The SA410 is an amplifier made by PIONEER in the 80’s that uses the feedbcak tone control feature in its power amplifier circuit.
The power amplifier uses a symmetrical voltage of 29VDC for the main circuit and an asymmetrical voltage of 19VDC to supply a longtail pair preamp circuit that uses a 5-legged transistor.
The circuit has used a servo driver circuit to determine the bias current in the power transistor.
The power transistor has a power dissipation capability of up to 40 Watt
( DATASHEETS ).