A simple transistor amplifier Bragin. AF power amplifier (improved version). Main parameters of the umzch

The AF amplifier offered to the attention of radio amateurs has very low coefficients of harmonic and intermodulation distortion, it is relatively simple, can withstand short-term short circuits in the load, and does not require remote elements for thermal stabilization of the current of the output stage transistors.

Main technical characteristics:
Maximum power at a load with a resistance of 4 Ohm, W 80
Maximum power at a load with a resistance of 8 Ohm, W 45
Rated input voltage UMZCH, V 0.8
Input impedance kOhm 100…120
Relative noise level dB no more than -90
Nominal frequency range, Hz 20…20,000
Harmonic distortion at maximum output power 80 W, %, at frequency:
1 kHz 0.002
20 kHz 0.004
Intermodulation distortion coefficient, % 0.0015
Maximum frequency at which maximum power is reduced by 1 dB, kHz 50
Output voltage rise rate (without capacitor C2), V/µs 40

The circuit diagram of the amplifier is shown in Fig. 1. Changes affected the output stage. To increase its input resistance, transistors VT1, VT2 are introduced into the AF amplifier. This facilitated the operation of op-amp DA1 and made it possible to ensure a stable base-emitter voltage of transistors VT3, VT4 when the temperature changes. In addition, the amplifier is complemented by a cascade on transistors VT5, VT6, which, together with current sensors R33, R34 and output stages on transistors VT7-VT10 in quiescent mode, respectively form two current generators, which eliminates the cutoff of the emitter current of the final stage transistors and reduces switching distortion. The latter, as is known, has a beneficial effect on the harmonic spectrum.

In addition to these changes, a deeper local feedback loop was introduced into each arm of the output stage by increasing the resistance of resistors in the emitter circuits of transistors VT3, VT4, which made the output stage more linear. Since resistors R20, R21 are connected to current sensors R33, R34, a fairly strict thermal stabilization of the quiescent current of the final stage transistors is obtained (when the temperature of the heat sinks of the output transistors fluctuates from 20 to 90 °C, the quiescent current varies within 150...180 mA). Availability of current sensors R33, R34, deep environmental protection DC and current-limiting resistors in the base circuits of transistors VT9, VT10 leads to their limitation collector currents to an acceptable value during short circuits in the load.

Resistor R14 sets the symmetry of the arms of the output stage. No other changes were made to the amplifier.

Nonlinear distortions were measured with an S1-68 oscilloscope using a GZ-118 AF signal generator (Kg - about 0.002%) and a precision double T-bridge included in the generator kit. The measurements were carried out according to the method outlined in the article by Yu. Mitrofanov “Economic mode A in a power amplifier” (see “Radio”, 1986, No. 5, pp. 40-43).

The intermodulation distortion coefficient was measured according to the recommendations given in the article by V. Kostin “Psychoacoustic criteria for sound quality and selection of UMZCH parameters” (see “Radio”, 1987, No. 12, pp. 40-43), using the measuring setup shown in rice. 2. The complete measuring circuit is also shown there.
Rice. 2

When testing the amplifier with a pulse signal, no emissions were observed at the output voltage.

About the power supply of the amplifier.

During testing, the author used an unstabilized power supply with filter capacitors with a capacity of 10,000 μF (50V). Noticeable improvement technical characteristics when powered from a stabilized source, it was not noted. During operation, it is permissible to reduce the supply voltage to +20 and -20 V, naturally, with the appropriate selection of resistors R12, R16 (the current through the zener diodes VD1, VD2 should be within 10...12 mA). The maximum output power at these supply voltages will decrease to approximately 12...13 W. It is not recommended to increase the supply voltage above the values ​​​​specified in the article (+35 and -35 V), as this will lead to a significant decrease in the reliability of the UMZCH.

Coil L1 data.

Coil L1 (inductance - 0.3 μH) is wound on the body of resistor R35 (MLT-2) and contains 12 turns of 0.8 mm PEL wire.

Replacement of parts.

Without deteriorating the technical characteristics of the UMZCH, it is possible to replace transistors KT3107A (VT1, VT6) with KT502V - KT502E; KT3102A (VT2, VT5) - on KT503V - KT503E; KT639D (VT7) and KT961A (VT8) - respectively on KT626B, KT626V and KT646A, KT646B; KT819GM ​​(VT9) and KT818GM (VT10) - respectively on KT819V, KT819G and KT818V, KT818G. Transistor KT3102A (VT3) can be replaced with KT3102B, and KT3107A (VT4) with KT3107B. Instead of K574UD1B you can use K574UD1A. A replacement for KD105 diodes (VD3, VD4) can be any diodes of the D220, D223, KD522, etc. series.

