplifier is already assembled, a low-frequency background is heard in the speakers, it must be removed. The primary task is the correct choice of the "ground" point. There are two ways to organize grounding:

• The connection of all wires going to the "ground" at one point - called "asterisk"
• Installing an energy-efficient electrical copper bus around the perimeter of the board, and solder the conductors to it.

It is necessary to verify the place for the ground point by experiment, listening for the presence of a background. To determine where the low-frequency background comes from, you need to do this: You need to use the method of sequential experiment, starting with the double triode of the pre-amplifier, to short the lamp grids to the “ground”. In the case of a noticeable decrease in the background, it will become clear which circuit of which lamp is “phoning”. And then, also empirically, you need to try to eliminate this problem. There are auxiliary methods that are mandatory for use:

Pre stage lamps

• Vacuum lamps of the preliminary stage must be closed with caps, and they, in turn, must be grounded
• Cases of trimmer resistors are also subject to grounding
• Lamp wires need to be twisted

Tube Audio Power Amplifier, or rather, the filament circuit of the pre-amplifier lamp can be powered with direct current. But in this case, you will have to add another rectifier assembled on diodes to the power supply. And the use of rectifier diodes in itself is undesirable, as it breaks the constructive principle of manufacturing a tube Hi-End amplifier without the use of semiconductors.

The pair placement of the output and mains transformers in a lamp device is quite an important point. These components must be installed strictly vertically, thereby reducing the background level from the network. One of the effective ways to install transformers is to enclose them in a housing made of metal and grounded. The magnetic circuits of transformers must also be grounded.

retro components

Radio tubes are devices from distant times, but again come into fashion. Therefore, it is necessary to complete tube audio power amplifier the same retro elements that were installed in the original lamp designs. If it concerns fixed resistors, then carbon resistors with high parameter stability or wire ones can be used. However, these elements have a large spread - up to 10%. Therefore, for a tube amplifier, the best choice would be to use small-sized precision resistors with a metal-dielectric conductive layer - C2-14 or C2-29. But the price of such elements is significantly high, then MLTs are quite suitable for them.

Particularly zealous adherents of the retro style get the "audiophile's dream" for their projects. These are BC carbon resistors, developed in the Soviet Union specifically for use in tube amplifiers. If desired, they can be found in tube radios of the 50-60s. If, according to the scheme, the resistor should have a power of more than 5 W, then PEV wire resistors coated with vitreous heat-resistant enamel are suitable.

Capacitors used in tube amplifiers are basically not critical to one or another dielectric, as well as to the element design itself. Any type of capacitor can be used in the tone control paths. Also in the rectifier circuits of the power supply, you can install any type of capacitors as a filter. When designing high-quality low-frequency amplifiers, isolation capacitors installed in the circuit are of great importance.

It is they who have a special influence on the reproduction of a natural, not distorted sound signal. Actually thanks to them we get an exceptional "tube sound". When choosing isolation capacitors to be installed in tube audio power amplifier, special care must be taken to keep the leakage current as low as possible. Because the correct operation of the lamp, in particular its operating point, directly depends on this parameter.

In addition, we must not forget that the decoupling capacitor is connected to the anode circuit of the lamp, which means that it is under high voltage. So, such capacitors must have an operating voltage of at least 400v. One of the best capacitors working as a transition capacitor are JENSEN capacitors. It is these capacitances that are used in top-end HI-END class amplifiers. But their price is very high, reaching up to 7500 rubles for one capacitor. If you use domestic components, then the most suitable would be, for example: K73-16 or K40U-9, however, they are significantly inferior in quality to branded ones.

Single Ended Tube Audio Power Amplifier

The presented tube amplifier circuit has three separate modules:

• Pre-amplifier with tone control
• The output stage, that is, the power amplifier itself
• Power supply

The preamplifier is made according to a simple scheme with the ability to adjust the signal gain. It also has a pair of separate bass and treble tone controls. To increase the efficiency of the device, an equalizer for several bands can be introduced into the design of the pre-amplifier.

Preamp Electronics

The preamplifier circuit presented here is made on one half of a 6N3P double triode. Structurally, the preamplifier can be made on a common frame with an output stage. In the case of the stereo version, then naturally two identical channels are formed, therefore, the triode will be fully involved. Practice shows that when starting to create any design, it is best to first use the circuit board. And after adjusting, already assemble in the main body. Provided that the preamplifier is assembled correctly, it starts to work synchronously with the supply voltage without any problems. However, at the setup stage, you need to set the voltage of the anode of the radio tube.

