Author Topic: How realistic is it to try and use the pi-Stomp as vocal/acoustic guitar fx  (Read 1673 times)

jvphotog

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I see there's two threads where members try to add a mic to the input of the pi-Stomp and there's a PB over on the mod devices forum that has decent vocal fx.
Would it be worth it to try and build something? Decent vocal/guitar pedals are around $300 so I wouldn't want to spend more than that. I would really only be interested in compression, eq, reverb, and delay. I wouldn't need midi or foot controls. Just something like a Fishman Loudbox. A mic preamp and a guitar input. Looks like the guitar input is already covered. And since this is stereo, one channel can be used for guitar, and the other for a mic. I've read through the FAQ and see that there is some suggestions for different sound cards - which might be a good idea since I think somewhere it also said that this design could be a little noisy.

Anyways, just thought Id ask if you had any suggestions. Im building a powered PA speaker and it would be awesome to have the mixer built in!
Thank you!

Randall (Admin)

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Processing-wise, I say with 350+ plugins, you'd have all you need and more.

The pi-Stomp inputs are buffered (1Meg impedance, "Instrument" to "Line" level).  Those should work for most acoustic guitar pickup outputs, but not ideal for mic inputs.  You'd most likely want a proper mic preamp.  Either an external unit (often those are expected to be phantom powered), or a simple small board to add within your pi-Stomp enclosure.  The issue you'll probably face with the latter is powering such a preamp.  Maybe you could find or design one powered via a unipoloar 5v supply, but most expect a bi-polar supply.  Please check out Rene's thread where he's building such a thing:
https://treefallsound.com/forum/index.php?topic=64.0

Regarding the rest of pi-Stomp.  If you have no need for controls or an LCD, you could save some money using the 99EU pisound:
https://blokas.io/pisound/

Same deal with that one regarding inputs except with 100k input impedance, you'd probably want a buffer or preamp for whatever you plug into it (even a guitar).

Cheaper yet, you could probably just have an RPi and an soundcard (and whatever preamp you choose).  For best sound/noise, I'd suggest the IQ Audio Codec Zero (I'm retooling pi-Stomp to be based on that). 



jvphotog

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Wow Randall, thank you for the detailed response, I appreciate it!

As luck would have it my friend had a spare rpi 3B v1.2 that he lent me. I was able to get Patchbox and MODEP running on it. I ordered an audio injector zero and got the outputs working. I couldn't get the inputs to work, not sure what the problem is.
I searched for a while and couldn't find any board level mic pre amps. This would have been the easiest option.

I ordered a Behringer UM2, a usb audio interface. The pi sees it, I can get vocals from a mic, and guitar in and out of the UM2 - with compression and delay from MODEP, BUT no matter what jack settings I try, I always get weird digital noise and pops and latency that would be iffy in a live situation.
Apparently the UM2 cannot be used as a stand alone preamp with analog out (incase I got the audio injector inputs to work). So I guess Ill be sending that back. $32 for a mic and guitar input seemed like a good deal.

Im not sure what to try now! Maybe work on getting the audio injectors inputs to work...
I love the pi-stomp. Ill probably get one eventually but I want to finish this project first.  Or maybe ill get one and try a xlr balance/terminator thing into the stomp and see if that's usable...

Randall (Admin)

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Yeah, USB audio interfaces rarely, if ever, have analog outs.  You can often pick up a cheap "mic preamp" on reverb.com.  I see plenty that would work from Art and Presonus that are less than $50.

Regarding PCB preamps, there are many simple ones you could build (I could send links if you want).  If you would power it from the same supply as your pi, the problem you'll likely run into the same as you found from your UM2... Raspberry Pi Power is F'n NOISY!  Your design has to have a very meticulous ground scheme, regulation and filtering.  And like I said before, you'd probably want to convert the 5v unipolar supply to a bipolar higher voltage supply for a mic.  That means adding a DC/DC converter, guess what?  Those can add a lot of noise.  Using an external unit which presumably addressed power/noise issues already, saves you a lot of headaches.

