STM32 Blackpill pedal schematic

Schematic for interfacing an audio (guitar) signal with the STM32 Blackpill board. Please note, I'm using 8 bit ADCs here, but the result is completely noise-free (as in gated) readings.

I'm using a ton of pre-gain before the ADC to maximise the 8 bit resolution. I have to assume the audio is clipping in the ADC but it's not noticeable to my ears.

The wet (digitized) signal is being inverted compared to the dry. This isn't an issue for me as I'm making delay-based FX. You could easily switch the pre-amp to non-inverting or even invert the wet signal again in your code.

You can play with the values in the Sallen Key filter to taste, increasing R7 & R8 for a lower cut-off or reducing those values for a higher cut-off.

It's important to use JFET based op amps like the TL07x series. I tried this design with an LM324 and the noise was atrocious.

This schematic works with the Blackpill board (both revisions) and assumedly the Bluepill as well.

Imgur link