[AudioI] Guitar Preamp for Isolated(Pro) soundcard

[Ted Rippert]

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