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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The AF power amplifier offered to the attention of readers, despite its simplicity, can be used in a household stereo complex. It has high thermal stability, operates from an unstabilized power source. The amplifier is easily repeatable, does not require the selection of transistors for the final and final stages.

The main technical characteristics of the amplifier: nominal input voltage - 0.7 V; input impedance - 50 kOhm; rated output power at a frequency of 1 kHz - 70 W at a load of 4 ohms and 35 W at a load of 8 ohms; the range of amplified frequencies with a frequency response unevenness of not more than 1 dB - 10 ... 30 Hz; harmonic coefficient at rated output power and in the specified frequency range - no more than 000%; relative noise level - no more than -0,1 dB; output impedance - 95 Ohm.

Schematic diagram of the amplifier is shown in the figure.

The voltage amplifier is assembled on a high-voltage op amp KR1408UD1 (DA1), the final stage is on transistors of different structures KT972A (VT1) and KT973A (VT2), and the final current amplifier is on transistors KT908A (VT3, VT4). The phase inverter functions are performed by a cascade on the transistor VT2. The amplifier is covered by the OOS circuit. The OOS voltage is removed from its output and fed through the divider R3R2 to the inverting input of the op-amp DA1. The output voltage is almost equal to the voltage at the output of the op-amp. The gain of the entire device is K= 1+R3/R2.

To eliminate distortions of the "step" type, a small bias voltage is applied to the bases of transistors VT1, VT2, which is created on the chain of diodes VD1-VD4.

The quiescent current of the device does not exceed 20 ... 30 mA, to stabilize it, the VD1-VD4 diodes must have thermal contact with the heat sinks of the output stage transistors. The R9C4 circuit prevents the amplifier from self-exciting at higher audio frequencies and when the load is turned off. For the same purpose, capacitors C2, C3 are installed. They must be placed near the DA1 chip.

When assembling the amplifier, fixed resistors MLT-0,125 (R1 - R4) and MLT-0.5 (R6 - R9) were used. All capacitors - KM-6. Diodes D220 can be replaced by any high-frequency. It is not recommended to use other microcircuits and transistors, as this will lead to a change in the parameters of the UMZCH. The terminal transistors are mounted on heat sinks with an area of ​​400 cm2.

Author: N.Rekunov

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