This is a simple and low cost 60W power amplifier. The optimal supply voltage is around 50V, but this amp can work from 30 to 60V. The maximum input voltage is around 0.8 – 1V. As you can see, in this design the components have a big tolerance, so you can build it with almost any components that you can find at home. The output power transistors can be any NPN types, but do not use Darlington types.
Capacitor C1 regulates the low frequencies (bass), as the capacitance grows, the low frequencies are getting louder. Capacitor C2 regulates the higher frequencies (treble), as the capacitance grows, the higher frequencies are getting quieter.
This is a class B amplifier, this means, that a current must flow through the end transistors, even if there is no signal on the input. This current can be regulated with the 500 Ohm trimmer resistor. As this current increases, the sound of the amplifier is better, but output transistors are dispatching more heat. If the current is decreased, the transistors are dispatching less heat, but the sound quality is decreased.