.There are many class AB audio amplifier circuits out there on the internet. Unfortunately the simpler ones often have bad biasing arrangements for the output transistors. This results in high standing current requirements and output transistors that are running hot even with no audio. The circuit shown above is a pure class B amplifier with a well defined standing current. It uses a high gain bootstrapped amplifier to drive the output transistors together with heavy negative feedback to give a good quality audio output with minimal crossover distortion It is also a genuinely useful alternative to using an audio output chip such as the LM386 or TDA2003 which often have too much gain and a familiar tendency to go into uncontrolled oscillation. Using cheap (or junkbox) 400mW transistors in the output stage will give a good output volume. Increasing R9 will increase the gain of the amplifier.