A low-distortion (<0.1% THD) 12W per channel class AB amplifier.
I've also included a LTSpice schematic file if you want to play around with the circuit.
It's a classic class AB amplifier topology. Previously it had a class A pre-amplifier stage but I decided to remove it and let the op-amp handle the voltage and current amplification stages.
The power supply is very simple since it's supposed to be powered by 6x18650 cells or 2x9V batteries.
This amplifier also includes a clipping/overload detector: