So the gain is the result of the power output (Pout) and power at the input to its function in the form of frequency. The size of the gain, (G) is usually the Decibel (dB).
2N3055 Power Amplifier
Skema 2N3055 Power Amplifier. The optimal supply voltage is around 30V, but this amp work from 24 to 32V. The maximal 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 almost of the components, which you find at home. The and transistors can be any NPN type power transistor, but do not use Darlington types. The output power is around 150 Watt.
capacitor C1 regulates the low frequencies (bass), as the capacitance grows, the low frequncies are getting louder.
capacitor C2 regulates the higher frequencies (treble), as the capacitance grows, the higher frequencies are getting quiter.
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 500Ω trimmer resistor. As this current incrases, the sound of the amplifier gets better, but the end transistors are more heating. But if this current decrases, the transistors are not heating so much, but the sound gets worse.