“I always wonder whether it is possible to make an amplifier of class D on ATtiny13 or not. Some time ago I found George Gardner’s project based on ATtiny85 – TinyD. It was a sign to start challenging it with ATtiny13. It took me a few hours but finally I made it! The code is very short and useses a lot of hardware settings which has been explained line-by-line in the comments. The project runs on ATtiny13 with maximum internal clock source (9.6MHz). It gave me posibility to use maximum of hardware PWM frequency (Fast PWM mode).
F_{PWM} = \frac{F_CPU}{N \cdot 256} = \frac{9.6\mathrm{MHz}}{1 \cdot 256} = 37.5\mathrm{kHz}
The PWM freqency of class D amplifiers should be of course much higher, a hundreds of kHz (some sources says 8 times more than sampling frequency). Unfortunately, in case of ATtiny13 it’s not posible to achieve such parameters. However, for experimentally selected settings of ADC sampling rate (~10kHz) and PWM frequency (37.5kHz) the amplifier sounds very good! The code is on Github, click here.”