Low-frequency transistor circuits. Two low-frequency circuits on transistors. Description of the low frequency amplifier circuit

On Habré there were already publications about DIY-tube amplifiers, which were very interesting to read. There is no doubt that they sound wonderful, but for everyday use it is easier to use a device with transistors. Transistors are more convenient, since they do not require warming up before operation and are more durable. And not everyone dares to start a lamp saga with anode potentials under 400 V, and transformers for transistor ones of a couple of tens of volts are much safer and simply more affordable.

As a circuit for reproduction, I chose a circuit from John Linsley Hood in 1969, taking the author's parameters based on the impedance of my speakers of 8 ohms.

The classic diagram from a British engineer, published almost 50 years ago, is still one of the most reproducible and receives extremely positive reviews about itself. There are many explanations for this:
- the minimum number of elements simplifies installation. It is also believed that the simpler the design, the better the sound;
- despite the fact that there are two output transistors, they do not need to be sorted into complementary pairs;
- 10 Watt output is enough for ordinary human dwellings, and the input sensitivity of 0.5-1 Volt matches very well with the output of most sound cards or turntables;
- class A - it is also class A in Africa, if we are talking about good sound. Comparison with other classes will be a little lower.

Interior design

The amplifier starts with power. Separation of two channels for stereo is best done from two different transformers, but I limited myself to one transformer with two secondary windings. After these windings, each channel exists by itself, so we must not forget to multiply by two everything mentioned below. On the breadboard we make bridges on Schottky diodes for the rectifier.

It is possible on ordinary diodes or even ready-made bridges, but then they must be shunted with capacitors, and the voltage drop across them is greater. After the bridges, there are CRC filters from two 33000 uF capacitors and a 0.75 Ohm resistor between them. If we take less capacitance and a resistor, then the CRC filter will become cheaper and less heated, but the ripple will increase, which is not comme il faut. These parameters, IMHO, are reasonable in terms of price-effect. The resistor in the filter needs a powerful cement one, at a quiescent current of up to 2A it will dissipate 3W of heat, so it is better to take it with a margin of 5-10W. For the rest of the resistors in the circuit, 2 W is sufficient.

Next, we move on to the amplifier board itself. A bunch of ready-made whales are sold in online stores, but there are no fewer complaints about the quality of Chinese components or illiterate layouts on the boards. Therefore, it is better to do it yourself, under your own "loose powder". I made both channels on a single breadboard, so that later I would attach it to the bottom of the case. Run with test items:

Everything except the Tr1 / Tr2 output transistors is on the board itself. Output transistors are mounted on radiators, more on that below. To the author's scheme from the original article, you need to make the following remarks:

Not everything needs to be soldered tightly right away. It is better to first put resistors R1, R2 and R6 with trimmers, after all adjustments, evaporate, measure their resistance and solder the final constant resistors with the same resistance. Setting is reduced to the following operations. First, with the help of R6, it is set so that the voltage between X and zero is exactly half of the voltage + V and zero. In one of the channels, 100 kOhm was not enough for me, so it's better to take these trimmers with a margin. Then, with the help of R1 and R2 (keeping their approximate ratio!), The quiescent current is set - we put the tester to measure the direct current and measure this very current at the point of entry of the power supply plus. I had to significantly reduce the resistance of both resistors to obtain the desired quiescent current. The quiescent current of the amplifier in class A is maximum and, in fact, in the absence of an input signal, all goes into thermal energy. For 8-ohm speakers, this current, according to the author's recommendation, should be 1.2 A at a voltage of 27 Volts, which means 32.4 Watts of heat per channel. Since setting the current can take several minutes, the output transistors must already be on the cooling heatsinks, otherwise they will quickly overheat and die. For they are mainly heated.

It is possible that, as an experiment, you will want to compare the sound of different transistors, so you can also leave the possibility of a convenient replacement for them. I tried 2N3906, KT361 and BC557C inputs, there was a slight difference in favor of the latter. In the pre-weekend we tried KT630, BD139 and KT801, stopped at imported ones. Although all of the above transistors are very good and the difference can be rather subjective. At the exit, I put 2N3055 (ST Microelectronics) right away, since many people like them.

