This a 24 Watt Class A Amplifier circuit made from discrete semiconductors like 2N3055. This audio amplifier can drive up to 24W Class A into 8 Ohm speaker with an audio bandwidth of 100mHz to 100kHz flat. The input power is 305mV for 24W output into 8 Ohm (33dB gain) and the THD is very low.
The supply voltage can be between 34V and 46V and the quiescent current should be set to 1.7A measured through R25 (a voltage of 0.75V must be measured over R25 for a quiescent current of just under 1.7A). R23 is a trimmer and must be set to maximum resistance (10kOhm) when powering up.
Then the resistance of R23 must be decreased until the the quiescent current is achieved. If the amplifier is mounted on a big enough heatsink ( 0.6K/W at most) then the amplifier is very safe from thermal runaway. Intelligence must be used when choosing power and voltage ratings of resistors and capacitors. Source: http://www.zen22142.zen.co.uk