At the first glance the circuit rather appears to be unsymmetrical in design, due to an unbalanced looking output stage. However a closer look will prove this wrong. Transistors T9, T10, T11 and T12, T13, and T14 form two well-balanced halves of the circuit, perfectly complementing each other.
The input stage employs the standard R/C filter configuration. R1 and R2 fix the input impedance, and the inclusion of C1 creates a high-pass filter that blocks all frequencies around 1.5 Hz. C1 also functions as an input stage DC bias isolator.
The presence of R2 and C2 ensures no frequency above 250 KHz makes its way into the circuit, thus blocking most of the high frequency RF intrusions.
Transistor T1 and T2 are wired up in a standard differential amplifier mode.
The remaining portion of the circuit is mainly the output stage and is responsible for amplifying the differential stage into the loud speakers.
Power output is 60 watts into 8 Ω and 100 watts into 4 Ω loudspeaker.
Total harmonic distortion is less than 0.01 %.
Frequency range is within 20 Hz and 20 kHz.
The input sensitivity is in the vicinity of 750 mV.
The frequency characteristics lie in between 1 dB from 15 Hz to around 100 kHz.
Due to its very high amplification factor of around 20,000, the output stage may have an ideally low quiescent current drain of about 40 mA.
The quiescent current can be set through P1 with a digital multimeter connected across resistors R6 and R7.
Adjust P1 until the meter reads about 40 mV, corresponding to 50 mA current.