[AudioI] Fwd: Guitar Preamp for Isolated(Pro) soundcard

[Ted Rippert]

I didn’t hit replay all, for this so I’m forwarding it to the list:

> Begin forwarded message:
> 
> From: Ted Rippert <ted.rippert at gmail.com>
> Subject: Re: [AudioI] Guitar Preamp for Isolated(Pro) soundcard
> Date: March 2, 2022 at 7:42:36 PM MST
> To: Randall Reichenbach <randaji at gmail.com>
> 
> Randall,  I am aware of your project from the archives here and your website. I actually used it to help design my simple buffer setup, especially the use of and LDO regulator. I did decide to use a dual output boost and two regulators to get a true dual supply going and get enough voltage to have an output buffer that can easily do +4dBu which all my power amps like to get. Thank you for the information you have put out there. It is very helpful.
> 
> You are right about MIDI being pretty simple, and the Pro card has both the UART and the 5V from the Pi on a header, so it should be straight forward to include. If you have any experience with particular optocouplers let me know, I haven’t actually built this yet. I’ve been using cheap USB to MIDI interfaces for my setup, and the work fine, just trying to come up with something more compact and “elegant”
> 
> I don’t really agree about the Pro card and preamp not fitting in a stomp box, but I’m used to using pretty large stomp boxes with lots of foot switches. I think of the Behringer FCB 1010 as a stomp box, so maybe I’m weird. I do agree you won’t be able to fit it into a Tube Screamer sized box.
> 
>> On Mar 2, 2022, at 7:10 PM, Randall Reichenbach <randaji at gmail.com <mailto:randaji at gmail.com>> wrote:
>> 
>> 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 <http://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 <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 <mailto: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
>> 
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