SA420 is an amplifier made by PIONEER which tone control is a feedback circuit from the power output to the input circuit.
This power amplifier requires a supply voltage that is not so large because it is intended for home amplifiers in the 80s era.
The following is an edited and cropped schema of the original schema:
The voltage inside the circuit uses 2 kind of voltage, namely 29VDC symmetrical for the main circuit and 19VDC asymmetrical for the preamp circuit.
Q2 is a 5-pins long tail pair preamp transistor.
VR1 is a potentio balance.
S2-3 is the switch to activate the LOUDNESS feature.
