This MOSFET amplifier uses two complementary MOSFETs (IRF9520 and IRF520) to deliver up to 20W into 8 Ω speaker. A TL071 op amp is used as an input amplifier. The MOSFETs should be heatsinked with a heatsink of better than 50C/W capability. THD is less than 0.15% from 100Hz to 10KHz.

Mosfet amplifier circuit schematic

20 watts mosfet amplifier