When adjusting and underestimating the resistance of the amplifier, the cutoff frequency of the low frequency can increase, therefore, for a capacitor at the input, it is better to use not 0.5 microfarads, but 1 or even 2 microfarads in a polymer film. A Russian picture-diagram "Ultra-linear amplifier of class A" is still walking on the Network, where this capacitor is generally proposed as 0.1 microfarad, which is fraught with a cut of all bass at 90 Hz:

They write that this circuit is not prone to self-excitation, but just in case, a Zobel circuit is placed between point X and the ground: R 10 Ohm + C 0.1 microfarad.
- fuses, they can and should be installed both on the transformer and on the power input of the circuit.
- it would be very appropriate to use thermal paste for maximum contact between the transistor and the radiator.

Locksmith and carpentry

Now about the traditionally the most difficult part in DIY - the case. The dimensions of the case are set by the radiators, and they should be large in class A, remember about 30 watts of heat on each side. At first, I underestimated this power and made a case with average radiators of 800 cm² per channel. However, with a set quiescent current of 1.2A, they heated up to 100 ° C in 5 minutes, and it became clear that something more powerful was needed. That is, you need to either install more radiators, or use coolers. I didn't want to make a quadrocopter, so I bought giant beauties HS 135-250 with an area of ​​2500 cm² for each transistor. As practice has shown, such a measure turned out to be a little redundant, but now the amplifier can be easily touched by hands - the temperature is only 40 ° C even in rest mode. Drilling holes in the radiators for fasteners and transistors became a certain problem - the originally purchased Chinese drills for metal were drilled extremely slowly, each hole would take at least half an hour. Cobalt drills with a sharpening angle of 135 ° from a well-known German manufacturer came to the rescue - each hole is drilled in a few seconds!

I made the body itself from plexiglass. We immediately order cut rectangles from the glaziers, make the necessary holes for fasteners in them and paint them with black paint on the back.

Plexiglass painted on the back looks very nice. Now all that remains is to collect everything and enjoy the muses ... oh yes, during the final assembly it is still important to properly dilute the ground to minimize the background. As it was found out decades before us, C3 must be connected to the signal ground, i.e. to the minus input-input, and all other minuses can be sent to the "star" near the filter capacitors. If everything is done correctly, then no background can be heard, even if you bring your ear to the speaker at maximum volume. Another "ground" feature that is characteristic of sound cards that are not galvanically isolated from the computer is interference from the motherboard, which can crawl through USB and RCA. Judging by the Internet, the problem is often encountered: in the speakers you can hear the sounds of the HDD, printer, mouse and the background of the system unit's power supply unit. In this case, the easiest way to break the earth loop is to tape the earth on the amplifier plug with electrical tape. There is nothing to fear here, tk. there will be a second ground loop through the computer.

I didn’t do the volume control on the amplifier, because I couldn’t get any high-quality ALPS, and I didn’t like the rustling of Chinese potentiometers. Instead, a conventional 47 kΩ resistor was installed between ground and the input signal. Moreover, the external sound card's regulator is always at hand, and every program also has a slider. Only the turntable doesn't have a volume control, so I attached an external potentiometer to the connecting cable to listen to it.

I will guess this container in 5 seconds ...

Finally, you can start listening. The sound source is Foobar2000 → ASIO → external Asus Xonar U7. Columns Microlab Pro3. The main advantage of these speakers is a separate block of their own amplifier on the LM4766 microcircuit, which can be immediately removed somewhere further away. Much more interesting with this acoustics was the amplifier from the Panasonic mini-system with the proud Hi-Fi inscription or the amplifier of the Soviet Vega-109 turntable. Both of the aforementioned devices operate in class AB. The JLH presented in the article outplayed all of the above comrades in one wicket in a blind test for 3 people. Although the difference could be heard with the naked ear and without any tests, the sound is clearly more detailed and transparent. It is quite easy, for example, to hear the difference between MP3 256kbps and FLAC. I used to think that the lossless effect was more like a placebo, but now the opinion has changed. Likewise, it has become much more pleasant to listen to files that are not compressed from loudness war - dynamic range less than 5 dB is not ice at all. Linsley Hood is worth the investment of time and money, as a similar branded amp will cost much more.

Material costs

Transformer 2200 r.
Output transistors (6 pcs. With a margin) 900 r.
Filter capacitors (4 pcs) 2700 rub.
"Loose" (resistors, small capacitors and transistors, diodes) ~ 2000 r.
Radiators 1800 r.
Plexiglas 650 r.
Paint 250 rub.
Connectors 600 rub.
Boards, wires, silver solder, etc. ~ 1000 r.
TOTAL ~ 12100 p.

