Mosfit field effect transistor power amplifier. Umzch with field-effect transistors irfz44 Scheme of high-quality umzch on field-effect transistors

The figure shows a 50W amplifier circuit with output MOSFETs.
The first stage of the amplifier is a differential amplifier based on transistors VT1 VT2.
The second stage of the amplifier consists of transistors VT3 VT4. The final stage of the amplifier consists of MOSFETs IRF530 and IRF9530. The output of the amplifier through the coil L1 is connected to a load of 8 ohms.
The circuit consisting of R15 and C5 is designed to reduce noise. Capacitors C6 and C7 power filters. Resistance R6 is designed to adjust the quiescent current.

Note:
Use a bipolar +/-35V power supply
L1 consists of 12 turns of insulated copper wire with a diameter of 1mm.
C6 and C7 should be rated at 50V, the rest of the electrolytic capacitors at 16V.
Requires heatsink for MOSFETs. Size 20x10x10 cm made of aluminium.
Source - http://www.circuitstoday.com/mosfet-amplifier-circuits

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UMZCH with complementary field effect transistors

We present to readers a variant of a hundred-watt UMZCH with field-effect transistors. In this design, power transistor packages can be mounted on a common heat sink without insulating pads, and this significantly improves heat transfer. As the second version of the power supply, a powerful pulse converter is proposed, which should have a sufficiently low level of intrinsic noise.

The use of field-effect transistors (FETs) in UMZCH until recently was constrained by a meager assortment of complementary transistors, as well as their low operating voltage. The quality of sound reproduction through the UMZCH on the FET is often rated at the level of the tube and even higher because, compared to amplifiers based on bipolar transistors, they create less non-linear and intermodulation distortion, and also have a smoother increase in distortion during overloads. They outperform tube amplifiers in both load damping and audio bandwidth. The cutoff frequency of such amplifiers without feedback is much higher than that of the UMZCH on bipolar transistors, which favorably affects all types of distortion.

Nonlinear distortions in the UMZCH are mainly introduced by the output stage, and a common OOS is usually used to reduce them. Distortion in the input differential stage, used as an adder of signals from the source and the circuit of the common OOS, may be small, but with the help of the general OOS it is impossible to reduce them.

The overload capacity of the differential stage on field-effect transistors is approximately 100 ... 200 times higher than with bipolar transistors.

The use of field-effect transistors in the UMZCH output stage makes it possible to abandon the traditional two- and three-stage Darlington repeaters with their inherent disadvantages.

Good results are obtained by using field-effect transistors with a metal-dielectric-semiconductor (MIS) structure in the output stage. Due to the fact that the current in the output circuit is controlled by the input voltage (similar to electrovacuum devices), then at high currents the speed of the cascade on MIS field-effect transistors in the switching mode is quite high (τ = 50 ns). Such cascades have good transmission properties at high frequencies and have the effect of temperature self-stabilization.

The advantages of field effect transistors include:

low control power in static and dynamic modes;
no thermal breakdown and low susceptibility to secondary breakdown;
thermal stabilization of the drain current, providing the possibility of parallel connection of transistors;
the transfer characteristic is close to linear or quadratic;
high speed in switching mode, thereby reducing dynamic losses;
the absence of the accumulation of excess carriers in the structure;
low noise level
small dimensions and weight, long service life.

But besides the advantages, these devices also have disadvantages:

failure due to electrical overvoltage;
thermal distortion may occur at low frequencies (below 100 Hz). At these frequencies, the signal changes so slowly that in one half-cycle the temperature of the crystal has time to change and, consequently, the threshold voltage and the slope of the transistors change.

The last of the noted shortcomings limits the output power, especially at low supply voltages; the way out is the parallel connection of transistors and the introduction of environmental protection.

It should be noted that recently foreign firms (for example, Exicon, etc.) have developed a lot of field-effect transistors suitable for audio equipment: EC-10N20, 2SK133-2SK135, 2SK175, 2SK176 with an n-type channel; EC-10P20, 2SJ48- 2SJ50, 2SJ55, 2SJ56 with p-channel. Such transistors are characterized by a weak dependence of the slope (forward transfer admitance) on the drain current and smoothed output I–V characteristics.

The parameters of some field-effect transistors, including those manufactured by the Minsk Production Association "Integral", are given in Table. 1.

Most transistor transformerless UMZCH are made according to a half-bridge circuit. In this case, the load is included in the diagonal of the bridge formed by two power supplies and two output transistors of the amplifier (Fig. 1).

When there were no complementary transistors, the UMZCH output stage was performed mainly on transistors of the same structure with a load and a power source connected to a common wire (Fig. 1, a) Two possible options for controlling the output transistors are shown in Fig. 2.

In the first of them (Fig. 2, a), the control of the lower arm of the output stage is in more favorable conditions. Since the change in the supply voltage is small, the Miller effect (dynamic input capacitance) and the Earley effect (collector current versus emitter-collector voltage) practically do not appear. The control circuit of the upper arm is connected here in series with the load itself, therefore, without taking additional measures (for example, cascode switching of devices), these effects are manifested to a large extent. According to this principle, a number of successful UMZCHs were developed.

According to the second option (Fig. 2.6 - MIS transistors are more suitable for such a structure), a number of UMZCHs were also developed, for example. However, even in such cascades it is difficult to ensure, even with the use of current generators, the control symmetry of the output transistors. Another example of balancing on the input resistance is the implementation of the arms of the amplifier according to a quasi-complementary circuit or the use of complementary transistors (see Fig. 1, b) c.

The desire to balance the arms of the output stage of amplifiers made on transistors of the same conductivity led to the development of amplifiers with an ungrounded load according to the circuit in Fig. 1 , g . However, even here it is not possible to achieve complete symmetry of the previous cascades. The negative feedback circuits from each arm of the output stage are unequal; the environmental protection circuits of these stages control the voltage at the load in relation to the output voltage of the opposite arm. In addition, such a circuit solution requires isolated power supplies. Due to these shortcomings, it has not found wide application.

With the advent of complementary bipolar and field-effect transistors, the output stages of the UMZCH are mainly built according to the circuits in Fig. 1, b, c. However, even in these variants, high-voltage devices must be used to drive the output stage. The transistors of the pre-output stage operate with a high voltage gain, and therefore are subject to the Miller and Earley effects and, without a common OOS, introduce significant distortion, which requires high dynamic characteristics from them. Feeding the preliminary stages with increased voltage also reduces the efficiency of the amplifier.

If in Fig. 1, b, c move the connection point with the common wire to the opposite shoulder of the diagonal of the bridge, we get the options in fig. 1e and 1e, respectively. In the structure of the cascade according to the scheme of Fig. 1, e automatically solves the problem of isolating the output transistors from the case. Amplifiers made according to such schemes are free from a number of the listed disadvantages.

Amplifier circuitry features

The attention of radio amateurs is offered an inverting UMZCH (Fig. 3), corresponding to the block diagram of the output stage in Fig. 1, e.

