Interfacing Guitar to Daisy Seed

I’ve been developing on the daisy platform for awhile however I want to be able to use instrument level with the pedal however I know the Daisy uses line in and out. How would I be able to make the Daisy use instrument in and out. I’m guessing I could use a simple non inverting Op-Amp circuit for the input, but I’m unsure as to how I could output at instrument level.

My 2 cents:

For a guitar, levels are more about impedance, because they actually can go quite high with humbucker pickups (mine outputs over 4 V peak-to-peak). After buffering the input, I would be more inclined to attenuate the signal before entering the ADC (which accepts 2.3 V ptp, based on the codec specs) instead of amplifying it, unless you’re not bothered with moderate clipping. I would also add protective Schottky diodes around the ADC inputs (protection which is slightly mitigated by the coupling caps on the Daisy board), unless you use the 3V3 supply to power the non-inverting op-amp.

For the outputs, you need to filter out the HF and buffer. Keep the cutoff high enough (150 kHz) so the phase in the audio passband is not affected too much. Quoting Douglas Self in Small Signal Audio Design: “Multiple-feedback filters are often preferred for this kind of application because they do not suffer from the failure of attenuation at very high frequencies that afflicts Sallen and Key filters […] The capacitors used must be linear; NP0 ceramic, polystyrene or polypropylene are the only types suitable.” Design tool here. At this point the signal is inverted, so if you want an additional gain, you can put an inverting amplifier. If you don’t, you can invert the output in software.

Don’t forget the final coupling cap if you use a virtual ground, a large resistor tied to the real ground to avoid pops when switching the circuit, and a protective serial resistor (100 to 1 kΩ).

3 Likes

Hi Holland!
I would like also to simply use my guitar as input ! Did you come up with a solution ?
kind regards,
Jean-Paul

If you’re looking for something you can just add parts to, the pedalpcb terrarium has all that you should need to interface your guitar to a seed - might be worth to have a look…!