High-quality umzc on transistors. Powerful umzc on field-effect transistors

The design presented here is a ready-made high-power monophonic amplifier module with very good parameters. This amplifier is modeled on the engineer's popular design. The circuit has low harmonic distortion, which does not exceed 0.05%, with a load power of about 500 W. This amplifier is useful and necessary when organizing various street concert events and has already proved indispensable many times during these events. The big advantage of the system is its simple design and inexpensive output stage, consisting of 10 combined MOSFETs. The UMZCH can work with speakers with an impedance of 4 or 8 ohms. The only adjustment that needs to be made during startup is to set the quiescent current of the output transistors.

The article provides only a diagram and description of the operation of the power amplifier itself, but do not forget that the complete audio complex also contains other modules:

• UMZCH end
• Preamplifier
• power unit
• Level indicator
• Soft start system
• Cooling control system
• Speaker protection unit

Schematic diagram of ULF on transistors 500 watts

The power amplifier circuit is shown in the figure above. This is a classic circuit design consisting of a differential input amplifier and a symmetrical power amplifier, in which 5 pairs of transistors operate. Transistors T2 (MPSA42) and T3 (MPSA42) operate in a differential amplifier circuit powered through resistors R8 (10k) and R9 (10k). The voltage in the middle of this divider is stabilized using zener diode D2 (15V/1W) and filtered by capacitor C4 (100uF/100V). The input signal is fed to the GP1 (IN) connector and filtered through elements R1 (470R), R3 (22k), C1 (1uF) and C2 (1nF), which limit the frequency range of the amplifier both above and below.

The load of the differential amplifier is transistors T1 (MPSA42) and T4 (MPSA42), operating in a system with a common base, as well as resistors R5 (1.2 k) and R6 (1.2 k). The polarity of the load is determined by the zener diode D1 (15V/1W) and resistor R7 (10k). The main task of the system consisting of transistors T1 and T4 is to match the impedance of the output signal for the ULF stage. Another stage, built on transistors T5 (MJE350) and T6 (MJE350), acts as a differential voltage amplifier. It is powered through resistor R11 (100P/2W). Its load will be transistors T14 (MJE340) and T15 (MJE340), resistors R13 (100P/2W) and R14 (100P/2W), and transistor T7 (BD139).

Capacitor C15 (47nF), connected in parallel with resistor R44 (10k/2W), improves the passage of pulse signals, while small capacitors C7 (56pF) and C8 (56pF) counteract self-excitation of the UMZCH. Transistor T7 together with resistors R10 (4.7 k), R45 (82R) and potentiometer P1 (4.7 k) allows you to set the correct polarity of output transistors T9-T13 (IRFP240), T17-T21 (IRFP9240) at rest. Potentiometer P1 can be used to set the quiescent current, which should be about 100 mA for each pair of output transistors. Transistors T9-T13, like T17-T21, are connected in parallel and work as voltage followers for a large maximum output current. Therefore, the previous amplifier stages must provide all the voltage gain, which is determined by the ratio of R4 (22k) to R2 (470R) and is about 47.

Resistors R30-R39 (0.33 R/5W) included in the sources of the output transistors provide protection against damage that could occur in the case of different resistances of the transistor channels. Resistors R20-P29 (470R), connected in series with the outputs of transistors T9-T13, T17-T21, serve to reduce the charging rate of the capacitor and, therefore, limit the frequency range of the amplifier.

The amplifier has two simple protections:

1. The first is aimed against overload and is implemented using zener diodes D3 (7.5 V/1W) and D4 (7.5 V/1W), which do not allow the voltage between the sources and outputs of powerful transistors to rise above 7.5 volts.
2. The second protection is built using transistors T7, T16 and (BD136), resistors R16-R17 (33k) and R18-R19 (1k) and diodes D7-D10 (1N4148). It prevents the power transistor current from increasing excessively, which could lead to exceeding the permissible power. A section of the circuit consisting of transistors T7, T16 monitors the voltage drop across R30 (0.33 R/5W) and R35 (0.33 R/5W) and limits the increase in voltage of powerful transistors if the permissible current passing through is exceeded.

The power supply is not stabilized, bipolar, consisting of a diode bridge Br1 (25A) and capacitors C9-C14 (10000uF/100V). The amplifier's power supply is protected by fuses F1-F2 (10A). Behind the fuses, the voltage is additionally filtered by capacitors C18-C19 (1000uF/100V). The power supply of the input circuits is separated from the power supply of the power amplifier using diodes D5-D6 (1N4009), resistors R12 (100P/2W), R15 (100P/2W) and filtered by capacitors C3 (100uF/100V) and C6 (100uF/100V). This prevents voltage surges that can occur during power peaks under heavy loads. LEDs D11-D12 together with their current-limiting resistors R40-R41 (16K/1W) are indicators of the presence of power in the circuit.

power unit

The figure below shows a diagram of the power supply - a source of several auxiliary voltages. It is not required to operate the power amplifier itself, but is very useful for powering the rest of the complete audio complex, such as the preamplifier, fans, level indicator, soft start system or speaker protection. All these modules are integrated into one common amplifier in a large housing.

