When I started this project about 4 years ago, I had two synthesizer firwares for the original Arduino in mind. Firstly, the Auduino granular synth, which I had built a dedicated version of, and secondly the DIY Polyphonic Drum Machine and Rhythm Looper by Sebastian Tomczak.
The Auduino uses a PWM-based audio output, but the drum machine was designed around an R2R DAC which I had to modify to use PWM as well. Luckily, this was not too much of a hassle. Here's an early prototype of the Drummachine:
Both synthesizers now run natively on the Synthboard, and even though they have done so for 4 years, I have yet to make some videos of that 🙈