When the supply voltage is reduced, instead of transistors with position designations VT1-VT6, you can use KT315V - KT315D and KT361V - KT361D. In the case of using transistors in plastic cases (KT818, KT819 series) in the final cascade, it is necessary to place copper pads with a diameter of 30 and a thickness of 0.5...0.8 mm, lubricated with heat-conducting paste, between their heat-conducting plates and heat sinks.

Transistors VT7 and VT8 must be installed on heat sinks with a cooling surface of at least 40 cm2.

The amplifier parts (with the exception of transistors VT9, VT10 and fuses FU1, FU2) are mounted on a printed circuit board (see Fig. 3) made of foil fiberglass laminate 1.5 mm thick. The board is designed for installation of permanent resistors MLT, trimmer SP3-38a, capacitors K53-1 (C3, C4, C6, C7), K50-6 (C13, C16), KD-1 (C5), K73-11 (C12, C15 ) and KM (the rest). The capacity of blocking capacitors Cbl (also KM), shunting zener diodes VD1, VD2, is 0.1 μF. Resistors R33 and R34 are made from pieces of nichrome wire with a diameter of 0.8 mm. To connect to the transistors of the final stage and the power source, an MRN-32 connector is used.
Radio No. 12 1990

I didn’t like the sound that Radiotekhnika-101U produces from the first listen. Purchased very cheaply for the occasion, this amplifier was lying around idle for more than 15 years. For a long time I couldn’t decide what to replace the built-in ULF-50-8 with, and in the end I settled on the Bragin amplifier. The arguments in favor were the relative simplicity with very decent quality. Viewing various modifications UMZCH Bragin, having driven them in the simulator, settled on the following scheme:

The circuit differs from the standard Bragin primarily in the wiring of the pre-output transistors. The use of transistors with a guaranteed gain of more than 100 made it possible to increase the resistance of the resistors, which reduced the heat generation on them, and accordingly made it possible to use lower power resistors. Another advantage of the 2SA1837/2SC4793 pair is their high frequency, which also has a positive effect on the quality of the amplifier. In addition, the plastic housing provides electrical insulation from the radiator. In addition, as the simulator shows, changing the feedback parameters reduces distortion.
Another one of essential elements affecting the quality of the amplifier is the op-amp. It must be fast-acting. Of ours, 544UD2A and 574UD1B are perfect. The use of low-speed op-amps like TL071 does not make sense; the result may be even worse than that of the native ULF-50-8.
Since the voltage amplification of the signal is carried out not only by the op-amp, but also by subsequent stages, there is no need to raise the supply voltage for the op-amp. +/-12…13V is quite enough.
In some variations of the amplifier, rectifier diodes of the 1N400X type are used as D3. This may not affect the quality, but I installed ultra-fast there.
A 2.7 pF capacitor is excluded from the feedback. The simulator showed the complex influence of this capacitance on the behavior of the amplifier; an inaccurately selected value does more harm than good.

To increase noise immunity, the resistance of the general feedback resistors has been reduced. To compensate for the decrease in the lower cutoff frequency, high-capacity capacitors are used in the feedback. In this regard, low-impedance capacitors from motherboards are excellent (they differ from ordinary ones in gold or silver inscriptions). In terms of voltage, it is enough to take 6.3V capacitors, since the voltage on them will be around zero. It is also clear from the diagram that the feedback is connected to ground through a resistor, and not a capacitor as usual. This rearrangement of the resistor and capacitors does not in any way affect the performance or parameters of the amplifier, but it simplifies the board layout.
Setting up the amplifier comes down to checking the voltage across resistors R20 and R21. There should be 0.2...0.3 volts on them. If necessary, it can be adjusted by selecting resistors R8* and R9*.

There is a different printed circuit board for each channel.

However, the differences are minimal; the ground connection is made from different sides. This allows you to create a “star” of mass for boards installed next to each other.
A slot in the bottom track geometrically separates the power mass from the signal mass.

About the white stripes on the board drawing. The standard thickness of foil on fiberglass laminate is 0.035 mm. To reduce the resistance of the power tracks, I recommend strengthening them by soldering copper wire ø0.8...1 mm on top. The location of this wire is indicated by white lines.
To reduce the resistance of the signal mass, it is enough to thicken it with solder.