The power supply is divided into several separate sections, each of which has its own separate ground circuit. The first section is a symmetrical 2x15V power supply, it is used to power the preamplifier. Connector A4 is used to connect a bipolar transformer winding. The voltage is rectified using bridge rectifier Br2 (1 A) and filtered by stabilizers U2 (LM317), U6 (LM337) using C1 (100nF), C7 (100nF) and C24-C25 (4700uF). The output filter is capacitors C8-C9 (100nF) and C19-C20 (100uF). The output voltage of this block is set using resistors R2-R3 (220R) and R9-R10 (2.4 k). Transistors T1 (BC546), T2 (BC556); resistors R4-R5 (10k) and R7-R8 (3.3 k) represent a power cut-off circuit, or rather, they reduce the supply voltage to 2 × 1.25 V, which will allow the preamplifier to be turned off. During normal operation, shorting the GP8 connector will ensure proper operation of the preamplifier.

The next two modules are 12 V power supplies, assembled using stabilizers U4 (7812) and U5 (7812) and designed to power other circuit elements. Two separate sources are necessary because the amplifier has two pairs of level meters, each on a separate ground. One pair works at the input, monitoring the input signal level, and the second pair is connected to the output and allows you to determine the current power level of the UMZCH.

Both power supplies are very simple, the first consists of a diode bridge Br3 (1A), filter capacitors C5-C6 (100nF), C18 (100uF) and C22 (1000uF) and a stabilizer U4. The transformer windings must be connected to connector A2, and the output of the power supply will be connectors GP6 and GP7.

The second 12V channel works exactly the same, and consists of elements: Br4 (1A), C10-C11 (100nF), C23 (1000uF), C21 (100uF) and U5.

The last module of the power supply system is the power supply circuits for other amplifier devices and cooling fans. A transformer should be connected to connector A1. The voltage is rectified using bridge rectifier Br1 (5A) and filtered by capacitors C27 (4700uF), C12 (4700uF) and C2 (100nF). The U1 microcircuit (LM317) works here as a stabilizer, which sets the required voltage using resistors R1 (220R) and R6 (2.7 k).

Capacitors C3 (100nF) and C16 (100uF) filter the voltage at the output of the stabilizer, which enters the fan control system through connectors GP1 and GP2. The same voltage is supplied through diode D1 (1N5819) to stabilizer U3 (7812), whose task is to provide power to other amplifier devices connected to connectors GP3-GP5. Capacitors C28 (4700uF), C13 (4700uF), C4 (100nF) and C17 (100uF) filter the voltage before the stabilizer.

Hi all! In this article I will describe in detail how to make a cool amplifier for your home or car. The amplifier is easy to assemble and configure, and has good sound quality. Below is a schematic diagram of the amplifier itself.

Resistors R7, R8, R10, R11, R14 - 0.5 W; R12, R13 - 5 W; the rest 0.25 W.
R15 trimmer 2-3 kOhm.
Transistors: Vt1, Vt2, Vt3, Vt5 - 2sc945 (usually c945 is written on the case).
Vt4, Vt7 - BD140 (Vt4 can be replaced with our Kt814).
Vt6 - BD139.
Vt8 - 2SA1943.
Vt9 - 2SC5200.

ATTENTION! Transistors c945 have different pinouts: ECB and EBC. Therefore, before soldering you need to check with a multimeter.
The LED is ordinary, green, exactly GREEN! He's not here for beauty! And it should NOT be super bright. Well, the rest of the details can be seen in the diagram.

And so, let's go!

To make an amplifier we need tools:
-soldering iron
-tin
-rosin (preferably liquid), but you can get by with regular
- metal scissors
- wire cutters
-awl
-medical syringe, any
- drill 0.8-1 mm
- drill 1.5 mm
-drill (preferably some mini drill)
-sandpaper
- and a multimeter.

Materials:
-one-sided textolite board measuring 10x6 cm
-sheet of notebook paper
-pen
-wood varnish (preferably dark color)
-small container
-baking soda
-citric acid
-salt.