The capacitor in the output circuit C7 can be used K73-16 with a rated voltage of 400v, but preferably from JENSEN, which will provide the best sound quality. Tube Audio Power Amplifier not particularly critical to electrolytic capacitors, so any type can be used, but with a voltage margin. At the stage of tuning work, we connect a low-frequency generator to the input circuit of the pre-amplifier and apply a signal. An oscilloscope must be connected to the output.

Initially, the input signal range is set within 10 mv. Then we determine the value of the voltage at the output and calculate the amplifying factor. With an audio signal in the range of 20 Hz - 20000 Hz at the input, you can calculate the bandwidth of the amplifying path and depict its frequency response. By selecting the capacitive value of the capacitors, it is possible to determine an acceptable proportion of high and low frequencies.

Setting up a tube amp

Tube Audio Power Amplifier implemented on two octal tubes. In the input circuit, a double triode with separate 6N9S cathodes is installed, connected in parallel, and the final stage is made on a rather powerful output beam tetrode 6P13S, connected as a triode. Actually, it is the triode installed in the final path that creates the exceptional sound quality.

To perform a simple adjustment of the amplifier, an ordinary multimeter will suffice, and in order to perform accurate and correct adjustment, you must have an oscilloscope and an audio frequency generator. You need to start by setting the voltage on the cathodes of the 6H9C double triode, which should be within 1.3v - 1.5v. This voltage is set by selecting a constant resistor R3. The current at the output of the beam tetrode 6P13S should be in the range from 60 to 65 mA. If a powerful constant resistor 500 Ohm - 4 W (R8) is not available, then it can be assembled from a pair of two-watt MLTs with a nominal value of 1 kOhm and connected in parallel. All other resistors indicated in the diagram can be installed of any type, but preference is still C2-14 is given.

Just like in the preamplifier, an important component is the decoupling capacitor C3. As mentioned above, the ideal option would be to install this element from JENSEN. Again, if there are none at hand, then Soviet film capacitors K73-16 or K40U-9 can also be used, although they are worse than overseas ones. For the correct operation of the circuit, these components are selected with the lowest leakage current. If it is impossible to perform such a selection, it is still advisable to buy elements from foreign manufacturers.

Amplifier power supply

The power supply is assembled using a 5Ts3S direct-heated kenotron, which provides AC rectification that fully complies with the design standards for HI-END class tube power amplifiers. If it is not possible to purchase such a kenotron, then two rectifier diodes can be installed instead.

The power supply unit installed in the amplifier does not require any adjustment - it turned on and that's it. The topology of the circuit makes it possible to use any chokes with an inductance of at least 5 H. As an option: the use of such devices from outdated TVs. The power transformer can also be borrowed from old Soviet-made lamp equipment. If you have the skills, you can make it yourself. The transformer must consist of two windings with a voltage of 6.3v each, providing power to the radio tubes of the amplifier. Another winding should be with an operating voltage of 5v, which are fed into the kenotron filament circuit and the secondary one, which has a midpoint. This winding guarantees two voltages of 300v and a current of 200mA.

Power amplifier assembly sequence

The assembly procedure for a tube sound amplifier is as follows: first, a power source and the power amplifier itself are made. After the settings are made and the necessary parameters are set, the preamplifier is connected. All parametric measurements with measuring instruments should be done not on a “live” acoustic system, but on its equivalent. This is in order to avoid the possibility of removing expensive acoustics from standing. The equivalent load can be made from powerful resistors or from thick nichrome wire.

Next, you need to deal with the case for the tube sound amplifier. The design can be developed independently, or borrowed from someone. The most affordable material for the manufacture of the case is plywood. Lamps for the output and preliminary stages and transformers are installed on the upper part of the housing. On the front panel there are devices for adjusting the timbre, sound and an indicator for supplying voltage. In the end, you may end up with devices like the models shown here.

Background:
Building a home audio system, I ran into difficulties. One of them - my tube power amplifier when connected to the source "directly" gives a boring, clamped sound. Without "tops" and "bottoms", one protruding lower middle. Moreover, the movie sound gives out good, but my music (black metal) plays badly.

Obviously, loudness is required. The purchase solved the problem, in general, but the sound quality (in general) deteriorated. The preamplifier went to gather dust on the mezzanine.

I decided to use a tone block in my system instead of loudness.
There are Chinese ones already assembled, for example, on two 6n1p and a kenotron:

But I took in Russia, from the site, this set is a tube timbre-preamplifier on a double triode 6n2p-ev.

