Uses the complex soft clipping stage.
This is the preamp. Power amp is build around a power IC...
(https://i.postimg.cc/R62wRf6b/Elektor-amp-1985.jpg) (https://postimg.cc/R62wRf6b)
A less complicated hard clipping configuration..
(threshold independently adjusted for positive & negative signal waveforms..)
(https://i.imgur.com/84ZQ8fN.gif)
It's a (multi-segment) piece-wise clipper. The transistors and resistors on the emitters set-up a chain of small DC voltages. The DC voltages add to the diode voltage, in effect increasing the clip point voltage and making the knee sharper. Then the series resistors (2k2, 8k2) set the softness/"gain" of the segment.
The basic idea is along the same lines of the sine-tri converter in the ICL8038 chip. The chip does a bit more in that it looks like it tries to reduce sine level drift due to temperature - not a big deal for an audio clipper but a PITA for a sine reference.
https://www.timstinchcombe.co.uk/index.php?pge=trisin