I will not list the radio components; they can be seen in the diagram.
Step 1 Preparing the board
And so, we need to make a board. Since I don’t have a laser printer (not at all), we will make the board “the old fashioned way”!
First you need to drill holes on the board for future parts. If you have a printer, just print this picture:

And so: turn on the amplifier, the LED should be on, measure the output voltage with a multimeter. There is no permanent situation, which means everything is fine.
Next, you need to set the quiescent current (75-90mA): to do this, short-circuit the input to ground, do not connect the load! Set the multimeter to 200mV mode and connect the probes to the collectors of the output transistors. (marked with red dots in the photo)

In this article, we present to you for self-assembly a power amplifier circuit produced by APEX Microtechnology, created in the early eighties in the United States by two smart engineers who left the Burr-Brown company. Currently, APEX has achieved a leading position in the production of very powerful operational amplifiers and hybrid circuits, which are characterized by super high reliability and stability relative to similar discrete components. Here it is important to explain that this enterprise has received quality standard certification, and the products it produces are according to the standard of the American military department MIL-PRF class H, this means a high degree of reliability of the technological solutions used in the production of APEX products. The presented model of the power amplifier is called APEX B500, as can be seen from the abbreviation, the manufacturer included in the name of the device a figure characterizing the output power of the amplifier. 500 W output, the model develops this power with a load resistance in the speakers of at least 2 Ohms.

Ready-made printed circuit boards

Schematic diagram of amplifier protection

High-quality and effective protection circuit against direct voltage in the final stage of the amplifier, short circuit, and temperature component on the output transistors of the amplifier with a built-in thermostat for the rotation speed of the cooling fan.

Printed circuit board protection circuit

Photos of finished amplifiers

Amplifier APEX H900

If half a kilowatt of output power is not enough for anyone, then these radio amateurs are invited to assemble an amplifier of the APEX N900 series operating in class H and with an output power of 900 W. The APEX B500 device you previously made will not be difficult to upgrade into an amplifier with a higher output power. To provide the B500 with a higher supply voltage, consisting of two stages, you will need to design a simple power driver board, which will do the job of supplying the main power circuit with two values, that is, two levels.

And this is the circuit diagram of the voltage driver itself

This is the power driver PCB

This diagram shows how to connect the driver to the amplifier

A long time ago, two years ago, I purchased an old Soviet speaker 35GD-1. Despite its initial poor condition, I restored it, painted it a beautiful blue and even made a box for it out of plywood. A large box with two bass reflexes greatly improved its acoustic qualities. The only thing left is a good amplifier that will drive this speaker. I decided to do something different from what most people do - buy a ready-made D-class amplifier from China and install it. I decided to make an amplifier myself, but not some generally accepted one on the TDA7294 chip, and not on a chip at all, and not even the legendary Lanzar, but a very rare amplifier on field-effect transistors. And there is very little information on the Internet about field amplifiers, so I became interested in what it is and how it sounds.

Assembly

This amplifier has 4 pairs of output transistors. 1 pair – 100 Watt of output power, 2 pairs – 200 Watt, 3 – 300 Watt and 4, respectively, 400 Watt. I don’t need all 400 watts yet, but I decided to install all 4 pairs in order to distribute the heating and reduce the power dissipated by each transistor.

The diagram looks like this:

The diagram shows exactly the values ​​of the components that I have installed, the diagram has been tested and works properly. I am attaching the printed circuit board. Lay6 format board.

Attention! All power paths must be tinned with a thick layer of solder, since a very large current will flow through them. We solder carefully, without snot, and wash off the flux. Power transistors must be installed on the heat sink. The advantage of this design is that the transistors do not need to be isolated from the radiator, but can be molded together. Agree, this saves a lot on mica heat-conducting spacers, because it would take 8 of them for 8 transistors (surprisingly, but true)! The heatsink is the common drain of all 8 transistors and the audio output of the amplifier, so when installing it in the case, do not forget to somehow isolate it from the case. Despite the fact that there is no need to install mica gaskets between the transistor flanges and the radiator, this place must be coated with thermal paste.

Attention! It’s better to check everything right away before installing the transistors on the radiator. If you screw the transistors to the heatsink, and there are any snot or unsoldered contacts on the board, it will be unpleasant to unscrew the transistors again and get smeared with thermal paste. So check everything at once.

Bipolar transistors: T1 – BD139, T2 – BD140. It also needs to be screwed to the radiator. They don't get very hot, but they still get hot. They also may not be isolated from heat sinks.

So, let's proceed directly to the assembly. The parts are located on the board as follows:

Now I am attaching photos of the different stages of assembling the amplifier. First, cut out a piece of PCB to fit the size of the board.