(click to enlarge)

The input differential stage is made on field-effect transistors (VT1, VT2 and DA1) according to a symmetrical circuit. Their advantages in the differential stage are well known: high linearity and overload capacity, low noise. The use of field-effect transistors greatly simplified this cascade, since there was no need for current generators. To increase the gain with an open loop OS, the signal is taken from both shoulders of the differential stage, and an emitter follower on transistors VT3, VT4 is installed in front of the subsequent voltage amplifier.

The second stage is made on transistors VT5-VT10 according to a combined cascode circuit with servo power. Such a power supply of the cascade with OE neutralizes the input dynamic capacitance in the transistor and the dependence of the collector current on the emitter-collector voltage. The output stage of this stage uses high-frequency BSIT transistors, which, compared to bipolar ones (KP959 versus KT940), have twice the cutoff frequency and four times the drain (collector) capacitance.

The use of an output stage powered by separate isolated sources made it possible to dispense with a low-voltage supply (9 V) for the preamplifier.

The output stage is made on powerful MIS transistors, and the conclusions of their drain (and heat-removing flanges of the housings) are connected to a common wire, which simplifies the design and assembly of the amplifier.

Powerful MIS transistors, unlike bipolar ones, have a smaller spread of parameters, which facilitates their parallel connection. The main spread of currents between devices arises due to the inequality of threshold voltages and the spread of input capacitances. The introduction of additional resistors with a resistance of 50-200 Ohm in the gate circuit provides almost complete equalization of the turn-on and turn-off delays and eliminates current spread during switching.

All stages of the amplifier are covered by local and general environmental protection.

Main technical characteristics

Open-loop feedback (R6 replaced by 22 MΩ, C4 excluded)
Cutoff frequency, kHz......300
Voltage gain, dB......43
Harmonic coefficient in AB mode, %, no more......2

With OS enabled

Output power, W at a load of 4 Ohm......100
at a load of 8 ohms......60
Reproducible frequency range, Hz......4...300000
Harmonic coefficient, %, no more......0.2
Rated input voltage, V......2
Quiescent current of the output stage, A ...... 0.15
Input impedance, kOhm.....24

Due to the relatively high cutoff frequency of an open-loop feedback amplifier, the feedback depth and harmonic distortion are nearly constant across the entire frequency range.

From below, the operating frequency band of the UMZCH is limited by the capacitance of capacitor C1, from above - by C4 (with a capacitance of 1.5 pF, the cutoff frequency is 450 kHz).

Construction and details

The amplifier is made on a board made of double-sided foil fiberglass (Fig. 4).

The board from the side where the elements are installed is maximally filled with foil connected to a common wire. Transistors VT8, VT9 are equipped with small plate heat sinks in the form of a "flag". Pistons are installed in the holes for the drain terminals of powerful field-effect transistors; the drain terminals of transistors VT11, VT14 are connected to a common wire from the side of the foil (marked with crosses in the figure).

Pistons are installed in holes 5-7 of the board for connecting the mains transformer leads and jumper holes. Resistors R19, R20, R22, R23 are made of manganin wire with a diameter of 0.5 and a length of 150 mm. To suppress the inductance, the wire is folded in half and folded (bifilar) is wound on a mandrel with a diameter of 4 mm.

The inductor L1 is wound with a PEV-2 wire 0.8 turn to turn over the entire surface of a 2 W resistor (MLT or similar).

Capacitors C1, C5, C10, C11 - K73-17, and C10 and C11 are soldered from the PCB side to the terminals of capacitors C8 and C9. Capacitors C2, C3 - oxide K50-35; capacitor C4 - K10-62 or KD-2; C12 - K10-17 or K73-17.

Field-effect transistors with an n-channel (VT1, VT2) must be selected with approximately the same initial drain current as the transistors in the DA1 assembly. In terms of cutoff voltage, they should not differ by more than 20%. Microassembly DA1 K504NTZB can be replaced by K504NT4B. It is possible to use a matched pair of transistors KP10ZL (also with indices G, M, D); KP307V - KP307B (also A, E), KP302A or transistor assembly KPC315A, KPC315B (in this case, the board will have to be reworked).

In positions VT8, VT9, you can also use complementary transistors of the KT851, KT850 series, as well as KT814G, KT815G (with a cut-off frequency of 40 MHz) of the Minsk Association "Integral".

In addition to those indicated in the table, you can use, for example, the following pairs of MIS transistors: IRF530 and IRF9530; 2SK216 and 2SJ79; 2SK133-2SK135 and 2SJ48-2SJ50; 2SK175-2SK176 and 2SJ55-2SJ56.

For the stereo version, power is supplied to each of the amplifiers from a separate transformer, preferably with a ring or rod (PL) magnetic circuit, with a power of 180 ... 200 W. Between the primary and secondary windings, a layer of shielding winding is placed with a wire PEV-2 0.5; one of its conclusions is connected to a common wire. The outputs of the secondary windings are connected to the amplifier board with a shielded wire, and the shield is connected to the common wire of the board. Windings for the rectifiers of the preamplifiers are placed on one of the network transformers. Voltage stabilizers are made on IL7809AC (+9 V), IL7909AC (-9 V) microcircuits - not shown in the diagram. The ONp-KG-26-3 (XS1) connector was used to supply the 2x9 V power board.

When setting up, the optimal current of the differential stage is set by a tuned resistor R3 to minimize distortion at maximum power (approximately in the middle of the working area). Resistors R4, R5 are designed for a current of about 2...3 mA in each arm with an initial drain current of about 4...6 mA. With a lower initial drain current, the resistance of these resistors must be proportionally increased.

The quiescent current of the output transistors in the range of 120 ... 150 mA is set by a trimming resistor R3, and, if necessary, by selecting resistors R13, R14.

Impulse power block

For those radio amateurs who have difficulty purchasing and winding large network transformers, a switching power supply is offered for the UMZCH output stages. In this case, the preamplifier can be powered from a low-power stabilized PSU.

A pulsed power supply unit (its circuit is shown in Fig. 5) is an unregulated self-generating half-bridge inverter. The use of proportional-current control of the inverter transistors in combination with a saturable switching transformer makes it possible to automatically remove the active transistor from saturation by the time of switching. This reduces the charge dissipation time in the base and eliminates the through current, as well as reduces power losses in the control circuits, increasing the reliability and efficiency of the inverter.

UPS specifications

Output power, W, no more......360
Output voltage......2x40
Efficiency, %, not less than ...... 95
Conversion frequency, kHz......25

An interference suppression filter L1C1C2 is installed at the input of the mains rectifier. Resistor R1 limits the inrush charging current of capacitor C3. A jumper X1 is provided in series with the resistor on the board, instead of which you can turn on a choke to improve filtering and increase the "hardness" of the output load characteristic.

The inverter has two positive feedback circuits: the first - by voltage (using windings II in transformer T1 and III - in T2); the second - by current (with a current transformer: turn 2-3 and windings 1-2, 4-5 of transformer T2).

