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@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

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