Then we put the image of the board on the PCB and drill holes for the radio components. Sand and degrease. We take a permanent marker, stock up on a fair amount of patience and draw paths (I don’t know how to do LUT, so I’m struggling).

We arm ourselves with a soldering iron, take flux, solder and tin.

We wash off the remaining flux, take a multimeter and check for short circuits between tracks where there should not be one. If everything is normal, we proceed to installing the parts.
Possible replacements.
First of all I will attach a list of parts:
C1 = 1u
C2, C3 = 820p
C4, C5 = 470u
C6, C7 = 1u
C8, C9 = 1000u
C10, C11 = 220n

D1, D2 = 15V
D3, D4 = 1N4148

OP1 = KR54UD1A

R1, R32 = 47k
R2 = 1k
R3 = 2k
R4 = 2k
R5 = 5k
R6, R7 = 33
R8, R9 = 820
R10-R17 = 39
R18, R19 = 220
R20, R21 = 22k
R22, R23 = 2.7k
R24-R31 = 0.22

T1 = BD139
T2 = BD140
T3 = IRFP9240
T4 = IRFP240
T5 = IRFP9240
T6 = IRFP240
T7 = IRFP9240
T8 = IRFP240
T9 = IRFP9240
T10 = IRFP240

The first thing you can do is replace the operational amplifier with any other one, even imported, with a similar pin arrangement. Capacitor C3 is needed to suppress the self-excitation of the amplifier. You can put more, which is what I did later. Any 15 V zener diodes with a power of 1 W or more. Resistors R22, R23 can be installed based on the calculation R=(Upit.-15)/Ist., where Upit. – supply voltage, Ist. – stabilization current of the zener diode. Resistors R2, R32 are responsible for the gain. With these ratings, it is somewhere around 30 - 33. Capacitors C8, C9 - filter capacitances - can be set from 560 to 2200 µF with a voltage not lower than Upit. * 1.2 so as not to operate them at their maximum capabilities. Transistors T1, T2 - any complementary pair of medium power, with a current of 1 A, for example our KT814-815, KT816-817 or imported BD136-135, BD138-137, 2SC4793-2SA1837. Source resistors R24-R31 can be set to 2 W, although it is undesirable, with a resistance from 0.1 to 0.33 ohms. It is not advisable to change power switches, although IRF640-IRF9640 or IRF630-IRF9630 are also possible; it is possible to use transistors with similar passing currents, gate capacitances and, of course, the same pin arrangement, although if you solder on wires, this does not matter. There seems to be nothing more to change here.

First launch and setup.

The first start-up of the amplifier is carried out through a safety lamp into a 220 V network break. Be sure to short-circuit the input to ground and do not connect the load. At the moment of switching on, the lamp should flash and go out, and go out completely: the spiral should not glow at all. Turn it on, hold it for 20 seconds, then turn it off. We check to see if anything is heating up (although if the lamp is not on, it is unlikely that anything is heating up). If nothing really heats up, turn it on again and measure the constant voltage at the output: it should be in the range of 50 - 70 mV. For example, I have 61.5 mV. If everything is within normal limits, connect the load, apply a signal to the input and listen to music. There should be no interference, extraneous hums, etc. If none of this is present, proceed to setup.

Setting up this whole thing is extremely simple. It is only necessary to set the quiescent current of the output transistors by rotating the trimmer resistor slider. It should be approximately 60 - 70 mA for each transistor. This is done in the same way as on Lanzar. The quiescent current is calculated using the formula I = Up./R, where Up. is the voltage drop across one of the resistors R24 - R31, and R is the resistance of this resistor. From this formula we derive the voltage drop across the resistor required to set such a quiescent current. Upd. = I*R. For example, in my case it = 0.07*0.22 = somewhere around 15 mV. The quiescent current is set on a “warm” amplifier, that is, the radiator must be warm, the amplifier must play for several minutes. The amplifier has warmed up, turn off the load, short-circuit the input to common, take a multimeter and carry out the previously described operation.

Characteristics and features:

Supply voltage – 30-80 V
Operating temperature – up to 100-120 degrees.
Load resistance – 2-8 Ohm
Amplifier power – 400 W/4 Ohm
SOI – 0.02-0.04% at a power of 350-380 W
Gain factor – 30-33
Reproducible frequency range – 5-100000 Hz