The trigger device is made on a unijunction transistor VT3. After starting the converter, it turns off due to the presence of the VD15 diode, since the time constant of the R6C8 circuit is much larger than the conversion period.

The peculiarity of the inverter is that when low-voltage rectifiers operate on large filter capacities, it needs a smooth start. The smooth start of the block is facilitated by the chokes L2 and L3 and, to some extent, the resistor R1.

The power supply is made on a printed circuit board made of one-sided foil fiberglass 2 mm thick. The board drawing is shown in fig. 6.

(click to enlarge)

The winding data of transformers and information about the magnetic circuits are given in Table. 2. All windings are made with PEV-2 wire.

Before winding the transformers, the sharp edges of the rings must be blunted with sandpaper or a bar and wrapped with varnished cloth (for T1 - rings folded together in three layers). If this pre-treatment is not done, then it is possible that the varnished fabric will be pressed through and the turns of the wire will be shorted to the magnetic circuit. As a result, the no-load current will increase sharply and the transformer will heat up. Between the windings 1-2, 5-6-7 and 8-9-10, a PEV-2 0.31 wire is wound in one layer turn to turn shielding windings, one end of which (E1, E2) is connected to a common UMZCH wire.

The winding 2-3 of the transformer T2 is a coil of wire with a diameter of 1 mm over the winding 6-7, soldered by the ends into the printed circuit board.

Inductors L2 and L3 are made on BZO armored magnetic cores made of 2000NM ferrite. The windings of the chokes are wound in two wires until the frame is filled with PEV-2 0.8 wire. Given that the chokes operate with DC bias, it is necessary to insert spacers of non-magnetic material 0.3 mm thick between the cups.

The L1 choke is of the D13-20 type, it can also be made on the B30 armored magnetic circuit similarly to the L2, L3 chokes, but without a gasket, by winding the windings in two MGTF-0.14 wires until the frame is filled.

Transistors VT1 and VT2 are mounted on heat sinks made of ribbed aluminum profile with dimensions of 55x50x15 mm through insulating gaskets. Instead of those indicated in the diagram, you can use the KT8126A transistors of the Minsk software "Integral", as well as MJE13007. Additional oxide capacitors K50-6 (not shown in the diagram) with a capacity of 2000 μF per 50 V are connected between the PSU outputs +40 V, -40 V and "its" midpoint (ST1 and ST2). These four capacitors are installed on a textolite plate with dimensions 140x100 mm, fixed with screws on the heat sinks of powerful transistors.

Capacitors C1, C2 - K73-17 for a voltage of 630 V, C3 - oxide K50-35B for 350 V, C4, C7 - K73-17 for 250 V, C5, C6 - K73-17 for 400 V, C8 - K10-17 .

The pulse PSU is connected to the PA board in close proximity to the terminals of the capacitors C6-C11. In this case, the VD5-VD8 diode bridge is not mounted on the PA board.

To delay the connection of acoustic systems to the UMZCH for the time of attenuation of transients that occur during power-up, and turn off the speakers when a constant voltage of any polarity appears at the output of the amplifier, you can use a simple or more complex protective device.

Literature

Khlupnov A. Amateur amplifiers of low frequency. -M.: Energy, 1976, p. 22.
Akulinichev I. LF amplifier with common-mode mode stabilizer. - Radio, 1980, No. Z.s.47.
Garevskikh I. Broadband power amplifier. - Radio, 1979, No. 6. p. 43.
Kolosov V. Modern amateur tape recorder. - M.: Energy, 1974.
Borisov S. MOS transistors in low-frequency amplifiers. - Radio. 1983, no. 11, p. 36-39.
Dorofeev M. Mode B in AF power amplifiers. - Radio, 1991, No. 3, p. 53.
Syritso A. Powerful bass amplifier. - Radio, 1978. No. 8, p. 45-47.
Syritso A. Power amplifier based on integrated op-amps. - Radio, 1984, No. 8, p. 35-37.
Yakimenko N. Field-effect transistors in the bridge UMZCH. - Radio. 1986, no. 9, p. 38, 39.
Vinogradov V. AU protection device. - Radio, 1987, No. 8. p. thirty.

Low-frequency amplifiers are very popular among fans of radio electronics. Unlike the previous scheme, this FET power amplifier consists mainly of transistors and uses an output stage on, which, with a bipolar supply voltage of 30 volts, can provide output power up to 70 watts on speakers with a resistance of 4 ohms.

Schematic diagram of an amplifier on field-effect transistors

The amplifier is assembled on the basis of the TL071 (IO1) operational amplifier or any similar one, which creates the main amplification of the differential signal. Amplified low-frequency signal from the output of the op-amp, most of which goes through R3 to the midpoint. The rest of the signal is sufficient for direct amplification on the IRF9530 (T4) and IRF530 (T6) MOSFETs.

Transistors T2, T3 and their surrounding components serve to stabilize the operating point of the variable resistor, since it must be correctly set in the symmetry of each half-wave on the load of the amplifier.

All parts are assembled on a single-sided printed circuit board. Please note that three jumpers must be installed on the board.

Amplifier setting

The best way to tune the amplifier is to apply a sinusoidal signal to its input and connect a load resistor with a value of 4 ohms. After that, the resistor R12 is set in such a way that the signal at the output of the amplifier is symmetrical, i.e. the shape and size of the positive and negative half-waves were the same at maximum volume.

Recently, designers of low-frequency power amplifiers are increasingly turning to tube circuitry, which allows, with a comparative simplicity of design, to achieve good sound. But you should not completely "write off" transistors, because under certain circumstances, the transistor UMZCH is still able to work quite well, and often better than lamps ... The author of this article had a chance to try a large number of UMZCH. One of these most successful "bipolar" options is offered to the readers. The idea of good work is based on the condition of symmetry of both arms of the UMZCH. When both half-waves of the amplified signal undergo similar conversion processes, one can expect satisfactory performance of the UMZCH in qualitative terms.

Even in the recent past, the introduction of deep environmental protection was considered an indispensable and sufficient condition for the good operation of any UMZCH. There was an opinion about the impossibility of creating high-quality UMZCH without deep general environmental protection. In addition, the authors of the designs convincingly assured that, they say, there is no need to select transistors to work in pairs (shoulders), the OOS will compensate for everything and the spread of transistors in parameters does not affect the quality of sound reproduction!

The era of UMZCH, assembled on transistors of the same conductivity, for example, the popular KT808. assumed the inclusion of output transistors UMZCH is already unequal, when one transistor of the output stage was turned on according to the scheme with OE, the second - with OK. Such an asymmetric inclusion did not contribute to the qualitative amplification of the signal. With the arrival of KT818, KT819, KT816. KT817 and others, it would seem that the problem of UMZCH linearity is solved. But the listed complementary pairs of transistors "for life" are too far from true complementarity.

