Note about the euxoloti schematic: It seems to me the opamp is powerd from 3.3V and ground there, just like in my schematic above. It works (I assume, haven't tested it myself) because for MCP6001/2/4 the input common mode range can go a bit below the negative rail (and for the inverting configuration, both inputs should be very close to 0V all the time). The output can then swing from 0V to 3.3V, and this is the main clipping used. Note that when the opamp output clips, the input is no longer at 0V and it will go out of spec; I don't know what happens with this particular opamp in that case. The 10k resistor is optional indeed, and maybe overkill, but you don't need low impedance in this case (the STM32F4 datasheet says 50k is acceptable at the highest ADC speed).
@Illuminate: To use clipping diodes, the general idea is this:
The diodes clip any voltage above 3.3V+x or below 0V-x, where x is the "diode drop" voltage, x=0.6V for standard (silicon) diodes. It is important to have some resistor R1 to limit the current through the diodes, otherwise you just blow up the diodes and you still have no protection. I again recommend at least 10k in this application (and at most 50k), it is high but at least it limits any substantial current injection into the axoloti power rails, which could be harmful. I would also recommend Schottky diodes, that have a lower voltage drop, the limit for axoloti is 0.3V if I remember correctly. An example of a Schottky diode is 1N5818.
The clipping above doesnt do scaling, only clipping so the input range is 0-3.3V. If you add a resistor from the axoloti pin to ground, it becomes a voltage divider, so you can scale down voltages, but still only positive ones. You would get about 0 - 4.8V range with R1=10k and 22k to ground. You can also make it work with negative inputs by adding 6.8k to the 3.3V rails (and keep the 22k to ground), which gives -4.8 to +4.8 V (not exactly symmetric) input range. But it has the downside that you get some nonzero reading when there is nothing plugged in. To fix that, you need an opamp.
Final note: you can use an opamp such as TL072 connected to the eurorack +/- 12V supplies, but then you need to do the clipping seperately, for example with the Schottky diodes. And then you definitely need the series resistor!
