LucienR About the voltage divider, I'm using homemade dirty modules, powered with +/- 12v, should I use a rather drastic ratio? I guess I'll be doing some trial and error, but I'm a bit worried I might damage my Bela with wrong values. Or am I safe as long as I use the diode?
trial and error here is probably not the best approach. You could end up damaging the board. The diode approach doesn't protect you against negative voltages and possibly not against very large current values on positive voltages. It was an expedient solution for that specific synth, which had a manageable output range and was guaranteed positive.
I recommend you use the high-value resistor dividers as per the Pepper schematics:
+-- Bela analog in
The scaling factor is (R2 / (R1 + R2)), which in this case is 0.32. You can tweak the values as needed, but ensure that R1 is always large enough (e.g.: > 30k).