We will not delve into the problems of non-complementarity of the above transistors, which are very widely used in various UMZCHs. It is only necessary to emphasize that fact. that, under equal conditions (modes) of these transistors, it is quite difficult to ensure their complementary operation in push-pull amplifying stages. This is well said in the book by N.E. Sukhov.

I do not at all deny the possibility of achieving good results when creating UMZCH on complementary transistors. This requires a modern approach in the circuitry of such UMZCH, with the obligatory careful selection of transistors for operation in pairs (keys). I had a chance to design such UMZCH, which are a kind of continuation of the high-quality UMZCH N.E. Sukhov, but about them some other time. Regarding the symmetry of the UMZCH, as the main condition for its good work, the following should be said. It turned out that the UMZCH, assembled according to a truly symmetrical circuit and certainly on transistors of the same type (with a mandatory selection of copies), has higher quality parameters. Selecting transistors is much easier if they are from the same batch. Usually copies of transistors from the same batch have fairly close parameters against "accidentally" purchased copies. From experience we can say that out of 20 pcs. transistors (the standard number of one pack) it is almost always possible to select two pairs of transistors for the UMZCH stereo complex. There were cases of more "successful catch" - four pairs of 20 pieces. I will talk about the selection of transistors a little later.

The schematic diagram of the UMZCH is shown in Fig. 1. As you can see from the diagram, it is quite simple. The symmetry of both arms of the amplifier is ensured by the symmetry of the inclusions of transistors

It is known that the differential stage has many advantages over conventional push-pull circuits. Without delving into theory, it should be emphasized that this circuit has the correct "current" control of bipolar transistors. The transistors of the differential stage have an increased output impedance (much greater than the traditional "buildup" according to the circuit with OK), so they can be considered as current generators (current sources). Thus, the current principle of controlling the output transistors of the UMZCH is implemented. It is very accurately said about the influence of resistance matching between transistor stages on the level of nonlinear distortion in: "It is known that the nonlinearity of the input characteristic of the transistor I b \u003d f (U be) is most pronounced when the amplifying stage is powered by a voltage generator, i.e. The output impedance of the previous stage is less than the input resistance of the next one.In this case, the output signal of the transistor - the collector or emitter current - is approximated by an exponential function of the emitter base voltage Ube, and the harmonic coefficient of the order of 1% is achieved at a value of this voltage equal to only 1 mV (!) This explains the causes of distortion in many transistor UMZCH. It is a pity that almost no one pays due attention to this fact. What is there, transistors "die" in UMZCH (like dinosaurs ?!), as if there is no way out of the circumstances, except how to apply lamp circuits...

But before you start winding a labor-intensive output transformer, you should still tinker with the UMZCH symmetrical transistor circuit. Looking ahead, I’ll also say that UMZCH on field-effect transistors were also assembled using similar circuitry, we’ll talk about this some other time.

Another feature of the circuit in Fig. 1 is the increased (compared to traditional UMZCH) number of power sources. You should not be afraid of this, since the capacitances of the filter capacitors are simply divided into two channels equally. And the separation of power sources in the UMZCH channels only improves the parameters of the stereo complex as a whole. The voltages of sources E1 and E2 are not stabilized, and a voltage stabilizer (40 volts) must be used as E3.

Speaking about the theoretical problems of push-pull circuits and transistor UMZCH in general, it is necessary to analyze one more cascade (or several such cascades) - a phase inverter. Long-term experiments confirm the fact of a significant deterioration in the quality of sound reproduction due to these cascades. Having assembled a completely symmetrical circuit, and even with painstakingly selected details, one has to face the problem of phase inverter circuits. It was found that these stages are capable of introducing very large distortions (the difference in the shape of the half-wave sine wave could be observed on the oscilloscope screen even without using any additional circuits). The foregoing fully applies to simple circuits of tube versions of phase inverter amplifiers. You select the ratings in the circuit in order to obtain the equality of the amplitudes of both half-waves (sinusoid) of the anti-phase signal on a high-end digital voltmeter, and subjective examination requires (by ear!) Turning the trimming resistors sliders away from this "instrument" way to adjust the levels.

Peering into the shape of a sinusoid on the oscilloscope screen, one can see "interesting" distortions - at one output of the phase inverter they are wider (along the frequency axis), at the other they are "thinner", i.e. the area of the figure of sinusoids is different for direct and phase-inverted signals. Hearing clearly captures this, you have to "de-adjust" the setting. It is extremely undesirable to equalize the sinusoid in phase-inverted stages with deep OOS. It is necessary to eliminate the causes of asymmetry in these cascades in other circuitry ways, otherwise the phase-inverted cascade can introduce "transistor" distortions that are very noticeable to the ear, the level of which will be comparable to the distortions of the UMZCH output stage (!). This is how it happens that the phase inverter is the main asymmetry node for any push-pull UMZCH (be it transistor, lamp or combined UMZCH circuits), if, of course, the amplifying elements in the arms are pre-selected with close parameters, otherwise it makes no sense to expect from such good sounding circuits.

Of the easiest to implement phase-inverted circuits that work well are tube options. Their simpler "analogues" are field-effect transistors, which (only!) With a competent circuit design approach, are quite capable of competing with tube amplifiers. And if audiophiles are not afraid of using matching transformers in the output stages, where this "iron" still "sounds", then transformers can be used in the previous stages with a clear conscience. I mean phase-inverted cascades, where the current amplitude (namely, this component adversely affects the hardware) is small, and the voltage amplitude reaches a value of only a few volts.

It is undeniable that any transformer is a kind of step back in terms of circuitry to the age of gigahertz Pentiums. But there are a few "buts" that are very appropriate sometimes to recall. First, a well-made transition or matching transformer will never introduce as many non-linear distortions as several "wrong" amplifying cascades can introduce a wide variety of distortions.Secondly, a transformer phase inverter really allows you to achieve real symmetry of antiphase signals, the signals from its windings are really close to each other both in shape and in amplitude.Besides, it is passive , and its characteristics do not depend on the supply voltages.And if your UMZCH is really symmetrical (in this case, we mean its input impedances), then the asymmetry of the UMZCH will already be determined more by the spread of the parameters of the radio components in the arms of the UMZCH than by the phase-inverted cascade.Therefore, it is not recommended to use in such an UMZCH, radio elements with tolerances of more than 5% (the only exceptions are the current generator circuits that feed the differential stage). You should be aware that with a spread in the parameters of transistors in the arms of the UMZCH of more than 20%, the accuracy of the resistors is already losing its relevance. Conversely, when well-chosen transistors are used, it makes sense to use resistors with a 1% tolerance. Of course, you can pick them up with a good digital ohmmeter.

One of the most successful circuit developments of a phase inverter is shown in Fig. 2. Seemingly too simple, it still requires close attention to itself, since it has several "secrets". The first one is the right choice. transistors by parameters. Transistors VT1 and VT2 should not have significant leakage between the electrodes (meaning gate-source junctions). In addition, transistors should have close parameters, especially with regard to the initial drain current - instances with I s.nach are most suitable here. 30-70 mA. The supply voltages must be stabilized, although the stabilization coefficient of the power supply does not play a significant role, in addition, a negative voltage can also be taken from the UMZCH stabilizer. In order for electrolytic capacitors to introduce their distortions less, they are shunted with non-electrolytic ones - such as K73-17.