The last point is worth dwelling on in more detail. Using this amplifier with noisy tone blocks such as the TDA1524 may result in seemingly unreasonable power consumption by the amplifier. In fact, this amplifier reproduces interference frequencies that are inaudible to our ears. It may seem that this is self-excitation, but most likely it is just interference. Here it is worth distinguishing between interference that is not audible to the ear and real self-excitation. I encountered this problem myself. Initially, the TL071 opamp was used as a preamplifier. This is a very good high-frequency imported op-amp with a low-noise output using field-effect transistors. It can operate at frequencies up to 4 MHz - this is sufficient for reproducing interference frequencies and for self-excitation. What to do? One good person, many thanks to him, advised me to replace the opamp with another one, less sensitive and reproducing a smaller frequency range, which simply cannot operate at the self-excitation frequency. So I bought our domestic KR544UD1A, installed it and... nothing has changed. All this gave me the idea that the variable resistors of the tone unit were making noise. The resistor motors rustle a little, which causes interference. I removed the tone block and the noise disappeared. So it's not self-stimulation. With this amplifier you need to install a low-noise passive tone block and a transistor preamplifier in order to avoid the above.

S. SAKEVICH, Lugansk
Radio, 2000, No. 11, 12

The described amplifier is designed for two-channel amplification of the signal power supplied from a mixing console or pre-amplifier. Each of the two inputs has an input signal level control that allows you to set the required sensitivity. A switch can be used to combine its inputs, and one of the two input connectors can be used as a line output to increase the number of amplifiers operating in parallel. Features of the UMZCH include a switchable speaker damping factor to optimize their sound in different acoustic conditions.

Main technical characteristics

Nominal input voltage. B................1.1
Rated output power of each of the two channels, W,
at Kg = 1% and load resistance
4 0m......400
8 0m...................220
Operating frequency range, Hz, with unevenness -0.5 dB...............20...20000
Output signal slew rate, V/µs........25
Signal harmonic distortion coefficient with a level of 1 dB, %, no more
at a frequency of 1 kHz.........0.01
in the operating frequency range...0.1
Signal/noise+background ratio, dB..........96
Maximum permissible voltage deviation in the network, V................170...270
Minimum load resistance. Ohm............2.5
Overall dimensions, mm........................430х90х482
Weight, kg, no more...............16

The amplifier has indicators of the output signal level and its limitations, output overload, as well as indicators for emergency shutdown of loudspeakers and overvoltage.

In Fig. Figure 1 shows a diagram of the right channel of the amplifier and the load protection unit.

The KR544UD2A OU is used at the UMZCH input. and circuits C4R4 and R1C3 limit the band of amplified frequencies. They reduce the penetration of infra- and ultrasonic frequency vibrations into the PA, which can lead to overload of the amplifier and dynamic heads. The voltage amplifier on VT1 - VT4 is similar to that used in. The output of the op-amp is connected to the emitter follower VT3, which, together with the circuit R6C15, performs the functions of a voltage-to-current converter. This current flows through the cascade from the OB to VT2 to the voltage amplifier at VT1.

Further, the structure of the amplifier is almost symmetrical: the load of transistor VT1 is the current generator on VT4, the input circuit of the subsequent cascade of current amplifiers, as well as resistor R12, which stabilizes the load resistance for VT1. This was done in order to slightly reduce the overall gain and increase the stability of the amplifier with a closed feedback loop. The subsequent current amplifier is made in three stages: VT5, VT10. further - VT11, VT17 and then VT12 - VT16, VT18 - VT22 (each arm has five parallel-connected transistors).

The short circuit (short circuit) protection unit in the load is made using transistors VT6, VT7 and VT8. VT9. connected according to a thyristor analog circuit, for the upper and lower arms, respectively. When turned off, this node has no effect on the output stage. When conditions arise for the protection to operate, the transistors of the corresponding arm of the output stage are completely closed. Thus, the current consumption of the PA during a short circuit and the rated input voltage will be even less than in idle mode, therefore, during a short circuit at the output, the power amplifier does not fail.

Resistor R14 is necessary for the correct operation of short-circuit protection. For example, when the upper arm of the circuit is overloaded, transistors VT6 open. VT7 and the residual voltage at the base of VT5 relative to the output does not exceed 0.8 V. If this resistor is not present, then the bias voltage on the diodes (approximately 2.6 V) will lead to an increase in the bias voltage for the lower arm of the output stage and its triggering.

Unlike other protection devices that turn off output transistors, the proposed unit automatically returns to its original state when the load with a resistance of 2.5...16 Ohms is restored and a useful signal is supplied to the input of the amplifier with a level of 25% of the nominal or higher. Circuits R18C13 and R19C14 eliminate the possibility of false operation of the protection due to a phase shift of the current in the load due to its reactive nature.

