Randall,

Like I said, I tend to use off the shelf stuff more than custom circuits. I also haven’t really made a small stomp box. My main amp is in a 19 inch rack mount case, so I just use a 3u faceplate and attach the Pi and circuits to the back and have a 7 inch touch screen on the front. So plenty of room for whatever. I use the FCB 1010 for foot control when playing. I really do like your setup with the small screen and the colored switches, just haven’t got that far yet. The biggest limitation of the FCB 1010 is that it really doesn’t give you any feedback about what the switches and pedals do.

For dual supply use I found this on Amazon:
https://www.amazon.com/dp/B08YYX14PS?psc=1&ref=ppx_yo2_dt_b_product_details

You can get it much cheaper direct from China too, but it takes a month or more to arrive:
https://www.amazon.com/Eletechsup-Positive-Negative-Boost-Buck-Converter/dp/B07NN49R4N/ref=pd_sbs_2/131-4075651-5797549?pd_rd_w=QXDPM&pf_rd_p=4b6b5072-e9bd-4f30-a3af-a1f5d52978ec&pf_rd_r=GYK4VN9XB60SKJ9G2FVN&pd_rd_r=17232d06-a502-419b-8443-556fb552b874&pd_rd_wg=20ilH&pd_rd_i=B07NN49R4N&psc=1

This provides a very quiet, adjustable dual output up to 30 VDC, boost or drop down. I’ve used it with a leftover 12VDC wall wort that is unusably noisy direct to the opamp, and it provides performance as quiet as a 9V battery. It also works feeding it from the 5V pin on the Pi, but gets noisy enough that you will want to use regulators to quiet it back down again. Just feedback of the noise from the Pi back into itself I guess. It uses an XL6019E1 chip and looks like the circuit is based of an example in the Datasheet.

Do you make your own PCBs? I’ve thought about getting a setup to do that. It probably would be much better, just don’t really have the gear as it is.

Thanks for the info on the MIDI parts. That's useful.

-Ted

On Mar 3, 2022, at 11:42 AM, Randall Reichenbach <randaji@gmail.com> wrote:

Hey Ted,

That's really cool that you've got a dual supply.  Thought about that, but couldn't justify the additional parts count and potential switching noise from the boost.  Are you having pretty good results, noise-wise?  Sounds like maybe not quite there with the perfboard prototyping?  Things get a lot quieter when you get them on a PCB with a nice ground plane and no wire mess.  I did many prototype PCB iterations just to address noise.  We're mixing super noisy digital with super sensitive analog, so what do we expect, right?  ;-)

I also own a FCB1010 ;-)  I guess it's technically a stompbox, but yeah, to work inside commonly available DIY enclosures (eg. Hammond 1590 series), I have to target smaller footprints.  Have you found a source for enclosures that accommodate 4+ footswitches that aren't huge in the other two dimensions?

Regarding optoisolators, I've just used the very typical 6N138 and 6N137.  There are oodles of midi-in schematics online using those.  I use one with 2x 220 ohm resistors and a 1N4188, and a single 220 for midi-out.

The 6N138 is significantly slower than the 6N137 (typical propagation delay of 6N138 is 7us, 50ns for the 137 - that's like 140x faster).  So I now opt for the 6N137.  They were hard to get last year but it looks like DigiKey, Mouser, have plenty now, fortunately.

Cheers,
Randall



On Wed, Mar 2, 2022 at 6:46 PM Ted Rippert via People <people@lists.audioinjector.net> wrote:
I didn’t hit replay all, for this so I’m forwarding it to the list:

Begin forwarded message:

From: Ted Rippert <ted.rippert@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@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@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).  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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