Let's take a closer look at the manufacturing features of the main node in this circuit - a phase-splitting (phase-inverted) transformer. Both the leakage inductance and the range of effectively reproduced frequencies depend on the accuracy of its manufacture, not to mention the level of various distortions. So, the two main secrets of the technological process for manufacturing this transformer are as follows. The first is the need to abandon the simple winding of the windings. I give two options for winding this transformer that I used. The first one is shown in Fig. 3, the second - in Fig. 4. The essence of the winding method is as follows. Each of the windings (I, II or III) consists of several windings containing exactly the same number of turns. Any error in the number of turns must be avoided, i.e. difference in turns between windings. Therefore, it was decided to wind the transformer in a long-proven way. According to Fig. 3, six wires are used (for example, PELSHO-0.25). The required length of the winding wire is calculated in advance (not always, and not every radio amateur will have six coils of wire of the same diameter at hand), put six wires together and wind all the windings at the same time. Further, it is only necessary to find the taps of the required windings and connect them in pairs in series. According to Fig. 4, nine conductors were used for this option. And yet, it is necessary to wind it so that the wires of one turn do not diverge in different directions far, wide from one another, but hold the common roll together. It is unacceptable to wind separate wires, the transformer will literally "ring" in the entire range of audio frequencies, the leakage inductance will increase, and the distortion of the UMZCH will also increase due to the asymmetry of the signals at the outputs of the transformer.

Yes, and it is very easy to make a mistake with separate methods of winding symmetrical windings. And an error of several turns makes itself felt by the asymmetry of antiphase signals. If we continue frankly, then a phase inverter transformer was made (in a single kind, copy) in ... 15 lived. There was an experiment that was included in the collection of great-sounding UMZCH designs. Once again, I would like to say that it is not the transformers that are to blame for the poor performance of some circuits, but their designers. All over the world, the production of tube UMZCHs has greatly expanded, the vast majority of them contain isolation transformers (or rather, matching ones), without which the tube stage (a typical push-pull output stage circuit contains 2-4 tubes) is simply impossible to match with low-impedance acoustic systems. There are, of course, instances of "superlamp" UMZCH, where there are no output transformers. Their place was taken by either powerful complementary pairs of field-effect transistors or ... a battery of powerful tube triodes connected in parallel. But this topic is already beyond the scope of this article. In our case, everything is much simpler. Transistor VT1 (Fig. 2) of the MOS type, connected according to a common drain circuit (source follower), works on a current generator (current source) made on a transistor VT2. Powerful field-effect transistors of the KP904 type should not be used, they have increased input and through capacitances, which cannot but affect the operation of this cascade.

Another stumbling block, a serious problem in the creation of a broadband transformer, awaits the designer when choosing a magnetic core. Here it is appropriate to add something to what can be found in the literature available to the radio amateur. Various design options for both amateurs and professionals offer the use of different materials for the magnetic cores of transformers, which would not cause trouble both in their acquisition and in their use. The essence of the methods is as follows.

If your UMZCH will operate at frequencies above 1 kHz, then you can safely use ferrite cores. But preference should be given to instances of magnetic circuits with the highest magnetic permeability, cores from line transformers of TVs work very well. Designers should be cautioned against using yokes that have already been in service for a long time. It is known that ferrite products lose their parameters with “age”, including the initial magnetic permeability, “unique” old age kills them no less than, for example, magnets of long-term used loudspeakers, which for some reason almost everyone is silent about.

Further, about the cores - if UMZCH is used as a bass option, then traditional W-shaped lamellar versions of magnetic circuits can be safely used. It must be emphasized that the shielding of all such transformers was a necessity and a requirement almost everywhere. What can you do, you have to pay for everything. It was usually sufficient to make a "cocoon" from ordinary roofing sheet with a thickness of 0.5 mm.

Toroidal cores also work well at low frequencies. By the way, their use simplifies the destruction of all kinds of pickups from network transformers. Here, the "reversibility" of the advantages of the toroidal core is preserved - in the network version it is distinguished by a small external radiation field, while in the input (signal) circuits it is insensitive to external fields. As for the broadband version (20 - 20,000 Hz), the most correct would be to use two different types of cores placed side by side, in one window of the frame for winding the transformer windings. At the same time, blockage is eliminated both at high frequencies (a ferrite core works here) and at low frequencies (transformer steel works here). An additional improvement in sound reproduction in the region of 1-15 kHz is achieved by coating the plates of the steel core with varnish, as is done in lamp UMZCH. In this case, each plate "works individually" as part of the core, which achieves a reduction in all kinds of losses due to eddy currents. Nitro-lacquer dries quickly, it is applied with a thin layer by simply dipping the plate into a dish with varnish.

To many, such a technology for manufacturing a transformer in a phase inverter may seem too painstaking, but take my word for it - "the game is worth the candle", because "you reap what you sow". And as for the complexity, "non-technological" we can say the following - in one day off it was possible to manufacture two such transformers without haste, and to unsolder their windings in the required order, which cannot be said about the output transformers for lamp UMZCH.

Now a few words about the number of turns. The theory requires an increase in the inductance of the primary winding (I), with its increase, the range of reproducible frequencies expands towards lower frequencies. In all designs, the winding of the windings was quite sufficient until the frame was filled, the wire diameter was used 0.1 for 15 cores, 0.15 for 9 cores and 0.2 for the 6-core version. In the latter case, the existing PELSHO 0.25 was also used.

For the same. who does not tolerate transformers, there is also a transformerless option - Fig. 5. This is the simplest. but a completely sounding version of the phase inverter cascade circuit, which was used not only in symmetrical UMZCH circuits, but also in powerful bridge UMZCH. Simplicity is often deceptive, so I will limit myself to criticism of such schemes, but I dare say that it is rather difficult to balance the areas of sinusoids, it is often necessary to introduce additional bias and balancing circuits, and the quality of sound reproduction leaves much to be desired. Despite the phase, amplitude-frequency distortions introduced by transformers, they make it possible to achieve an almost linear frequency response in the audio frequency region, i.e. over the entire range of 20 Hz - 20,000 Hz. From 16 kHz and above, the capacitance of the windings can affect, but an additionally increased cross-sectional area of the magnetic circuit allows you to partially get away from this problem. The rule is simple, similar to network transformers: by increasing the cross-sectional area of the magnetic circuit of the transformer core, for example, twice. boldly reduce the number of turns of the windings by half, etc.