Regarding getting your AudioInjector inputs to work... Yeah, there are a number of things that can cause that.  You might be able to try some things in the pi-Stomp troubleshooting guide since most of those apply for any pi+audiocard hardware:

https://www.treefallsound.com/wiki/doku.php?id=troubleshooting_guide

If none of that works.  Let me know, and I'll try to help you.  It could of course just be a bad card.  Out of roughly 100 pi-Stomps built with AudioInjectors, two have, so far, been defective.


jvphotog

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Some success! I reloaded Patch OS and used the manual method of enabling the audioinjector (which is written out on the last page of the set up thread on the audio injector forums). Audio in and out works great now and is much faster than the USB interface. Plugging my acoustic guitar straight into the audio injector's input even seems to work fine.

All of those cheaper used mic preamps seem to have tubes in them. This will be installed inside a PA amp and I definitely don't want to mess around with that.

Im not sure what my next step is. The amp runs off of a Mean Well 36v 10A power supply - which supplies the amp/dsp for the speaker. I've been looking at some bipolar ~40vdc to 15v and 5v convertors to power the pi and any preamp. Some mixed reviews on amazon. But Im having a hard time finding a solid mic pre that isn't too expensive. A buffered high z input for the guitar would be nice too. Too bad that cheap behringer didn't work out!

I've read through Rene's thread! I think id rather buy a pcb and just populate it. I've been searching for ideas! I don't know if something like this would work for the mic: https://www.electronics-lab.com/project/tiny-proffesional-microphone-preamplifier/

ReneMadeira

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Hello, jvphotog.
Glad you read my thread, I hope it was helpful for you.
I didn't have time to write my progress there yet, but I've already finished the mic preamp of my project, and I'm quite pleased with the results.
The preamp I used is based on typical applications from Burr Bown's INA217 datasheet.
It's referred to in many places as "the famous $5 preamp", and you can read more about it here: https://www.audiomasterclass.com/blog/the-famous-5-preamp-everything-you-need-to-know
I bought a PCB for it from the following link: https://www.ebay.com/itm/114326622246. I just populated with the components, as you intend to do.
For the positive/negative psu, I bought a small board called DD1718PA, you can find it in several places. It is very cheap and I was impressed with it. To take care of the noise, I just had to put a 10mH in series with the Vin, which was taken from the 5V that I used to power the pi-stomp.
I believe that the board that you linked will work similarly, as it is based on the same opamp.

This project is also worth mentioning: https://sound-au.com/project122.htm
Building it would be even cheaper, and I started designing a circuit for a universal board, which I provide attached. Please mind that I didn't build it, it is just a concept. If you try it, please let me know how it works.

Randall, I've provided some links here, please tell me if you want them removed. I only intend to help, there are no affiliate links at all.

If you need further information, I'll be glad to help.

Regards,
Rene
« Last Edit: July 18, 2022, 11:43:02 AM by ReneMadeira »

Randall (Admin)

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Good stuff Rene!  Thanks a lot for sharing that.  The vision for pi-Stomp has always been a platform to foster hacking, modding and innovation.  So yes, please post whatever ideas and links you have.

It looks like your "universal board" is based on the sound-au design, correct?

Did you find any need to regulate the output of your boost converter or just the inductor?

I hope to find the time to try out some of this stuff myself.  Such a thing would be a great optional add-on.  One feature to make it more universally useful would be phantom power.  It would be nice to not require yet another boost converter to produce the high voltage.  Have you seen any supplies that offer both a bi-polar (12 or 15v) plus an additional high voltage (30+)?

With all the pro-sumer gear that includes mic inputs with phantom power (eg. USB audio interfaces), it would seem there's a common solution they tend to use.  My google searches haven't been successful yet, but maybe I'll just rip some piece of gear open to see what they do.

ReneMadeira

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Hello, Randall,

It looks like your "universal board" is based on the sound-au design, correct?
Yep, the "universal board" is based on the sound-au project. It was actually a bad translation, and by "universal board" I was trying to say it's for a perf board.

