HiFi AMPLIFIER

Power amplifier circuit
which is depicted in the figure, in its electric
satisfies performance requirements for Amplifiers HiFi class.

The output power of the amplifier at a load of 4 ohms : 25 watts.
Harmonic coefficient in the reproduced frequency band from
5 Hz to 46 kHz does not exceed 0.1%.

The difference between this scheme and
the most common is that the voltage amplifier on the op amp A1
connected to the terminal amplifier through the buffer stage
on transistors V2, V3.

The transistor V2 plays the role of servo voltage follower,
and the transistor is ultrasonic, included according to the scheme with a common base,
matches the low output impedance of the repeater with a two-stage current amplifier.

With this construction of the amplifier, it becomes possible
get output amplitude signal at load close
to the supply voltage (rails) - not of an operational amplifier;
with minimal distortion of form.

Elements V4 - V9, V11 - V14
set the initial offset of the operating point of the current amplifier when
changes in ambient temperature and supply bias.

Diodes V17 and V18 provide reliable protection of the amplifier against
overloads and short circuits in the load, implementing a "soft limit"
collector currents of transistors.

Radio Fernsehen (GDR), 1977, No. 14

As the opamp you can use K1UT531A (uA709, TAA521).

Transistors:
Original: V2, V10, V16: KFY18 (Silicone!)  (1W, 60Vce, 0.5A, Hfe=90, 50Mhz), 2N2222, BFY18

Russians recommend: V2, V10, V16: GT602W/B  (Germanium!! Why??)

V3, V11, V15: SF128D, KT602A/B
V21, V22: KT808

Diodes:
V4-V9, V12-V14, V17,V18: SAY18, (1Vfwd, fast silicone diode 25V, 100mA, 2ns), KD103A, 1N483, BA128

V19,V20: Sy400, BY255, KD202M (0.9V - 1.1V Vfwd)
V23,V24: KD202B