Expand the region of effectively reproducible frequencies down, i.e. below 20 Hz, you can use the following method. Field-effect transistors (VT1, VT2 - Fig. 2) are used with large values of I s.nach. and increase the capacitance of the capacitor C4 to 4700 microfarads. Electrolytic capacitors operate much cleaner when a forward polarizing voltage of a few volts is applied to them. It is very convenient in this case to proceed as follows. An instance is installed in the upper (according to the scheme) transistor VT1 with an initial drain current greater than that of the transistor VT2. You can do it even more "efficiently" by using a balancing resistor for the transistor VT2, a fragment of the circuit with such a resistor is shown in Fig. 6. Initially, the engine of the tuning resistor R2 "is in the lower (according to the scheme) position, moving its engine upward causes an increase in the drain current of the transistor VT2, the potential on the positive plate of the capacitor C4 becomes more negative. The reverse process occurs with the opposite movement of the slider of the resistor R2. Thus, it is possible to adjust the cascade according to the most suitable modes, especially when there are no transistors (VT1 and VT2) with similar values of I s.nach. , and you have to install what is at hand ...

In some detail, I dwelled on such a seemingly very simple scheme. It is simple, but not primitive. It also has undeniable advantages over "all-transmitting" galvanically connected circuits of phase inverter amplifiers. The first such plus is the suppression of infra-low-frequency interference (for example, in EPU), the second is the "cut-off" of ultrasonic interference such as powerful radio stations, various ultrasonic installations, etc. And one more positive property of such a scheme should be emphasized. We are talking about the absence of any problems when docking excellent symmetrical circuits with an asymmetric input. It is worth looking at Fig. 5, and it immediately becomes clear (if a person has dealt with this!) that the problem of potentials here simply has not been solved in any way. It is partially solved by replacing the electrolytic capacitor with a battery of non-electrolytic connected in parallel, they say, the time delay in connecting the AC will solve everything. The time delay of connecting acoustic systems to the UMZCH really eliminates clicks and surges when turned on, but it cannot solve the issue of additional distortion due to different potentials and different output impedances of the phase inverter. This phase inverter amplifier circuit (Fig. 2) was successfully used with various UMZCH, including symmetrical lamp ones.

Recently, in periodicals, you can find UMZCH circuits on powerful KP901 and KP904. But the authors do not mention that field-effect transistors should be rejected at "leakage" currents. If, for example, VT1 and VT2 (in the circuit of Fig. 2) it is unambiguously necessary to use high-quality specimens, then in cascades with large amplitudes of voltages and currents, and most importantly, where the input resistance of the MOS transistor (its decrease) does not play a role, you can apply the worst instances. Once the maximum leakage values are reached, MOSFETs are generally stable in the future and do not degrade further over time (in most cases).

The number of transistors with increased leakage in the gate circuit, for example, in one pack (standard - 50 pieces) can vary from 10 to 20 pieces. (or even more). It is not difficult to reject powerful transistors - it is enough to assemble a kind of stand, for example, according to Fig. 6 and include a digital ammeter in the gate circuit (in this case, pointer instruments are too sensitive to overloads and inconvenient due to the need for multiple switching from range to range).

And now, when the phase inverter has already been made, you can proceed to the scheme of Fig. 1, i.e. return directly to UMZCH. Widespread connectors (sockets) SSH-3, SSH-5 and the like cannot be used at all, as many designers and manufacturers do. The contact resistance of such a connection is significant (0.01 - 0.1 Ohm!) And still fluctuates depending on the flowing current (resistance increases with increasing current!). Therefore, powerful connectors (for example, from old military radio equipment) with low contact resistance should be used. The same applies to the relay contacts in the AC protection unit against the possible appearance of a constant voltage at the UMZCH output. And it is not necessary to cover them (contact groups) with any feedback to reduce distortion. Take my word for it that by ear (subjective examination) they are practically inaudible (with sufficiently low contact resistances), which cannot be said about the "electronic" distortions introduced by all amplifying stages, capacitors and other components of the UMZCH, which certainly bring bright colors to the overall sound reproduction picture. All kinds of distortions can be minimized by the rational use of amplifying stages (this is especially true for voltage amplifiers - the fewer of them, the better the quality of the amplified signal). In this UMZCH, there is only one voltage amplification stage - this is the transistor VT3 (left shoulder) and VT4 (right shoulder). The cascade on transistors VT6 and VT5 is just matching (current) emitter followers. Transistors VT3 and VT4 are selected from h21 e more than 50, VT6 and VT5 - more than 150. In this case, there will be no problems when operating the UMZCH at high powers. The negative feedback voltage for direct and alternating current is supplied to the bases of transistors VT6 and VT5 through resistors R24 and R23. The depth of this OS is only about 20 dB, so there are no dynamic distortions in the UMZCH, but such an OS is quite enough to maintain the modes of the output transistors VT7 and VT8 within the required limits. UMZCH is sufficiently resistant to RF self-excitation. The simplicity of the circuit allows it to be quickly unmounted, since it allows independent power off (-40 V) for the driver and terminal transistors (2 x 38 V). The complete symmetry of the amplifier helps to reduce non-linear distortion and reduce sensitivity to supply voltage ripples, as well as additional suppression of common mode noise coming to both UMZCH inputs. The disadvantage of the amplifier is the significant dependence of nonlinear distortions on h21 e of the transistors used, but if the transistors have h21 out \u003d 70 W) is 1.7 V (effective value).

On transistors VT1 and VT2, a source (current generator) is made that feeds the differential cascade (driver). The value of this current 20 ... 25 mA is set by a trimming resistor R3 (470 Ohm). Since the quiescent current also depends on this current, then for the thermal stabilization of the latter, the transistor VT1 is placed on the heat sink of one of the transistors of the output stage (VT7 or VT8). The increase in the temperature of the heat sink of the output transistor, respectively, is transferred to the transistor VT1 placed on this heat sink, while the latter is heated, the negative potential decreases on the basis of the transistor VT2. This closes the transistor VT2, the current through it decreases, which corresponds to a decrease in the quiescent current of the output transistors VT7 and VT8. Thus, the stabilization of the quiescent current of the output transistors is carried out with significant heating of their heat sinks. Despite the apparent simplicity of implementing such thermal stabilization, it is quite effective and there were no problems in the reliability of the UMZCH. It is very convenient to control the currents of differential transistors (VT3 and VT4) by the voltage drop across resistors R7 and R15 or R21 and R26. Trimmer resistor R11 - balancing, serves to set the zero potential on the loudspeaker (at the UMZCH output).

The scheme of the loudspeaker protection unit (Fig. 7) is made according to the traditional scheme. Since the design for placing the UMZCH in separate buildings was chosen, then each UMZCH had its own speaker protection nodes. The speaker protection scheme is simple and reliable, this option has been tested in many designs for a long time and has proven itself to be good and reliable, more than once "saving" the life of expensive loudspeakers. Satisfactory operation of the circuit can be considered the operation of relay K1 when a constant voltage of 5 V is applied between points A and B. It is very easy to check this using an adjustable power supply (with a variable output voltage). In different designs, various types of relays were used, the voltage of the power supply of this node also changed within 30-50 V (for large values of this voltage, transistors VT1 and VT2 should be replaced with higher-voltage specimens, for example, KT503E, etc.)