To enlarge, click on the image (opens in a new window)

In the output stage, the transistors of the pre-final stage operate in AB mode with a quiescent current of about 100 mA, determined by the bias voltage on the diodes VD9-VD12 and resistors R24, R35. Their relatively low resistance allows this stage to operate in small signal mode directly to the load and reduces the discharge time of the capacitance SBE of the terminal stage transistors, reducing its switching distortions. These transistors operate in mode B, so they do not require thermal compensation circuits or quiescent current regulation.

The indicator for limiting the output signal and short circuit at the output is powered by pulses of negative polarity at the output of the op-amp DA1, which arise as a result of a break in the OS loop when the output signal is limited or the protection unit is triggered.

The device for delaying the connection of the load and disconnecting it when a constant voltage appears at the output of the amplifiers is common to both channels. When the power is turned on, capacitor C19 is charged through resistor R49. providing a delay in the opening of transistors VT25, VT27 and the activation of relay K1 by 2 s. When a constant voltage appears at the output of one of the amplifiers, with positive polarity, transistor VT23 will open, and in the case of negative polarity, VT24 will open, locking transistors VT25, VT27 and turning off the relay.

The loudspeakers are turned off by the protection unit and when the voltage in the network increases above 250 V (VT26. VD17-VT19. R51-R53). As practice shows, exceeding the supply voltage happens much more often than one might expect. When the supply voltage of the protection unit increases, the current flowing through the zener diodes VD17-VD19 opens the transistor VT26, as a result, the indication of excess network voltage turns on and the transistor VT23 opens, which leads to disconnection of the load. Continued operation is possible after moving the mains voltage switch to the “250 V” position.

The diagram of the power supply, display unit and interconnections of both channels is shown in Fig. 2. The numbering of the interconnect connections of the PA board and the AC protection board, as well as the indicator board, corresponds to the numbering of the pins of the contact pads in the corresponding drawings of the placement of elements on printed circuit boards. Each of the two inputs of the amplifier has an input signal level regulator (variable resistors R1, R2), which allows you to set the required sensitivity. Push-button switch SB1 can combine its inputs.

In UMZCH it is possible to switch the degree of damping of loudspeakers used in different acoustic conditions. When the amplifier is switched to high output impedance mode (switch button SB2 "Out. N/V" is pressed), the output impedance of the amplifier increases to 8... 10 Ohms due to the introduction of current feedback into the amplifier from resistors R3, R4. This. as practice shows, this is the optimal value for most loudspeakers. However, it can be easily changed in any direction by selecting resistor R2 on the amplifier board.

Note that the mode of increased output impedance significantly increases the reliability of the speakers. The fact is that increasing the output impedance of the amplifier helps to reduce active losses in the loudspeaker, which allows you to more fully use its capabilities and, in addition, significantly reduce intermodulation distortion. The high output impedance mode also reduces the phase shift of the current in the output stage relative to the input signal.

The amplifier is equipped with indicators for monitoring the operating mode. These are indicators for turning on the power supply (HL9), emergency shutdown of the loudspeakers (HL7) and the HL8 indicator. indicating forced shutdown of the load due to a dangerous excess of supply voltage. Signal strength indicators HL2 and HL3. HL5 and HL6 have threshold values ​​of 5, 20 dB, and also show its limitation (LEDs HL1, HL4) for each channel separately. In addition to the limitation, the same indicators signal a short circuit at the output of any channel (if the other level indicators are not illuminated).

The amplifier's power supply is simplified as much as possible. The UMZCH itself is powered from a rectifier with a voltage of 70 V; the protection and indication unit uses its own rectifier, connected to a separate winding of the power transformer. Fans Ml, M2 are designed for blowing heat sinks of powerful transistors.

Apparently, the purpose of the SB5 switch also requires explanation: in a sound reinforcement system it is installed in a position in which the minimum background noise from power supply interference is achieved.

Construction and details

The appearance of the amplifier is shown in Fig. 3 (from the rear panel). Its main components are placed on a metal chassis with a lid. On the front panel with slotted holes there are fans for forced ventilation of the heat sinks of the amplifier's powerful transistors, as well as an operating mode indication board. The rear panel has connectors for connecting signal cables and a three-wire power cable, switches for the mains voltage limit and speaker damping factor, and a fuse holder.

The amplifier is mounted mainly on three boards - the amplifier board, the indication board and the power rectifier board. On the amplifier board there are two PA channels with heat sinks for output transistors and a speaker protection unit. The printed circuit board (its dimensions are 355x263 mm) and the arrangement of elements that are usually depicted in life-size in the magazine are shown in Fig. 4 (p. 40,41) on a scale of 85%.

To enlarge, click on the image (opens in a new window)

In the load protection unit, you can use relay RP21, which has four groups of contacts (two in parallel), or REK34 or similar with an operating voltage of 24 V. “Radiators” of type P1, produced by Vinnytsia PA “Mayak” (TU 8.650.) are used as heat sinks. 022) with milled platforms for installing two powerful transistors (KT8101A or KT8102A) each.