The transistor amplifier, despite its already long history, remains a favorite subject of research for both beginners and venerable radio amateurs. And this is understandable. It is an indispensable part of the most popular and low-frequency (audio) frequency amplifiers. We will look at how the simplest transistor amplifiers are built.

Amplifier frequency response

In any TV or radio receiver, in every music center or sound amplifier, you can find transistor sound amplifiers (low frequency - LF). The difference between transistor audio amplifiers and other types lies in their frequency characteristics.

The transistorized audio amplifier has a uniform frequency response in the frequency range from 15 Hz to 20 kHz. This means that the amplifier converts (amplifies) all input signals with a frequency within this range in approximately the same way. The figure below shows the ideal frequency response curve for an audio amplifier in coordinates “amplifier gain Ku - input frequency”.

This curve is practically flat from 15 Hz to 20 kHz. This means that such an amplifier should be used specifically for input signals with frequencies between 15 Hz and 20 kHz. For input signals with frequencies above 20 kHz or below 15 Hz, the efficiency and quality of its operation decreases rapidly.

The type of frequency response of the amplifier is determined by the electrical radioelements (ERE) of its circuit, and above all by the transistors themselves. An audio amplifier on transistors is usually assembled on the so-called low- and medium-frequency transistors with a total bandwidth of input signals from tens and hundreds of Hz to 30 kHz.

Amplifier class

As you know, depending on the degree of continuity of current flow throughout its period through the transistor amplifier stage (amplifier), the following classes of its work are distinguished: "A", "B", "AB", "C", "D".

In class of operation, current "A" flows through the stage for 100% of the input signal period. The operation of the cascade in this class is illustrated in the following figure.

In the class of operation of the amplifier stage "AB", the current flows through it more than 50%, but less than 100% of the period of the input signal (see figure below).

In the class of operation of the "B" stage, the current flows through it exactly 50% of the period of the input signal, as illustrated in the figure.

And finally, in the class of operation of stage "C", the current flows through it for less than 50% of the period of the input signal.

LF amplifier on transistors: distortion in the main classes of work

In the working area, the class "A" transistor amplifier has a low level of nonlinear distortion. But if the signal has pulse voltage surges, leading to saturation of transistors, then higher harmonics (up to the 11th) appear around each "standard" harmonic of the output signal. This causes the phenomenon of the so-called transistor or metallic sound.

If the LF power amplifiers on transistors have an unstabilized power supply, then their output signals are modulated in amplitude near the mains frequency. This leads to a harsh sound at the left end of the frequency response. Various methods of voltage stabilization make the amplifier design more complex.

The typical efficiency of a single-ended class A amplifier is less than 20% due to the constantly open transistor and the continuous flow of the DC component. You can make a class A amplifier with a push-pull, the efficiency will slightly increase, but the half-waves of the signal will become more asymmetrical. The transfer of the same stage from the class of work "A" to the class of work "AB" quadruples the nonlinear distortions, although the efficiency of its circuit increases.

In amplifiers of classes "AB" and "B", the distortion increases as the signal level decreases. You involuntarily want to turn on such an amplifier louder for the fullness of the feeling of power and dynamics of music, but often it does not help much.

Intermediate work classes

Work class "A" has a variation - class "A +". In this case, the low-voltage input transistors of an amplifier of this class operate in class "A", and the high-voltage output transistors of the amplifier, when their input signals exceed a certain level, go into classes "B" or "AB". The efficiency of such stages is better than in the pure class "A", and the harmonic distortion is less (up to 0.003%). However, their sound is also "metallic" due to the presence of higher harmonics in the output signal.

Amplifiers of another class - "AA" have even lower degree of nonlinear distortion - about 0.0005%, but higher harmonics are also present.

Return to Class A transistor amplifier?

Today, many experts in the field of high-quality sound reproduction advocate a return to tube amplifiers, since the level of nonlinear distortions and higher harmonics introduced by them into the output signal is obviously lower than that of transistors. However, these advantages are largely offset by the need for a matching transformer between the high-impedance tube output stage and low-impedance speakers. However, a simple transistor amplifier can be made with a transformer output, which will be shown below.

There is also a point of view that the ultimate sound quality can only be provided by a hybrid tube-transistor amplifier, all stages of which are single-ended, are not covered and operate in class "A". That is, such a power follower is a single-transistor amplifier. Its circuit can have the maximum achievable efficiency (in class "A") no more than 50%. But neither the power nor the efficiency of the amplifier are indicators of the quality of sound reproduction. In this case, the quality and linearity of the characteristics of all EREs in the circuit are of particular importance.

