[AudioI] Guitar Preamp for Isolated(Pro) soundcard

[Joseph DiVerdi]

Excellent points raised by Ted. I'm going to write in-line here.

> On Mar 2, 2022, at 2:41 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.

This is very good latency and I hope to be able to get the same in my own devices.

> 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 true. Power supplies (USB style and others) for digital devices are awful generally speaking for analog work. I do find that it is possible to use a very simple low pass filter (10 to 100 ohms series and 10 to 100 µF to ground) to clean up a lot. This by itself is not sufficient and good layout and control of ground are critical to success.

> 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.

I mentioned earlier that I used a unipolar power in my (internal) guitar amplifier and tone control and now I'm suffering from "buyer's remorse" as I wish I had gone bipolar. The power on/off transient is substantial. I'm going to try to attach a schematic of a very simple bipolar supply that I have used for analog circuitry based on a 555 AC and a pair of voltage doublers. Careful choice of the operating frequency and layout and output filtering make this a great choice for analog work. If the image does it make it through the mail list I'll be happy to share it directly with anyone who wants it.


Joseph

> 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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