This is the circuit diagram of 150W power amplifier which built using power transistors. The circuit above is essentially a mono channel circuit. To make it into a stereo channel, you have to build the two identical circuits. 150W is high enough for the audio system in a small field or in the hall/meetinghouse.
150W Power Amplifier Note
To maintain performance remains excellent and prevent damage to the power transistor caused by excessive heat, then you must install the heatsink on the transistor Q7 to Q12.
On 150W power amplifier circuit schematic above is included the simple power supply circuit that very easy to built. Transformer to use is 3A minimum for mono applications, and 5A minimum for stereo applications. For better performance, maybe you could try using a regulated power supply that is certainly capable of providing a large current and voltage as needed by this power amplifier circuit.