I thought I'd post the link here as the synth is digital and maybe purely analogue builders wouldn't be interested. The construction notes outlines the main features, but to summarise;

5 oscillator auto-harmonizing chord VCO
Single oscillator mono synth (chromatic)
Dual oscillator mono synth (chromatic)
9 waveforms - saw, saw+octave, square, square with PCM, FM1, FM2, FM3, random noise, triangle, sine
Note -tracking ringmod
Multi-function harmonized arpeggiator
Envelope generator with 4 modes
Glide with selectable time
Hammer-on/slide (selectable)
Transpose (natural, -1 and -2 octave)
Clipping/bypass/signal LED
Infinite note/chord hold
Bypass
Wet/drymix
Input gain adjustment
Internal FX loop

No sound clips right now, but will upload later. Updated link to correct missing file.

https://drive.google.com/file/d/1RwaHBN9I9VE4IKOL94ja4zjIvzWFW2jf/view?usp=sharing

Here are a few recordings I did. Having real difficulty recording with my phone - no matter how low I turn the amp I get distortion, but you can get the overall type of sounds. I forgot though to record any detuned dual oscillator stuff.

https://drive.google.com/drive/folders/1PUwsxuIfZztUcgo4vuSY_O5qXTqJAWIx?usp=drive_link

Thanks Mick! I'll be investigating this 

Sounds pretty cool and looks pretty simple.  Amazing!

I just noticed a logical error with button 3 where glide time was changing along with the arpeggiator start point, now fixed in v1.0.1.

BTW, sound clips are recorded with just reverb - what sounds like repeats on the arpeggio clips as the note decays is the arpeggiator running - not delay repeats. This can be changed by selecting a different envelope.

Interesting project. I didn't know about the RP2040, looks like a good platform.
Are you using PWM as a DAC?
How is the ADC performing?

I'm using PWM to output audio directly, instead of a DAC. The ADC is excellent and rather unexpected in how well it performs. Way better than you might expect, especially as it's configured as 8 bit. This gives much better tracking than the RP2040's native 10 bit.

Remember the PIC16F88 EPE magazine guitar to midi project? You described how this worked and this stuck with me for years. I built that project and cascaded it into an Arduino Midi to CV circuit for my modular synth. It worked well, but latency was a big issue. I have no programming skills, but managed to learn enough to implement my own version of the code in C++ and expand it into a fully functioning standalone synth. The hammer-on function is based on your suggestion for that magazine project. Latency is no longer an issue with this build.