Two-stage amplifier. The first stage is built on IC op amp type TL071, for stereo can use a dual op amp TL072 with any letter indices. Cascade provides amplification of the signal voltage in R7/R65 time (~ 26 dB).
The output stage is built on a pair of complementary field effect transistors Q1 and Q2. The transistors are mounted on heat sinks, an area of not less than 700 square centimeters each. In principle, changing the output circuitry, the cascade can be applied to the same type of transistors (or even change the upper arm, and so far …). Cascade Q2 – source follower, collected in Q1 current generator, the magnitude of the quiescent current is given by the resistor R41. Load Impedance (speaker) is indicated on the diagram and R1 can be any within 4-16 ohms. The quiescent current is set for 4 ohm load equal to 3A, for an 8-ohm load – 1.5 A. Output in the first case will be equal to 7W, in the second case – 4W. Mode amplifier for constant voltage is set automatically at the expense of environmental protection at a constant voltage across the resistor R89.
AC voltage – NFB absent due to the shunting effect of C23, which together with low output impedance op amp does not allow the signal from the amplifier output to go to the inverting input of op amp. Capacitors C3 and C23 is better to use a series of K73, or MBM. Electrolytic capacitor C27, the polar for a voltage not less than 16 V. The diodes D1, D5, D6 silicon type D220, KD503-509 installed in thermal contact with the heatsink of Q1, and perform temperature stabilization of the quiescent current amplifier. The capacitor C19 is selected, giving the amplifier input square wave frequency of 20 kHz and an amplitude of 100 mV on the front undistorted form of output voltage.