Bringing non-Eurorack gear’s audio into Eurorack is an issue, as Eurorack levels are a LOT hotter, so your keyboard / guitar pedal output’s going to end up really quiet if you want to run it through your modular.
Euro audio level is 10 volts peak to peak (so a swing of +5v to -5v). Other gear ranges from consumer line level (normally written as -10db) where a lot of non-modular synths are at (about 1v p2p), +4dbu (about 3.7v p2p), and Instrument level (around 10-100mv).
I wanted something to amplify Line level or Instrument level to Euro’s 10v p2p, and something to go the other way and bring Euro audio down to 1v p2p.
Down is easy. Euro to line is a negative gain (attenuation) of 10, bringing 10v peak to peak down to 1v p2p. A voltage divider (R3 and R4 on the schematic) would do it – I added a pot to this as an attenuator (fully anticlockwise it’s 3.5v to 10vp2p, and about 3/4s it’s 1v to 10v p2p) so it could handle the different line levels.
Going up to Eurorack just needs an op-amp in a non-inverting configuration, with resistors chosen for the appropriate gain.
I’ve mentioned this before, but if you want to get your head around op-amps, MFOS’s Virtual Op-Amp Lab is great (in this instance, change the top range from 10vp2p to 1vp2p – Line Level -and change the first resistor – the one not in the feedback loop – to 10K from 100K, and then what this circuit does will be shown on the virtual scope).
To get from 1v to 10v is a gain of 10. From 10mv to 10v needs a gain of 1000. Switch SW1 swaps a 10K (ok, this is a gain of 11, but close enough to avoid weird resistor values) and a 100 ohm resistor (again, gain of 1001) to ground into the circuit with the 100k feedback resistor. I didn’t put an attenuator on this simply because virtually all synths and electric / electronic instruments have a volume control.
So I can check the levels going out aren’t clipping, I added a LM393 comparator which compares the audio signal, rectified by a zener diode to remove the negative voltages, to a reference voltage (4.5v on the 10v p2p OUT, and 0.5V on the 1v p2p Line level). In use, adjust until the LED lights and then dial it down a tiny bit.
This certainly isn’t perfect, but it works and does the job. I’m using this circuit as an attempt to learn PCB fabrication in KiCad and will post the Gerber files here once I’ve built it on PCB.