For 4000 rubles I received (all parts are new):

1100+1100 rubles - Two sets of parts for assembling two mono channels.
1000 rubles - TAN 15-01, toroidal power anode-filament transformer.
130 rubles - Power supply board.
270 rubles - Choke D15N (50mA, 10H).
400 rubles - shipping (from St. Petersburg to Novosibirsk).

Parcel content:


Close-up on the components of the power supply:


Choke, and two double triodes 6n2p-ev - 1972 and 1976 of release - which is strange. Thought they would be one year old. And these differ structurally even by eye:


(P.S.: The author wrote that he has all the lamps of 1976. Mine 1972 slipped into his set in an unknown way, and he did not put it to me on purpose. I suggested listening to it. Didn't offer free bulb replacement. He did not apologize for the missing radio components. In general, the seller does not use any polite words (“thank you”, “hello”, “goodbye”) in correspondence, probably for reasons of principle).

Preamp shrouds, two mono channels:


Parts set #1:


Parts set #2:


"Manuscript" (Xerox copy in A4) with handwritten blots, which I could not completely decipher. Just evaluate the level of performance:


Almost soldered boards (differences from the original photo on the site are immediately visible - isolation capacitors and lamp sockets):


The amplifier was assembled on a breadboard (I apologize for the quality of the photos):




Sound quality:

Average.

But the tone block, as it seemed to me, is not quite optimally designed for high-quality acoustic systems. A little "narrow" or something.

Adjustment within: ±8dB.
LF: 300 Hz.
RF: 3 kHz.
band: 20-20000Hz. (±0.3dB).
THD: 0.05%.
out: 2V、Maximum 20V or more.

Because of this, the adjustment takes place in a limited range, which is clearly audible.

I would like an adjustment LF: 100 Hz And RF: 10 kHz maybe even wider.
The seller said that the scheme, and suits many.

He suggested replacing capacitors C3 at low frequencies, instead of the original 15 nF, put 10 nF, like Manakov's.

For high frequencies, I suggested changing the capacitor C1 to 1 nF (according to the scheme at Manakov, at Matyushin C2) in the direction of decrease.

Advantages:

Pretty inexpensive.

Easy assembly.

Flaws:

You need two mono channels for the stereo version, which increases the inconvenience of adjustment, and twice the number of "twists".

The instructions could have been more precise.

Variable resistors are used the most ordinary, with the characteristic "B", so the timbres are not regulated smoothly, but abruptly, abruptly.

Complete radio components in a set are the cheapest.

The kit was missing 4 resistors. The radio tubes were not paired.

There is no assembly diagram, so I could not assemble it correctly until I myself found an error in the markup applied to the board.

It turned out to be the “output” block at the back. It has reverse polarity compared to other pads on the board:

In general, the scheme proposed by Matyushin is less successful than Manakov's scheme.

Manakov's scheme is much simpler, the gain is less (which is good), since Matyushin has it redundant.

In addition, Matyushin's circuit requires three expensive coupling capacitors per channel, instead of Manakov's one.

P.S.
I decided to make a Manakov timbre block out of Matyushin's timbre block. According to the scheme, we remove the following elements:


We get this kind of payment:


The most strongly affecting the sound quality of this preamplifier is the coupling capacitor and capacitor C2 in the tone block. I put the paper-oil K40U-2 (0.1uF 350V) instead of the film Wima, because I did not find anything more suitable. On C2 you need to put either high-voltage ceramic or mica. I put SGM-1.

The sound quality has greatly increased compared to the original circuit, but the K40U-2 capacitor begins to sound good only after its “warm-up” (at least half an hour). What caused this, I do not know, but it is a fact.

P.P.S.
K40U-2 changed to Taiwanese polypropylene:


The sound has changed compared to the K40U-2 - on my black metal, the “middle” has become more dynamic and harsh. But at the same time, the sound became less "melodious" and "soulful" on rock ballads, etc.

P.P.P.S.
The 6N2P-EV lamp can be replaced with a 6N1P-EV lamp without changing the circuit - just pulled one out and inserted the other (as you can see, I also shunted the electrolytes in the anodes with 1 microfarad 250V film capacitors, I didn’t hear a difference, but let them be):


The only difference I heard is that 6N1P-EV plays a little quieter. Well, inside they are different in design:


P.P.P.P.S.
As a result of my barbaric, "at random" experiments, one of the two 6N2P-EV lamps fell victim. Interestingly, a newer lamp, 1976, burned out.

Stay tuned.