Preference for use in the protection unit should be given to relay instances with the most high-current groups of contacts, with a large area of contact contact surfaces. But the RES-9 or RES-10 relays should not be used at all - at high output powers of the UMZCH, they begin to introduce their "unique" colors into the amplified signal. The AC protection unit is powered from a separate rectifier, and it is necessary to exclude any galvanic connections of this unit with the UMZCH, with the exception of only output voltage sensors - points A and B are connected to the UMZCH outputs.

Drivers of both channels can be powered from one common voltage regulator. In this case, both UMZCH channels are combined into one housing, and the power supplies are assembled in another housing. Naturally, there is a wide field of choice for each specific case, to whom what is more suitable in the design. A diagram of one of the stabilizer options for powering drivers is shown in Fig. 8. On the transistor VT1 assembled the current generator that feeds the transistor VT2, the required voltage at the output of the stabilizer is set by the tuning resistor R6. It should be emphasized that the maximum output power of the UMZCH primarily depends on the voltage of this stabilizer. But it is not recommended to increase the voltage above 50 V due to the possible failure of the transistors VT3 and VT4 of the driver. The total stabilization voltage of the zener diodes should be in the range of 27-33 V. The current through the zener diodes is selected by resistor R4. Resistor R1 is limiting (current), prevents failure of the regulating transistor VT2. The latter is quite likely in the process of establishing, while increasing the power supply of the driver will be able to disable the entire UMZCH. After setting up the UMZCH, the resistor R1 in the stabilizer can be closed with a piece of wire, or you can not do this, since the drivers consume a current of only a little more than 50 mA - the effect of the resistor R1 on the stabilizer parameters is insignificant at low load currents.

With a block design, you will have to completely separate the power supply of both UMZCH, including drivers. But in any case, a separate rectifier with its own winding in the transformer is required to power the driver. The rectifier circuit is shown in Fig.9. Each UMZCH channel uses its own power transformer. This design option has several advantages over the traditional use of a single transformer. The first thing that succeeds is to reduce the height of the unit as a whole, since the dimensions (height) of the mains transformer are significantly reduced with separate supply transformers for each UMZCH. Further, it is easier to wind, since the diameter of the winding wires can be reduced by 1.4 times without compromising the power of the UMZCH. In this regard, the network windings can also be turned on in antiphase to reduce network interference (this helps a lot to compensate for the radiation of transformer fields, especially when other amplifier circuits - tone blocks, volume control, etc.) are placed in the same housing with the UMZCH. Separation of the supply circuits of the output transistors UMZCH allows you to increase the quality of the reproduced signal, especially at low frequencies (transient distortion in the channels at low frequencies is also reduced). To reduce the level of intermodulation distortion caused by mains power, electrostatic shields are introduced into the transformers (one layer of wire wound coil by coil).

In all variants of UMZCH designs, toroidal magnetic cores for transformers are used. Winding was done manually using shuttles. You can also recommend a simplified version of the design of the power supply. To do this, use a factory LATR (a nine-amp copy is well suited). The primary winding, as the most difficult in the winding process, is already ready, you just need to wind the screen winding and all the secondary ones and the transformer will work fine. Its window is spacious enough to accommodate the windings for both UMZCH channels. In addition, in this case, drivers and phase inverter amplifiers can be powered from common stabilizers, "saving" two windings in this case. The disadvantage of such a transformer is a large height (except, of course, for the above circumstances).

Now for the details. It is not necessary to install low-frequency diodes (like D242 and the like) to power the UMZCH - distortion at high frequencies (from 10 kHz and above) will increase, in addition, ceramic capacitors were additionally introduced into the rectifier circuits to reduce intermodulation distortion caused by a change in the conductivity of the diodes in the moment of their switching. Thus, the influence of mains power on the UMZCH is reduced when it operates at high frequencies in the audio range. The situation is even better with the quality when shunting electrolytic capacitors in high-current rectifiers (UMZCH output stages) non-electrolytic. At the same time, by ear, both the first and second additions to the rectifier circuits were quite clearly perceived by the subjective examination - by hearing the operation of the UMZCH, its more natural operation was noted when reproducing several high-frequency components of different frequencies.

About transistors. It is not worth replacing transistors VT3 and VT4 with copies that are worse in terms of frequency properties (KT814, for example), the harmonic coefficient increases at the same time by at least two times (in the HF section and even more). By ear, this is very noticeable, the mid frequencies are reproduced unnaturally. In order to simplify the design of the UMZCH, composite transistors of the KT827A series were used in the output stage. And although they, in principle, are quite reliable, they still need to be checked for the maximum withstand (each instance has its own) collector-emitter voltage (meaning the forward voltage U ke max. for a closed transistor). To do this, the base of the transistor is connected to the emitter through a 100 ohm resistor and the voltage is gradually increased: plus to the collector, minus to the emitter. Instances that detect the flow of current (the ammeter limit is 100 μA) for Uke = 100 V are not suitable for this design. They may work, but not for long... Instances without such "leaks" work reliably for years without creating any problems. The scheme of the test stand is shown in Fig.10. Naturally, the parameters the KT827 series wants to be the best, especially in terms of their frequency properties. Therefore, they were replaced by "composite" transistors assembled on KT940 and KT872. It is only necessary to select KT872 with the largest possible h21e, since Ikmax is not large enough for KT940. This equivalent works just fine in the entire audio range, and especially at high frequencies. The circuit for switching on two transistors instead of one composite type KT827A is shown in Fig. 11. Transistor VT1 can be replaced with KT815G, and VT2 - with almost any powerful one (P to > 50 W and with U e > 30).

Resistors used types C2-13 (0.25 W), MLT. Capacitors of types K73-17, K50-35, etc. Setting up the assembled UMZCH correctly (without errors) consists in setting the quiescent current of the UMZCH output stage transistors - VT7 and VT8 within 40-70 mA. It is very convenient to control the value of the quiescent current by the voltage drop across the resistors R27 and R29. The quiescent current is set by resistor R3. A constant output voltage close to zero at the output of the UMZCH is set by a balancing resistor R11 (a potential difference of no more than 100 mV is achieved).

LITERATURE

Sukhov N.E. etc. Technique of high-quality sound reproduction - Kyiv, "Technique", 1985
Sukhov N.E. UMZCH high fidelity. - "Radio", 1989 - No. 6, No. 7.
Sukhov N.E. On the issue of assessing non-linear distortions UMZCH. - "Radio", No. 5. 1989.

