This simple mosfet power audio amplifier circuit, with TL071C and 2 MOSFETs (IRF9530 and IRF530) can deliver up to 45W on 8Ω speaker or 70W on 4Ω speaker. This schematic is based on Siliconix application and on variations of voltage on the 2 resistors that are serial inserted on the voltage supplier of the operational amplifier driver. The MosFet transistor must be mounted on a heatsink with at least 1K/W.
The efficiency of this amplifier is 70%, distortions at cut frequency were at most 0.2% at 20Hz on 8Ω and 10W. With a power supply of ± 30V the mosfet audio amplifier can deliver 45W on 8 Ω and 70w on 4Ω. Remember that this audio amplifier is not protected on short-circuits so everytime you switch on check to see if the speaker is connected.