The heat sinks are cooled using exhaust ventilation by two VVF71 fans. installed behind the front panel of the amplifier. It is highly undesirable to install them on the rear panel due to the high level of interference from their motors.

The design of the board also allows the use of homemade heat sinks for six transistors (for each arm) with a heat-dissipating surface of at least 600 cm and forced cooling. The amplifier board is placed in the body of the amplifier itself like this. that the signal inputs and outputs of both channels are located on the rear panel.

As already indicated, the amplifier has a switchable damping factor, implemented by turning on the OO loop. Resistors R3. R4 in Fig. 2 - load current sensors used to change the damping factor are made of ten MLT-0.5 resistors connected in parallel with a resistance of 1 Ohm. The use of wirewound resistors is undesirable.

Choke L1 (see Fig. 1) is wound directly on resistor R55 MLT-2 with PEV-2 0.8 mm wire in one layer (before filling). Blocking capacitors - K73-11. in the power filter - K50-18. The power transformer is made on a strip magnetic core type ШЛ40Х45 mm. Its winding data is given in the table.

The output stage transistors KT8101A and KT8102A must be selected according to the gain - no less than 25 and no more than 60, and most importantly - according to the maximum voltage and ^ to determine this parameter it is necessary to assemble a simple device consisting of an alternating voltage rectifier up to 300...350 V, a resistor with a resistance of 24...40 kOhm (power 2 W) and a voltmeter with a limit of 500 V (Fig. 5). A transistor with closed base and emitter terminals is connected through a current-limiting resistor to the source. A voltmeter connected in parallel to the transistor records the avalanche breakdown voltage of the transistor being tested, which will be its limit. Transistors should be selected with a breakdown voltage of at least 250 V. Ignoring this requirement may lead to failure of the amplifier during operation.

The power rectifier board (shown in Fig. 6 on a scale of 1:2) is installed on the terminals of the rectifier filter capacitors and secured with the appropriate screws.

To enlarge, click on the image (opens in a new window)

Installation of the common wire and power circuits is carried out using stranded wire with a cross-section of 1.2 mm2. In addition, the installation of the common wire from the rectifiers to the amplifier board and the load disconnection unit is carried out using separate wires that are as short as possible.

In Fig. Figure 7 shows a drawing of the printed circuit board of indicators and the location of the elements. The LEDs are installed so that their ends protrude slightly on the surface of the front panel of the amplifier.

TURN ON AND SETUP

To configure the amplifier you will need an oscilloscope and a 3H generator. LATR autotransformer for voltage 0 - 250 V at load current up to 2 A and resistive load equivalents. The amplifier is connected to the output terminals of the autotransformer through an auxiliary cable, which makes it possible to connect an AC voltmeter and ammeter to the power circuit.

First, you should set the mains voltage switch to the “220 V” position and check the operation of the power supply, then the operation of the load protection unit by applying a constant voltage of 2...3 V (alternately of different polarities) to the left terminal of resistors R47 or R48 according to the diagram. After making sure that the unit is working, you need to set the load disconnection threshold using an adjusted resistor R52 when the network voltage increases to 250 V and higher.

The next stage is the most crucial. Having connected one of the amplifier channels via ±70 V circuits (mains power must be supplied through a fuse with a maximum current of no more than 1 A) and monitoring the current consumption with an ammeter and the output signal with an oscilloscope, you need to very slowly increase the supply voltage from the autotransformer from zero to nominal. The current consumption of the output stage should not exceed 250 mA, otherwise, immediately turn off the power and carefully check the installation.

Initially, a constant voltage of positive polarity will appear at the output of the amplifier. When its value reaches approximately half of the rated supply voltage, the output voltage will abruptly be close to zero due to the activation of the OOS action. The voltage drop across resistors R24 and R25 should be 200...250 mV, which corresponds to the quiescent current of transistors VT11, VT17 within 60...85 mA. If necessary, diodes VD9-VD12 are selected or one of VD9 - VD11 is replaced with germanium.

After this, check the operation of the UMZCH without a load from the 3CH generator. Having set the frequency to 1...2 kHz, smoothly increase the signal at the amplifier input and make sure that it is correct. that the amplitude of its output voltage is at least 50 V. The overload indicator should light up when the output signal begins to be limited. Next, having replaced the fuse with another (for a current of 5 - 7 A), use an oscilloscope to observe the operation of the amplifier under a load on a powerful resistor with a resistance of first 8 and then 4 Ohms. The amplitude of the unlimited signal must be at least 46 and 42 V, respectively. Possible excitation at HF ​​in some cases is eliminated by selecting capacitors C9, SY. C15, and when replacing powerful transistors - C11, C12.