The board was developed for KR544UD2A. In the case of using KR574UD1B, the track between legs 1 and 8 of the microcircuit should be removed, and a 5...15 pF capacitor should be soldered to legs 5 and 6.

There are no elements for balancing the op-amp on the board. In my amplifier, the output constant was 5 mV in one channel and 12 mV in the other, which is significantly lower than the permissible 30 mV. If anyone wants to make adjustments, I advise you to do this by soldering constant resistors with reverse side fees. I don’t think it’s advisable to install a trimmer for these purposes. The trimmer is good for mass production when productivity is important. For personal purposes, it is better to spend time selecting permanent resistors once, but get rid of the surprises of the moving contact.

It was not possible to install the C17-R26 chain on the board beautifully and efficiently. Soldering it to the bottom of the board turned out to be the best solution.

Typically the capacitor in this chain is set to 0.1 µF or more. My boards with 0.01 uF capacitors installed showed the absolute stability of the amplifier, and I did not load the output with additional useless load.

The board was developed for the installation of domestic MLT type resistors. If imported resistors are used, resistors with twice the power should be used (applies only to those resistors for which the power is indicated on the diagram).
Resistors R24 and R25 are structurally composed of 4 resistors 1.2 ohm per 0.5 watt. First, 2 resistors are soldered, and then the pairs are soldered into the board. Here I didn’t invent anything, but used resistors from the outputs of the ULF-50-8. The coil was also taken from there.

Installing a new amplifier requires altering the power supply. A 100 W transformer is installed in Radiotekhnika-101U, but is used at 80 W. The main power secondary winding is designed to receive a constant voltage of +/-31V, and has a tap to receive +/-26V. In the native circuit, only +/-26V is supplied to the output stages. It’s better to file a lawsuit against Bragin more voltage nutrition. Therefore, you should swap the wires coming from the transformer to diode bridges. Naturally, you will need to transfer the power wires from units operating on +/-26V voltage to another bridge.

The mass distribution was radically changed. All wires soldered in different places to the case were removed. The grounds of the protection unit and indicator are connected to the ground on the power supply board. The masses of the left and right channel boards were connected by three jumpers made of copper wire ø0.8 mm and soldered together. This spike became the star of mass breeding.

The wire coming down from the star comes from the ground of the power supply. The wire coming up from the star goes to the amplifier body to the ground socket. The shield of the network cable is soldered to the same socket.

The ground wires from the speakers are soldered near the mass star on the back side of the boards, each to its own channel, respectively.

A wire is soldered to the bottom of the mass star that goes to the ground of the preamplifier-timbre block. Next, the mass goes from the tone block to the input selector.
Thus, we obtain a mass that diverges from the star and does not have closed contours.

A couple of general photos

A small instrument test of the amplifier was carried out. Results and .

Listening. The amplifier accurately replicates the input signal. With a high-quality source, the sound is clear and transparent, you want to listen and listen. But it’s better not to include mp3s with low bitrates. The amplifier will produce all the artifacts of mp3 encoding, which in bad amplifiers are lost against the background of the amplifier’s own distortions and are not audible.

The proposed UMZCH (Fig. 1) is built on the basis of the KR544UD2 operational amplifier.

Operational amplifier DA1 is powered through transistors VT1 and VT2, which reduce the supply voltage to the values ​​specified by the dividers R3, R4 and R5, R6. The bias voltages of transistors VTZ, VT4 are determined by the voltage drop across resistors R8, R9. If necessary, DA1 can be balanced using a divider R14, R15.

Rice. 1. UMZCH diagram

The quiescent current of the pre-terminal transistors VT3, VT4 determines the bias voltage on resistors R11, R12 (0.35...0.4 V), which, at low signal levels, maintains transistors VT5, VT6 in the closed state even when the supply voltage increases by 10.. .15% or overheating by 60...80°. Resistors R11, R12 simultaneously stabilize the operating mode of the pre-final stage VTZ, VT4, creating local negative feedbacks(OOS) by current. The overall voltage feedback is formed by the divider R7, R10.

Main parameters of UMZCH

Filters low frequencies R2, C2 and R13, C7 with cutoff frequencies in the 60 kHz region prevent self-excitation of the amplifier at high frequencies. Capacitors C5, Sb correct the phase-frequency characteristics of the pre-terminal and final cascades. Coil L1 increases the stability of the amplifier when operating a load with increased reactivity.

Assembly and installation

When assembling the structure, you must use a soldering iron with good insulation and a power of no more than 40 W. A drawing of the UMZCH printed circuit board is shown in Fig. 2, and the assembly drawing is in Fig. 3.

