[AudioI] Guitar Preamp for Isolated(Pro) soundcard

[Randall Reichenbach]

I would also love if AudioInjector made a card with "instrument" inputs.
The Pro and Isolated cards are great, but the physical footprint is just
too large for stomp boxes IMO.

I hit the issues you list in developing my project, pi-Stomp (
treefallsound.com).  Yes, the pi 5v power rail is quite noisy, but I was
able to tame it enough using a LDO regulator (AMS1117-3.3 same as the Zero
card uses) and a 10uF oscon cap on the output side.  Having a separate
analog ground (for your buffer preamp and its regulator) from your digital
ground (pi and everything attached) helps a ton too.  The AudioInjector
card is the single place where my grounds are connected.

Starting with a rather clean power supply helps too.  The one I supply with
all of my kits has 1% line regulation, 4% load regulation and 50mVp-p
ripple.  If the supply manufacturer doesn't list their regulation specs,
you can guess it's pretty noisy.

Once I had clean power, the next challenge was maintaining headroom.  Even
a LDO reduces 5 to 3.3, so you'll want a rail-to-rail opamp.  I tried 6
dual opamps and settled on the MCP6292 due to its noise performance
(speced, heard and measured).  I configure it as just a simple
non-inverting buffer.  Same as the second to last circuit here
http://www.muzique.com/lab/buffers.htm   but with 2M resistors to yield 1M
input impedance.  You could easily configure it with some gain as well by
adding a feedback resistor.

I've also dealt with adding MIDI (DIN and USB).  USB you get for free via
the pi USB connector.  DIN MIDI just requires an opto isolator, a few
resistors and a diode.  If ya wanna chat about any of that, let me know.

Cheers,
Randall





On Wed, Mar 2, 2022 at 1:41 PM Ted Rippert via People <
people at lists.audioinjector.net> wrote:

> Hello,
> I’ve seen several threads on this list relating to using Guitar or Bass
> with an Audio Injector card. I’ve been doing that for years using a rack
> mount mixer with a couple High-Impedance inputs interfacing into an Audio
> Injector Stereo card. Works very well, and I can get round trip latencies
> on the Pi as low as 2.1 ms running Guitarix amp sim if I use some CPU
> isolation techniques.
>
> I’ve been playing around with making a small preamp/buffer that runs on
> the 5V from the Pi, and found just what others have, that it’s too noisy.
> So I worked up a setup with a dual supply converter that then goes into a
> couple linear regulators. It does work, but it’s a mess using just perf
> board and through hole parts. Also, the Zero sound card I used has the same
> usable but high noise floor that the stereo card does.
>
> So I want to try making a preamp for the Isolated card (really a Pro card
> from Amazon, but same interface). I would like to talk to Matt and any
> others with experience about the design of such a card, and any possibility
> of an “official” Audio Injector version being made. I just do perf
> board/through hole stuff, so I can only make one-offs myself.
>
> To get things started, let me list some specs for such a card:
>
> Input impedance: > 500kohm, 1 Mohm nominal
> Input voltage: ~ 1 VDC pk-pk, preferably with an adjustable gain and
> significant headroom
> Input connector: 1/4 inch TS socket
> Output impedance: low audio interface level, preferably less than 300 ohms
> Output voltage: at least +4dBu, preferably with adjustable gain, since the
> codec has an adjustable analog output volume, this may be in software
> Output connector: 1/4 inch socket, preferably a TRS type that will work as
> both a balanced and unbalanced output depending on what is connected to it
>
> It could be as simple as a modified RCA preamp with different connectors
> and higher impedance input circuits, or you can think about adding things
> like a DIN based MIDI interface and a headphone amplifier output. All of
> this seems quite doable using the AI-ABS standard, but I need some more
> details about the specs of that, and about the practicalities of how to
> design opamp buffers to interface to the codec on the Pro board.
>
> So, if anyone is interested, lets talk about it.
>
> Thanks,
> Ted
>
> --
> People mailing list
> People at lists.audioinjector.net
> https://lists.audioinjector.net/mailman/listinfo/people
>
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