Since single-ended circuits get such a perspective, we will consider their possible options below.

Single-ended single transistor amplifier

Its circuit, made with a common emitter and R-C-couplings on the input and output signals for operation in class "A", is shown in the figure below.

It shows an n-p-n transistor Q1. Its collector is connected to the positive terminal of + Vcc through a current-limiting resistor R3, and the emitter is connected to -Vcc. The pnp transistor amplifier will have the same circuit, but the power supply pins are swapped.

C1 is a coupling capacitor by which the AC input source is separated from the DC voltage source Vcc. In this case, C1 does not interfere with the passage of the alternating input current through the base-emitter junction of the transistor Q1. Resistors R1 and R2 together with the resistance of the transition "E - B" form Vcc to select the operating point of the transistor Q1 in the static mode. Typical for this circuit is R2 = 1 kΩ and the operating point is Vcc / 2. R3 is the pull-up resistor of the collector circuit and is used to create an output signal on the collector of an alternating voltage.

Suppose that Vcc = 20 V, R2 = 1 kΩ, and the current gain is h = 150. The voltage at the emitter is Ve = 9 V, and the voltage drop across the "E - B" junction is taken equal to Vbe = 0.7 V. This value corresponds to the so-called silicon transistor. If we were considering an amplifier based on germanium transistors, then the voltage drop across the open junction "E - B" would be equal to Vbe = 0.3 V.

Emitter current approximately equal to collector current

Ie = 9 V / 1 kΩ = 9 mA ≈ Ic.

Base current Ib = Ic / h = 9 mA / 150 = 60 μA.

Voltage drop across resistor R1

V (R1) = Vcc - Vb = Vcc - (Vbe + Ve) = 20V - 9.7V = 10.3V,

R1 = V (R1) / Ib = 10.3 V / 60 μA = 172 kΩ.

C2 is needed to create a circuit for the passage of the alternating component of the emitter current (in fact, the collector current). If it were not for it, then the resistor R2 would severely limit the AC component, so that the considered amplifier on a bipolar transistor would have a low current gain.

In our calculations, we assumed that Ic = Ib h, where Ib is the base current flowing into it from the emitter and arising when the bias voltage is applied to the base. However, collector leakage current Icb0 always flows through the base (both with and without bias). Therefore, the real collector current is Ic = Ib h + Icb0 h, i.e. the leakage current in the OE circuit is increased 150 times. If we were considering an amplifier based on germanium transistors, then this circumstance would have to be taken into account in the calculations. The point is that they have a significant Icb0 of the order of several μA. In silicon, it is three orders of magnitude lower (about several nA), so it is usually neglected in calculations.

Single-ended amplifier with MIS transistor

Like any amplifier on field-effect transistors, the considered circuit has its own analogue among amplifiers on Therefore, consider an analogue of the previous circuit with a common emitter. It is made with a common source and R-C connections for input and output signals for operation in class "A" and is shown in the figure below.

Here C1 is the same blocking capacitor by which the AC input source is separated from the DC voltage source Vdd. As you know, any amplifier based on field-effect transistors must have a gate potential of its MOS transistors below the potentials of their sources. In this circuit, the gate is grounded by resistor R1, which usually has a high resistance (from 100 kΩ to 1 MΩ) so that it does not bypass the input signal. There is practically no current through R1, so the gate potential in the absence of an input signal is equal to the ground potential. The source potential is higher than the ground potential due to the voltage drop across the resistor R2. Thus, the potential of the gate turns out to be lower than the potential of the source, which is necessary for the normal operation of Q1. Capacitor C2 and resistor R3 have the same function as in the previous circuit. Since this is a common source circuit, the input and output signals are 180 ° out of phase.

Amplifier with transformer output

The third single stage simple transistor amplifier, shown in the figure below, is also a common emitter design for Class A operation, but is coupled to the low impedance speaker through a matching transformer.

The primary winding of transformer T1 loads the collector circuit of transistor Q1 and develops an output signal. T1 sends the output signal to the speaker and matches the output impedance of the transistor to a low (on the order of a few ohms) speaker impedance.

The voltage divider of the collector power supply Vcc, assembled on resistors R1 and R3, provides the choice of the operating point of the transistor Q1 (supply of a bias voltage to its base). The purpose of the remaining elements of the amplifier is the same as in the previous circuits.