The assembly order is as follows: jumper S1, resistors, capacitors, coil L1, operational amplifier(DA1), transistors VT1 ... VT4, after pre-adjustment- transistors VT5, VT6. Frameless coil L1 contains 10 turns of any copper winding wire with a diameter of 1 ... 2 mm. It is wound on a temporary mandrel with a diameter of 4...6 mm, for example on a thin ballpoint pen or pencil.

Rice. 2. Printed circuit board

Rice. 3. Assembly drawing

In order to minimize nonlinear distortions, transistors VT3...VT6 should be connected to the printed circuit board with conductors no longer than 50 mm. The optimal design of the UMZCH is shown in Fig. 3. Using two corners, the board is screwed to the heat sink, and the transistors are soldered directly into the board. The most convenient way to do this is in the following sequence:

Mark the heat sink, drill the necessary holes and cut M3 threads into them. The design of the heat sink can be arbitrary, but its surface area for a maximum output power of 60 W must be at least 500 cm2;

Screw the board to the heatsink;

Install transistors VT3, VT4 into the corresponding holes on the board, then screw them to the heat sink, and then solder them;

After preliminary adjustment, mount transistors VT5, VT6 in the same way;

After this, solder wires for connecting power and load with a cross-section of at least 0.5 mm2.

Setup

To set up the amplifier, you need an oscilloscope, a low-frequency generator, a tester, a load equivalent and a bipolar power supply with an output voltage of ±30 V at a load current of at least 4 A.

The high stability of the UMZCH allows it to be powered from a simple unstabilized power source. During its adjustment and operation, power is supplied to the amplifier through 5 A fuses. Adjustment begins with transistors VT5, VT6 turned off and the input shorted (points 1 and 2 are connected).

Connect an oscilloscope to the output of the UMZCH without load in maximum sensitivity mode and briefly apply power. If there is no output AC voltage, i.e. the amplifier is not excited, measure the operating modes VTZ, VT4; voltage at pins 7 and 4 DA1. They should be within 13.4... 14 V and differ from each other by no more than 0.3 V. The voltage drops across resistors R11, R12 should be within 0.35...0.4 V. If they differ by more than 10%, it is necessary to select resistors R8, R9. At the same time, their new values ​​should still be approximately equal to each other.

In the case of self-excitation of the amplifier, you should increase the capacitance of capacitors C5, Sb, or, by cutting the track connecting pins 1 and 8 of DA1, solder a capacitor of the KM-5 type with a capacity of 5...10 pF to them.

Measure constant voltage at the output and if it exceeds 30 mV, balance DA1. To do this, solder a variable resistor with a resistance of 100...200 kOhm instead of resistors R14 and R15 (with the middle terminal at the point of their connection with pin 7 of DA1). By rotating the axis of this resistor, achieve the desired output voltage value, measure the resulting resistance values ​​and solder the corresponding fixed resistors R14 and R15. It is undesirable to use a trimming resistor as a balancing resistor - due to the aging of this resistor, the balancing of the amplifier may be disrupted during its operation.

Install transistors VT5, VT6 on the heat sink and on the board. By briefly applying power, make sure that the UMZCH is not excited.

Connect a resistor with a resistance of 16 Ohms with a power of 10...15 W to the output of the UMZCH, and apply a signal with a level of 0.05 V with a frequency of 1 kHz from the generator to the input (disconnect points 1 and 2). Gradually increasing the input signal level to 1.0 V, check the symmetry of the clipping of both half-waves of the sine wave.

If necessary, by final balancing DA1, achieve the minimum constant voltage at the output of the UMZCH.

Connect a rated load - a resistor with a resistance of 4...8 Ohms with a power of at least 50 W (for example, a rheostat) - and again measure the main characteristics of the UMZCH.

After final adjustments, connect the music source and the actual speaker system.

To operate a power amplifier from signal sources with a standard 250 mV linear output (tape recorder, player, etc.), you should use a pre-amplifier with the ability to adjust volume and tone.

If the input signal source is assembled using a single-supply circuit, you may hear “clicks” in the speaker systems when you turn on the amplifier. To eliminate this phenomenon, you can assemble a connection delay circuit speaker system and protecting speakers from short circuits, for example according to the diagrams given in.