To date, many UMZCH options have been developed with field-effect transistor output stages. The attractiveness of these transistors as powerful amplifying devices has been repeatedly noted by various authors. At audio frequencies, field-effect transistors (FETs) operate as current amplifiers, so the load on the pre-stages is negligible and the output stage on an IGFET can be directly connected to a pre-amplifier stage operating in a class A linear mode.
When using powerful FETs, the nature of non-linear distortions changes (less higher harmonics than when using bipolar transistors), dynamic distortions decrease, and the level of intermodulation distortions is significantly lower. However, due to the lower transconductance than bipolar transistors, the non-linear distortion of the source follower is large, since the slope depends on the level of the input signal.
The output stage on powerful FETs, where they withstand a short circuit in the load circuit, has the property of thermal stabilization. Some drawback of such a cascade is the lower utilization of the supply voltage, and therefore it is necessary to use a more efficient heat sink.
The main advantages of high-power FETs include the low order of nonlinearity of their pass-through characteristics, which brings together the sound features of FET and tube amplifiers, as well as a high power gain for signals in the audio frequency range.
Of the latest publications in the journal about UMZCH with powerful PTs, articles can be noted. The undoubted advantage of the amplifier from is the low level of distortion, and the disadvantage is low power (15 W). The amplifier has more power, sufficient for residential premises, and an acceptable level of distortion, but it seems to be relatively difficult to manufacture and configure. Hereinafter, we are talking about UMZCH, designed for use with household speakers with a power of up to 100 watts.
The UMZCH parameters, focused on compliance with the international IEC (IEC) recommendations, determine the minimum requirements for hi-fi category equipment. They are fully justified both from the psychophysiological side of human perception of distortions, and from the realistically achievable distortions of audio signals in acoustic systems (AS), for which the UMZCH actually works.
In accordance with the requirements of IEC 581-7 for speakers of the hi-fi category, the total harmonic distortion factor should not exceed 2% in the frequency range of 250 ... 1000 Hz and 1% in the range above 2 kHz at a sound pressure level of 90 dB at a distance of 1 m. characteristic sensitivity of household speakers, equal to 86 dB / W / m, this corresponds to an output power of the UMZCH of only 2.5 W. Taking into account the peak factor of musical programs, which is taken equal to three (as for Gaussian noise), the output power of the UMZCH should be about 20 watts. In a stereo system, the sound pressure at the bass is approximately doubled, which allows you to move the listener away from the speakers already by 2 m. At a distance of 3 m, the power of a stereo amplifier of 2 × 45 W is quite sufficient.
It has been repeatedly noted that distortions in UMZCH on field-effect transistors are mainly due to the second and third harmonics (as in serviceable speakers). If we assume that the causes of the occurrence of non-linear distortions in the speakers and UMZCH are independent, then the resulting harmonic coefficient for sound pressure is determined as the square root of the sum of the squares of the harmonic coefficients of the UMZCH and AC. In this case, if the total harmonic distortion factor in the UMZCH is three times lower than the distortion in the speaker (i.e., does not exceed 0.3%), then it can be neglected.
The range of effectively reproducible UMZCH frequencies should not be already audible to humans - 20 ... 20,000 Hz. As for the slew rate of the output voltage of the UMZCH, in accordance with the results obtained in the work of the author, a speed of 7 V / μs is sufficient for a power of 50 W when operating at a load of 4 ohms and 10 V / μs when operating at a load of 8 ohms.
The proposed UMZCH was based on an amplifier in which a high-speed op-amp with tracking power was used to “build up” the output stage in the form of composite repeaters on bipolar transistors. The tracking power was also used for the bias circuit of the output stage.

The following changes were made to the amplifier: the output stage based on complementary pairs of bipolar transistors was replaced by a stage with a quasi-complementary structure based on inexpensive FETs with an insulated gate IRFZ44 and the depth of the total SOS was limited to 18 dB. The circuit diagram of the amplifier is shown in fig. 1.

Op-amp KR544UD2A with high input impedance and increased speed was used as a preamplifier. It contains an input differential stage on a FET with a p-n junction and an output push-pull voltage follower. Internal frequency correction elements provide stability in various feedback modes, including in a voltage follower.
The input signal is fed through the RnC 1 low-pass filter with a cutoff frequency of about 70 kHz (here, the internal resistance of the signal source = 22 kΩ). which is used to limit the spectrum of the signal entering the input of the power amplifier. The R1C1 circuit ensures the stability of the UMZCH when the value of RM changes from zero to infinity. To the non-inverting input of the op-amp DA1, the signal passes through a high-pass filter built on the elements C2, R2 with a cutoff frequency of 0.7 Hz, which serves to separate the signal from the DC component. The local OOS for the operational amplifier is made on the elements R5, R3, NW and provides a gain equal to 43 dB.
The voltage stabilizer of the bipolar supply of the op amp DA1 is made on the elements R4, C4, VDI and R6, Sat. VD2, respectively. The stabilization voltage is chosen to be 16 V. Resistor R8, together with resistors R4, R6, form a UMZCH output voltage divider to supply “tracking” power to the op-amp, the range of which should not exceed the limit values of the common-mode input voltage of the op-amp, i.e. +/- 10 V "Tracking" power allows you to significantly increase the range of the output signal of the op-amp.
As you know, for the operation of a field-effect transistor with an insulated gate, in contrast to a bipolar one, a bias of about 4 V is required. To do this, in the circuit shown in fig. 1, for the transistor VT3, a signal level shift circuit was applied on the elements R10, R11 and UOZ.U04 to 4.5 V. The signal from the output of the op-amp through the circuit VD3VD4C8 and resistor R15 goes to the gate of the transistor VT3, the constant voltage on which relative to the common wire is +4, 5 V.
The electronic analogue of the zener diode on the elements VT1, VD5, VD6, Rl2o6ecne4H-vaet voltage shift of -1.5 V relative to the output of the op-amp to ensure the required operating mode of the transistor VT2. The signal from the output of the op-amp through the circuit VT1C9 also enters the base of the transistor VT2 connected according to the scheme with a common emitter, which inverts the signal.
On the elements of R17. VD7, C12, R18, an adjustable level shift circuit is assembled, which allows you to set the required offset for the VT4 transistor and thereby set the quiescent current of the final stage. Capacitor SU provides "tracking power" to the level shift circuit by supplying the output voltage of the UMZCH to the junction point of resistors R10, R11 to stabilize the current in this circuit. The connection of transistors VT2 and VT4 forms a virtual field-effect transistor with a p-type channel. i.e., a quasi-complementary pair is formed with the output transistor VT3 (with an n-type channel).
The C11R16 circuit increases the stability of the amplifier in the ultrasonic frequency range. Ceramic capacitors C13. C14. installed in close proximity to the output transistors serve the same purpose. UMZCH protection against overloads in case of short circuits in the load is provided by fuses FU1-FU3. since the IRFZ44 field effect transistors have a maximum drain current of 42 A and withstand overloads before the fuses blow.
To reduce the DC voltage at the output of the UMZCH, as well as to reduce non-linear distortion, a common OOS was introduced on the elements R7, C7. R3, NW. The AC OOS depth is limited to 18.8 dB, which stabilizes the harmonic coefficient in the audio frequency range. For direct current, the op-amp, together with the output transistors, operates in the voltage follower mode, providing a constant component of the UMZCH output voltage of no more than a few millivolts.