Checking operation in the mode of increased output resistance should be done with a load with a resistance of 4 ohms: it is with such a load that the signal from the current sensor is approximately equal to the input and no noticeable change in the gain occurs. If, after turning on this mode, self-excitation is detected, you need to increase the capacitance of the phase correction capacitor C10 in the OOS circuit.

Next, you need to make sure that the short circuit protection unit in the load circuit is working (this test is best carried out in low output resistance mode). To do this, first, under a load with a resistance of 8 Ohms and an output voltage swing of 20...30 V, jumper the bases VT6, VT7. and then VT8, VT9. In this case, the positive and negative half-waves should be “cut off” on the output signal oscillogram, respectively.

After this procedure, you need to check the response of the amplifier to a load with a resistance of 0.33 Ohm and a power of 3 - 6 W, simulating a short circuit. Remove the input signal, connect an ammeter to the power circuit of one of the arms, and a voltmeter to the output. With this load connected to the output, slowly increase the input voltage while monitoring the output voltage, current consumption, and waveform. At an output voltage level of 2.1...2.3 V, the protection for one arm (usually the upper one in the circuit, the signal shape is shown in Fig. 8,a) should be triggered; with a further increase in voltage, the protection for the other arm should be triggered (Fig. 8.6 ). The current consumption should drop to 160...200 mA. After this, checking the operation of the UMZCH can be considered complete.

The transistors in the final stage of the amplifier's output stage operate with virtually no initial bias. Converting them to class AB mode makes it possible to reduce nonlinear distortions at high frequencies by approximately 6...8 times. The simplest version of the displacement unit is shown in Fig. 9. It is turned on instead of four bias diodes, point “A” - to the collector VT1. point "B" - to the VT4 collector. Resistor R12 is also excluded in this case. The temperature sensor (transistor VT28) is installed on the heat sink as close as possible to the powerful transistor of the output stage, which is in the worst cooling conditions. When using this unit, it is necessary to increase the resistance of resistors R24, R35 to 12 - 15 Ohms.

The quiescent current adjustment is as follows. First, the motor of the variable resistor R58 is brought to the top position in the diagram. Once the power is supplied, the quiescent current is set to 150...180 mA. After this, with the load connected and the rated output voltage, the amplifier is warmed up for 10...15 minutes. The quiescent current is measured again. If it is lower than the original one, you need to slightly increase the resistance R60 in the VT28 emitter circuit and repeat the adjustment procedure until you obtain approximately the same quiescent current in cold and hot states. The disadvantages of this unit are the presence of a tuning resistor and the large inertia of the environmental protection thermal circuit.

The device for automatic regulation of the quiescent current according to the circuit shown in Fig. is free from these shortcomings. 10. The principle of its operation is to measure the voltage drop across resistors R63, R64 - quiescent current sensors of the output transistors, with subsequent control of the current of the optocoupler transistors U1, connected instead of biasing diodes. With a sufficiently large signal, transistors VT29 and VT30 operate almost alternately: when one of the nicks is in a saturation state, the other is in an active state, controlling the optocoupler and the quiescent current. And vice versa. The unit does not require settings, however, it is possible to correct the quiescent current by selecting resistor R58. After turning on the power, the UMZCH quiescent current is zero for 8...10 s, and then gradually increases to normal. In an amplifier with automatic regulation of the quiescent current, the resistance of resistors R24, R35 can be increased to 12-15 Ohms.

It is possible to introduce smooth adjustment of the output impedance in the amplifier. To do this, it is enough to replace the damping switch SB2 with a dual variable resistor with a resistance of 2...4 kOhm and reduce the resistance R2 to 100 Ohms to expand the range of adjustment of the output resistance (increasing).

Power transistors of the output stage can be replaced with 2SC3281 and 2SA1302. 2SA1216 and 2SC2922, 2SA1294 and 2SC3263 (in this case it is not necessary to select transistors). KT940A and KT9P5A can be replaced with KT851 and KT850 with any letter index.

LITERATURE
1. Kletsov V. Low-frequency amplifier with low distortion. - Radio, 1983. No. 7. p. 51-53.
2. Sukhov N. UMZCH of high fidelity. - Radio. 1989. No. 6. p. 55 - 57.
3. Zuev P. Amplifier with multi-loop feedback. - Radio. 1984. No. 11. p. 29-32.
4. Ageev S. Should the UMZCH have a low output resistance? - Radio. 1997, no. 4, p. 14-16.