The design is a self oscillating amp (SODA … read more here, use Google to reanslate from German), and it simply didn’t want to start oscillating.
Just running the modulator without inclusion of the MosFet output stage, it worked like a charm, but including the output stage just didn’t work.
The remedy is to feed back a little current from the modulator / comparator output to the input of the modulator, then it just workes
So one more resistor, and it just plays.
Well I also found some other smal faults and improvements. One being the necessary level shifter, that shifts the +-5V volt output from the modulator / comparator to 5V above the negative rail, to feed it into the MosFet driver ( here IR2110). It simply didn’t work fast enough.
I altered the circuit with a small 1N4148 and a 1k resistor in parallel with the input resistor to allow for at high current return path.
Take a look at a video of the beast her on YouTube: DIY Class-d SODA amplifier test