Literature:

1. Radio, 1990, No. 8, p. 63.
2. Radio, 1991, No. 1, p. 59.
3. Radio, 1992, No. 4, p. 37.

I found the circuit of this amplifier along with a signet (installation) in a radio magazine of the year 1987. The author of the amplifier is G. Bragin. He later upgraded the circuit, adding 20 watts of output power, while reducing the harmonic distortion by an order of magnitude. True, more radio components were added.

I settled on the first version of the amplifier. I wanted to put together a diagram about eight years ago! - far from the only radio-electronic device that had to be assembled. However, it was precisely the shortage of components necessary for the assembly of ULF that slowed down the entire process. And of course, over time I modernized, or rather, over time, the opportunity arose to replace our domestic components - large ones, with smaller ones. Naturally, the size of the entire structure of Bragin’s amplifier was constantly decreasing.

Difficulties began when all the parts were soldered, but the amplifier did not work properly, and specifically, it was necessary to select the gain according to the parameters of KT816G(V) with KT817G(V), and the output KT819 and KT818 with the same letters. I couldn’t even imagine that these h21e data are so different from those written in the reference tables. That is, as I understand it, our transistors are produced domestically without observing any standards. You come to the store where they sell them with your tester and pick them up. And most often, I’ll tell you, 200-400 difference is noticeable, and this difference was enough for the incorrect unstable operation of Bragin’s amplifier. The transistors simply overheated without having time to really work! I turn on the power and use resistor R6, changing the resistance, to achieve the required voltage readings - as indicated in the amplifier circuit. Everything is great! As soon as I apply a signal to the input, the quiescent current goes off scale, the transistors heat up and, if we continue, it all ends in thermal breakdown. When, nevertheless, after repeated haze I came to this decision, the problem was solved. Now I know that it’s better to pick it up on the spot - who knew that these h21e were so airy.

Just six months ago, the home stretch was the fifth modernization of Bragin’s amplifier. The case is made of sheet duralumin, large powerful diodes were replaced with bridge diodes, which are much smaller. The capacitances were 10,000 uF x 50 volts, four pieces. I bought a Chinese 20,000 uF x 63 V, five times smaller in size. The transformer costs 250 watts from a tube TV, double-coil. I rewound the secondary one. At one time I wanted to change it to a toroidal one - for 1000 rubles, and I’ll also have to rewind the secondary - let it work that way! And also, so as not to install large radiators, although the output transistors do not heat up very much, relatively slightly, I installed forced cooling. 400 mm square is without a fan for each transistor, and needle ones are 150 mm for each. Fine. Pre-amplifier with volume and tone control assembled on the TDA1524 chip. The sound is pleasant - lows, mids and high frequencies I'm auditioning, I like it, I'm just having fun. Bombboxes that are commercially available are not comparable to Bragin's ULF. They do not have soft deep bass, and the output power is not the same. For home use, cranking it up to half is enough, adding the desired timbre according to your mood. It's just nice to dry!

Now, having achieved the result, what else would I like is, of course, amplification power of up to 200 watts. Having looked at a set of amplifier circuits on this site from the article UMZCH 125-200-500, I noticed a similarity. The last output transistors are bipolar or field effect. Having assembled the basic circuit, adding or decreasing the quantity changes the output power with a corresponding change in the supply. I have a question. Is it possible to do something similar in relation to the Bragin amplifier? Well, let's say, increase the sound power from 80 to 200 W? Or is it better not to bother, but to immediately assemble a ready-made one, selecting it according to power? Please advise.

The AF power amplifier (UMZCH) brought to the attention of readers has a low harmonic coefficient with a relatively simple circuit design, is capable of withstanding short-term load short circuits and does not require thermal stabilization of the quiescent current of the final stage transistors.

Main technical characteristics

Rated (maximum) power at a load with a resistance of 4 Ohms, W. . . 60 (80)

Nominal frequency range, Hz. . . 20...20 000

Harmonic coefficient at rated output power in the rated frequency range, %. . . 0.03

Rated input voltage, V. . . 0.775

Output impedance in the nominal frequency range, Ohm, no more. . . 0.08

Output voltage rise rate (without capacitor C2), V/µs. . . 40

The circuit diagram of the amplifier is shown in Fig. 1. The main voltage gain is provided by a cascade based on high-speed op-amp DA1. The pre-terminal stage is assembled using transistors VT1-VT4. The output emitter follower is made of transistors VT5, VT6 operating in mode B.

