The photo shows a prototype class D power amplifier using 4 Hexfets in class D operation. Power output is 575W from 24 volt power supply. Input requirement is a few milliwatts. The single turn primary of the 5:1 output transformer is centre tapped and the quad complimentary hexfet pairs alternatively short respective half turns. 24v is therefore alternatively applied to the 1/2 turn putting a maximum voltage of 48v on the hexfet drains. The secondary voltage is a squarewave with maximum peak amplitude Vpeak=i/j x Vcc where i/j is the turns ratio which in this case is 10, and Vcc is 24v. The fundamental frequency component of this squarewave is selected by the o/p filter. Measured values are slightly above theoretical.