The modulator works as follows. A CD4060 clock divider runs an 8MHz crystal clock and divides it down by 2 to the power of 5 (32) to 250kHz. This clock drives a transistor Miller integrator for a 2Vpp sawtooth waveform. The sawtooth is compared to a line level audio signal on the inputs of a comparator (LM311) which modulates the 250kHz clock duty cycle with the audio (via a TL071 op amp), creating a pulse duration modulated waveform, with a 50% resting duty cycle. A small amount of negative feedback can be aplied for error correction. The resulting PWM waveform drives a high and low-side gate driver (IR2110), which directly drives the gates of the power switching FETs (IRFP260), which alternately switch the full DC supply across the Low Pass Filter which presents a 12 to 16 ohm load.

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