Building on top of Rouslan Dimitrov's Microcontroller Class D Amplifier, I developed the idea to feed the power amplifier with PCM data directly. Rouslan does some dithering and noise shaping, but I'm convinced that proper interpolation would sound better. The ATtiny is too slow to do even cubic interpolation so I'll move that into the host.

The result should be an ALSA driver, that does upsampling of 44.1 or 48 kHz to about 240 kHz (the Class-D PWM frequency) and sends it over preferably, but not limited to, SPI towards the AVR, which will drive the power FETs with its PWM outputs. This can give any (embedded) linux system the right audio "punch" with a fairly short BOM.