Push-pull audio amplifier

A push-pull low-frequency amplifier on two transistors splits the input frequency into two antiphase half-waves, each of which is amplified by its own transistor stage. After performing this amplification, the half-waves are combined into a complete harmonic signal, which is transmitted to the speaker system. Such a conversion of the low-frequency signal (splitting and re-merging) naturally causes irreversible distortions in it, due to the difference in the frequency and dynamic properties of the two transistors in the circuit. This distortion degrades the sound quality at the amplifier's output.

Push-pull amplifiers operating in class "A" do not reproduce complex sound signals well enough, since a constant current of increased magnitude continuously flows in their arms. This leads to unbalanced signal half-waves, phase distortion and ultimately loss of intelligibility. When heated, two powerful transistors double the distortion of the signal in the region of low and infra-low frequencies. But still, the main advantage of the push-pull circuit is its acceptable efficiency and increased output power.

A push-pull circuit of a transistor power amplifier is shown in the figure.

This amplifier is designed to work in class "A", but class "AB" and even "B" can be used.

Transformerless transistor power amplifier

Transformers, despite the success in their miniaturization, are still the most bulky, heavy and expensive ERE. Therefore, a way was found to eliminate the transformer from the push-pull circuit by performing it on two powerful complementary transistors of different types (n-p-n and p-n-p). Most modern power amplifiers use this principle and are designed to work in class "B". The diagram of such a power amplifier is shown in the figure below.

Both of its transistors are connected according to the scheme with a common collector (emitter follower). Therefore, the circuit transfers the input voltage to the output without amplification. If there is no input signal, then both transistors are on the border of the on state, but at the same time they are off.

When the harmonic signal is applied to the input, its positive half-wave turns on TR1, but puts the pnp transistor TR2 completely into cutoff mode. Thus, only the positive half-wave of the amplified current flows through the load. The negative half-wave of the input signal opens only TR2 and locks TR1, so that the negative half-wave of the amplified current is supplied to the load. As a result, a full power-amplified (due to current amplification) sinusoidal signal is released at the load.

Single transistor amplifier

To master the above, we will assemble a simple transistor amplifier with our own hands and figure out how it works.

As the load of the low-power transistor T of the BC107 type, we turn on headphones with a resistance of 2-3 kOhm, we supply the bias voltage to the base from a high-resistance resistor R * with a value of 1 megohm, decoupling electrolytic capacitor C with a capacity of 10 μF to 100 μF we include in the base circuit T. Power the circuit We will be powered by a 4.5 V / 0.3 A battery.

If R * is not connected, then there is neither base current Ib nor collector current Ic. If the resistor is connected, then the voltage at the base rises to 0.7 V and a current Ib = 4 μA flows through it. The current gain of the transistor is 250, which gives Ic = 250Ib = 1 mA.

Having assembled a simple transistor amplifier with our own hands, we can now test it. Plug in the headphones and place your finger on point 1 of the circuit. You will hear noise. Your body receives radiation from the mains supply at a frequency of 50 Hz. The noise you hear from the headphones is this radiation, only amplified by the transistor. Let us explain this process in more detail. A 50 Hz AC voltage is connected to the base of the transistor through a capacitor C. The base voltage is now the sum of the DC bias voltage (approximately 0.7 V) from resistor R * and the finger AC voltage. As a result, the collector current receives an alternating component with a frequency of 50 Hz. This alternating current is used to move the speaker membrane back and forth at the same frequency, which means we can hear a 50 Hz tone at the output.

Listening to the noise level of 50 Hz is not very interesting, so you can connect low-frequency signal sources (CD-player or microphone) to points 1 and 2 and hear amplified speech or music.

To everyone who finds it difficult to choose the first circuit for assembly, I want to recommend this amplifier with 1 transistor. The circuit is very simple, and can be done both by mounting and printed wiring.

