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

Thu Mar 3 18:42:18 UTC 2022

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 at 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 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> 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 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
>>
>> --
>> People mailing list
>> People at lists.audioinjector.net
>> https://lists.audioinjector.net/mailman/listinfo/people
>>
>
>
>
> --
> People mailing list
> People at lists.audioinjector.net
> https://lists.audioinjector.net/mailman/listinfo/people
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.audioinjector.net/pipermail/people/attachments/20220303/5ee2572d/attachment.htm>