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

Sat Dec 26 07:58:05 UTC 2020

Sure, a battery solves the noise problem and a 9V certainly offers a lot
more headroom.  I did prototype a solution using a battery.  Since I didn't
want to have the battery drain when not in use or require the user to
unplug the input jack like some pedals do, I created a two transistor
switch which turns on the buffer when the pi receives power.  That worked
great, dead quiet, but it seemed odd requiring the user to add (and
eventually replace) a battery to a pedal that has external power.

The buffer circuit I used is from this handy tutorial about guitar buffers:
http://www.muzique.com/lab/buffers.htm

I tried most all of them, but settled on the second from the last.  A
simple Op amp with voltage-divider biasing.  The TL071/TL072 won't work
with 3.3v and somewhat noisy even with properly powered, so I just swapped
it for the MCP6292-E/P.  I also changed R1/R2 to 2Meg for an input
impedance of 1Meg.  Also, since the AudioInjector card has 10uF input caps,
I didn't need that on the output of the buffer.  The MCP6292 is a dual
opamp, so you get 2 channels per 7 parts (4 resistors, 2 caps, 1 opamp).
Not bad.

For the power regulation, I just use the AMS1117-3.3.  Ground the adj
terminal.  A 220uF (could probably be as small as 10uF) and 0.1uF bypass
ceramic across the 5V input.  A 10uF oscon across the 3.3v output and
another 0.1uF bypass ceramic near the opamp.  Standard regulation.  I
believe the Audioinjector boards include an inductor on the output to form
a simple LC low-pass filter.  13uH should roll-off at 14kHz.  6uH at 20kHz
so somewhere in there should help, but it didn't seem to make a noticable
difference so I left it out.

Regarding a commodity widget.  Tons of guitar effects include an input
buffer similar to those AMZ presents.  You can also buy little boards that
do that:
http://www.muzique.com/pcb.htm
https://guitarpcb.com/product/3pdt-buffer-board/

So I'm guessing maybe you're creating an audio interface for recording?
I'd be interested in hearing what you come up with.

Good luck!

-Rand

On Fri, Dec 25, 2020 at 11:00 PM hugh crawford <hugh at hughcrawford.com>
wrote:

> Randy,
> Thanks for such a prompt response!
> Yes a schematic would be nice to have and very much appreciated. Thanks
> for the warning about the power noise. For proof of concept purposes, a
> battery would make a pretty clean power supply I suppose.
>
> I am amazed that there isn't a commodity widget for guitar to line level.
> Of course I may be overestimating the market since I need 6 x everything.
>
> Thanks
> Hugh
>
> On Fri, Dec 25, 2020 at 8:23 PM Randy Reichenbach <randaji at gmail.com>
> wrote:
>
>> 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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