Did you find any need to regulate the output of your boost converter or just the inductor?
I tried many things on the input and output of the boost converter, including regulating the input with the AMS1117 and the outputs with LM7812 and 7912. The only thing that made a difference was the inductor at the input. It is worth mentioning that the DD1718PA has many versions with outputs ranging from +/-5V to +/-24V, and I bought two of +/-12V and two of +/-15V. The latter is what I used with the regulators.

Have you seen any supplies that offer both a bi-polar (12 or 15v) plus an additional high voltage (30+)?
I haven't found any board that has all these output options built in, but since the requirements for a preamp and phantom power are very low in terms of current, even two cards would take up little space. In the case of an addon, I believe the best would be to make a board with everything (preamp and converters) included, I may try to do this in a perfboard in the future, after finishing my project.

Thank you!

Rene

jvphotog

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Rene and Randall, thank you so much for the replies! And reading through your thread was very helpful. I ordered a breadboard kit and am waiting parts from mouser! I also found a 15v bipolar supply on ebay that im going to check out. It seems like that's the biggest hurdle. I don't need phantom power, so I don't need to worry about 48v.... but most usb interfaces have that option. I wonder how they power it?

Randall (Admin)

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Very cool.  Please let us know how your build turns out.

I also recently purchased a bi-polar 15v supply board.  Not sure if I'll use the preamp PCB that Rene did or roll my own.  I would like to have phantom power.

You might have read that a full 48 volts of phantom power is not required by most condenser mics.  This article:
https://acousticguitar.com/everything-you-need-to-know-about-microphone-phantom-power/

says that the ANSI standard (IEC 61938) includes phantom power as low as 12v.  Then says:
"Most mics work fine over the voltage range of 12-48 volts, although many mic manufacturers specify 48 volts for best performance, and some mics may be more problematic than others when encountering lower levels of phantom power—you may notice some loss of output or sensitivity with lower voltages. In any case, a lower voltage won’t damage the mic, so you can always see for yourself if a lower voltage will work."

So, I'm thinking 15v might be just fine for most pi-Stomp users.

This article:
https://www.instructables.com/Build-the-Four-Channel-SSM2019-Phantom-Powered-Mic/

explored that a bit and added 15v phantom from their bi-polar supply.  This makes practical sense to me.  My plan is to use my variable bench supply wired similarly, attach the condenser mics that I have, and do some test recordings/measurements with 15v vs 48v.  If 15v seems just as good, I'll just use that positive rail from the bipolar supply that I use for the preamp.  Two birds, one stone.


ReneMadeira

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Hello, jvphotog!

I am working on a full preamp with phantom power on stripboard.
If you have DIY Layout Creator installed, you can open the phantom power section on the attached file. (check out DIY Layout Creator on google, is a very cool open source tool for DIYers).
The XL6007 must be put in a DIP-8 adapter.
Again, I did not test it yet.

There is also project 193 from sound-au (https://sound-au.com/project193.htm), it says that it could power up to two microphones from 5V.


jvphotog

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Thank you for the links!

Just fyi, I successfully bread boarded a mic preamp and powered it using this CINCON bipolar supply - supplied from a 5v pin on the pi. I DID get a low voltage warning but it went away and didn't seem to effect anything. I also didn't hear any noise - I compared it to an external wallwart power supply.

https://www.ebay.com/itm/392688097203?_trkparms=amclksrc%3DITM%26aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908105057%26meid%3D9b4fd128701d4a39a5e8f9d3d05c018a%26pid%3D100675%26rk%3D3%26rkt%3D15%26sd%3D114326622246%26itm%3D392688097203%26pmt%3D0%26noa%3D1%26pg%3D2380057%26brand%3DCincon&_trksid=p2380057.c100675.m4236&_trkparms=pageci%3A500a79f0-17fa-11ed-bcf8-feac809dfa30%7Cparentrq%3A834868e31820a60c134fa8a9ffff7398%7Ciid%3A1

ReneMadeira

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Very nice!
I had a hard time dealing with noise in my preamp, but in the end the problem was something in my raspberry pi that was injecting noise, and the noise was gone when I swapped the board.
I tried a similar bipolar psu, but it did not have common ground.
Are you satisfied with the sound quality of your design? Which chip is it based?
Regards,
Rene