Here the circuit diagram of 50W power amplifier. The amplifier uses a pair of power transistors that are MJ2955 and 2N3055 to amplify the input signal. You must use a heatsink on those transistors. The below design diagram uses for single input/output channel, just built two similar circuit to make it dual channel (stereo).
Power Supply Circuit:
This amplifier require dual polarity output power supply (+, ground and -). Use this power supply circuit:
Transformer: 3-5 Ampere, 24V secondary output
Diode: Diode bridge 5A or 4 piece of single diode 5A