This is the reverse engineered schematics of a repaired PA amplifier,
part of an old professional 3-way active studio monitor. The schematics
has been tidied up to get rid of a couple of nonsensical precision
resistors and obsolete transistors/FETs.

R5 is used to adjust the class B idle current somewhere within 5-50 mA
range. R2 is the thermal protection absorbing the Q2,Q3 current mirror
input. Its output current goes into a phase splitter (Q6 and Q7) driving
the 2 NMOS FETs. The splitter is controlled by an old-skool differential
amplifier consisting of Q4, Q5 and Q8 which compares input with output
voltage. DC gain is 1. AC gain is ~15 >20Hz due to feedback attenuation
(R19, R15, C6).

After the repair:
The IRF520 FETs replace broken MTP8N10.
The bc556b are actually still bc307a.
Zeners may well be spec'd at 9V, I just measured. Much more to tell but this is it.