[AudioI] high impedance guitar input with Octo RCA sound card

[Randy Reichenbach]

I have experience with using the AudioInjector Zero for guitar processing.
It works great at long as you have a preamp or unity gain buffer feeding
the input.  I created an open-source project around it:
http://treefallsound.com

I tried many different op-amps (5) and discrete transistor buffers (2).
The biggest problem I had was noisy power since I needed it to be powered
from the same supply as the Raspberry Pi.  The 5v rail is simply unusable
without regulation.  The 3.3v pi rail is regulated and less noisy, but
still not usable IMO.  If you could steal 3.3v from the AudioInjector, that
would probably be your easiest option.  That would have required a hack to
the Zero card, so I settled on regulating the 5v rail with a AMS1117-3.3
low-dropout regulator (same regulator the AudioInjector Zero uses).

For the buffer itself, I found the MCP6292-E/P opamp (
https://www.mouser.com/ProductDetail/579-MCP6292-E-P) to be the best
performing.  Best noise floor and output swing.  You'll want a rail-to-rail
opamp cuz even 3 volts is not much headroom for a guitar signal.  If I
could justify the extra cost and board footprint for my project, I'd go
with their "isolated soundcard" which has almost double the headroom and
I'm sure the noise and audio quality are much better too.

If you'd like a schematic for the opamp buffer and supply regulation I use
for pi-Stomp, let me know.

Cheers,
Rand


On Fri, Dec 25, 2020 at 5:23 PM hugh crawford via People <
people at lists.audioinjector.net> wrote:

> The Octo RCA sound card supports six audio inputs and I would like to use
> it and a Pi card for electric guitar signal processing preferably built
> into the body of the guitar eventually but outboard is fine for now.
>
> I am using passive high impedance guitar pickups. A coil of wire
> wrapped around a magnet for each string.
>
> It seems like an obvious application, and I would imagine that I am not
> the first to try this. Does anyone have any experience with this? Some sort
> of minimalist signal boosting / impedance matching / buffering perhaps?
>
> I could make a balanced connection on the pickup side if that helps
>
> Thanks
> Hugh
>
>
> --
> People mailing list
> People at lists.audioinjector.net
> https://lists.audioinjector.net/mailman/listinfo/people
>
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