When designing an amplifier special attention was given to the pre-terminal cascade. In order to reduce nonlinear distortions, the AB mode with a relatively large quiescent current (about 20 mA) was selected. Temperature stability is achieved by including relatively high resistance resistors R19, R20 in the collector circuits of transistors VT3, VT4. However, due to the absence of 100% OOS in the pre-final stage, when its temperature changes, fluctuations in the quiescent current are possible within 15...25 mA, which are quite acceptable, since they do not violate the operational reliability of the amplifier as a whole. To compensate for possible instability of the base-emitter voltage of transistors VT1, VT2 when the temperature changes, diodes VD3-VD5 are included in their base circuits. Each arm of the pre-terminal stage is covered by a local feedback loop with a depth of at least 20 dB. The OOS voltage is removed from the collector loads of transistors VT3, VT4 and through dividers R11R14 and R12R15 is supplied to the emitter circuits of transistors VT1, VT2. Frequency correction and stability in the OOS circuit are provided by capacitors C10, C11. Resistors R13, R16 and R19, R20 limit maximum currents pre-final and final stages of the amplifier at short circuit loads. For any overload, the maximum current of transistors VT5, VT6 does not exceed 3.5...4 A, and in this case they do not overheat, since fuses FU1 and FU2 have time to burn and turn off the power to the amplifier.

Diode VD6, connected between the bases of transistors VT5, VT6, reduces step-type distortion. The voltage falling across it (about 0.75 V) narrows the range of voltages at the emitter junctions of the transistors at which they are closed. This ensures their opening at a lower signal amplitude and at the same time reliable closing in its absence. At small signals, the current of the pre-final stage flows into the load, entering through resistor R21. A low-pass filter L1, C14 and R23 is connected to the output of the final stage, which reduces the amplitude of sharp signal bursts (lasting about 1 μs) at the moment of switching the transistors of the output stage and eliminates oscillatory processes in the output stage. The filter does not have a noticeable effect on the slew rate of the output signal.

The reduction in harmonic distortion was achieved by introducing a deep (at least 70 dB) general feedback loop, the voltage of which is removed from the output of the amplifier and, through a divider C3-C5, R3 and R4, supplied to the inverting input of op-amp DA1. Capacitor C5 adjusts the frequency response of the amplifier through the OOS circuit.

Strict stabilization of the DC output voltage at a level of no more than ±20 mV was achieved by using 100% DC feedback in the amplifier. To reduce this voltage to ±1 mV or less, it is necessary to balance op-amp DA1. by connecting to the corresponding terminal (depending on the sign of the voltage) a resistor R24 ​​or R25 with a resistance of 200... 820 KOhm.

The R1C1 circuit connected at the amplifier input limits its bandwidth to 160 kHz. The maximum possible linearization of the frequency response of the UMZCH in the 10...200 Hz band was achieved by appropriate selection of the capacitance of capacitors C1, C3 and C4.

The amplifier can be powered from both a stabilized and an unstabilized power source, and its functionality is maintained when the supply voltage is reduced to ±25 V (of course, with a corresponding decrease in output power). When using a stabilized power source, one should take into account the possibility of large (up to 10 V) ripples appearing at the output of the stabilizers with the frequency of the amplified UMZCH signal at a power close to the rated one.

The amplifier is assembled on a board made of foil fiberglass 2 mm thick, connected to external circuits with an MPH32-1 connector. Transistors VT3, VT4 are equipped with heat sinks (Fig. 2), bent from sheet aluminum alloy 1 mm thick, and installed on the board. The final stage transistors VT5, VT6 are mounted outside the board on heat sinks with a cooling surface area of ​​400 cm2 each. The amplifier uses MLT resistors, capacitors K73-17 (C1), KM (C2, C8-C11), K53-1 (C3, C4, C6, C7), KD (C5), MBM (C14) and K73-16V ( C12, C13). Coil L1 is wound with PEV-2 0.8 wire in three layers on the body of resistor R22 (MT-1) and contains 40 turns.

Instead of those indicated in the diagram, you can use op-amps K574UD1A, K574UD1V and transistors of the same types, but with the indices G, D (VT1, VT2) and B (VT3-VT6).

An amplifier assembled from serviceable parts requires almost no adjustment. As mentioned above, the quiescent current of transistors VT3, VT4 is set, if necessary, by selecting resistor R6, and the minimum constant voltage at the amplifier output is set by resistor R24 ​​or R25.

The harmonic coefficient was measured in the range of 20...20,000 Hz using the compensation method. The first surge in the output voltage (with capacitor C2 disconnected) did not exceed 3%, which indicates good stability of the amplifier.

On import:

Magazine "Radio" 4/87, G. Bragin, Chapaevsk, Kuibyshev region.