I must say right away that the assembly of this amplifier is justified only as an experiment, since the sound quality will, at best, be at the level of cheap Chinese receivers - scanners. If someone wants to build themselves a low-power amplifier with better sounding, using a microcircuit TDA 2822 m , can go to the following link:

Portable speaker for player or phone on tda2822m chip Amplifier check photo:

The following figure provides a list of required parts:

Almost any of the medium and high power bipolar transistors can be used in the circuit. n - p - n structures, for example KT 817. It is desirable to put a film capacitor at the input with a capacity of 0.22 - 1 MkF. An example of film capacitors in the following photo:

I give a drawing of a printed circuit board from the program Sprint-Layout:


The signal is taken from the output of an mp3 player or telephone, ground and one of the channels are used. In the following figure, you can see the wiring diagram of the Jack 3.5 plug for connecting to a signal source:


If desired, this amplifier, like any other, can be equipped with a volume control by connecting a 50K ohm potentiometer according to the standard scheme, 1 channel is used:


In parallel with the power supply, if there is no large electrolytic capacitor in the power supply after the diode bridge, you need to supply the electrolyte for 1000 - 2200 MkF, with an operating voltage greater than the supply voltage of the circuit.
An example of such a capacitor:

You can download a printed circuit board of a single transistor amplifier for the sprint - layout program in the My files section of the site.

You can evaluate the sound quality of this amplifier by watching the video of its work on our channel.

Circuit of a simple transistor audio amplifier, which is implemented on two powerful composite transistors TIP142-TIP147 installed in the output stage, two low-power BC556B in the differential path and one BD241C in the signal pre-amplification circuit - only five transistors for the whole circuit! Such a design of the UMZCH can be freely used, for example, as part of a home music center or for swinging a subwoofer installed in a car, at a disco.

The main attraction of this sound power amplifier lies in its ease of assembly even by novice radio amateurs, there is no need for any special tuning, there are no problems in purchasing components at an affordable price. The PA circuit presented here has electrical characteristics with high linearity in the frequency range from 20Hz to 20000Hz. p>

When choosing or self-manufacturing a transformer for a power supply, the following factor must be taken into account: - the transformer must have a sufficient power reserve, for example: 300 W per channel, in the case of a two-channel version, then naturally the power doubles. You can use a separate transformer for each, and if you use a stereo version of the amplifier, then you will generally get a device of the "double mono" type, which will naturally increase the efficiency of sound amplification.

The operating voltage in the secondary windings of the transformer should be ~ 34v alternating, then the constant voltage after the rectifier will turn out to be in the region of 48v - 50v. In each power supply arm, it is necessary to install a fuse designed for an operating current of 6A, respectively, for a stereo when operating on one power supply unit - 12A.

The amplifier offered to your precious attention is simple to assemble, terribly easy to set up (it actually does not require it), does not contain particularly scarce components and, at the same time, has very good characteristics and easily pulls on the so-called hi-fi, so dearly loved by most citizens ...The amplifier can operate on a load of 4 and 8 ohms, it can be used in a bridge connection to a load of 8 ohms, while it will give 200 watts to the load.

Main characteristics:

Supply voltage, V .............................................. .................. ± 35
Consumption current in silent mode, mA ................................ 100
Input impedance, kOhm .............................................. ........... 24
Sensitivity (100 W, 8 Ohm), V ........................................ ...... 1.2
Output power (KG = 0.04%), W ...................................... ........ 80
The range of reproducible frequencies, Hz ............................. 10 - 30000
Signal-to-noise ratio (unweighted), dB .............................. -73

The amplifier is completely on discrete elements, without any op-amp or other tricks. When operating at a load of 4 Ohms and a 35 V supply, the amplifier develops a power of up to 100 W. If there is a need to connect an 8 Ohm load, the power supply can be increased to +/- 42 V, in this case, we will get the same 100 watts.It is strongly discouraged to increase the supply voltage more than 42 V, otherwise you may be left without output transistors. When operating in bridge mode, an 8 ohm load must be used, otherwise, again, we lose all hope of the survival of the output transistors. By the way, it should be taken into account that there is no protection against short-circuit in the load, so you need to be careful.To use the amplifier in bridge mode, the MT input must be screwed to the output of another amplifier, to the input of which the signal is supplied. The remaining input is shorted to the common wire. Resistor R11 is used to set the quiescent current of the output transistors. Capacitor C4 determines the upper limit of the gain and should not be reduced - get self-excitation at high frequencies.
All resistors are 0.25 W with the exception of R18, R12, R13, R16, R17. The first three are 0.5 W, the last two are 5 W. The HL1 LED is not for beauty, so there is no need to stick a super-bright diode into the circuit and output it to the front panel. The diode should be the most common green color - this is important, since LEDs of other colors have a different voltage drop.If suddenly someone is unlucky and he could not get the output transistors MJL4281 and MJL4302, they can be replaced with MJL21193 and MJL21194, respectively.The variable resistor R11 is best taken with a multiturn resistor, although the usual one will do. There is nothing critical here - it's just more convenient to set the quiescent current.