Building a Pi-Rack

[Andrew_S]

Hi all,  I've had a couple of chats with Randall about my idea to build a Pi-Rack, instead of building a pi-stomp.  My reason for doing this is because I'm not a gigging guitar player, I mess about in my studio at home.  I'm also studying for a BSc in Audio Engineering and will be using this project as part of my degree. 

My idea is to take all the exellent work that's been done for the pi-stomp but to then build it into a rack/desk enclosure.  This change will allow the use of more control buttons and switches than have been used in the stomp design.  As I've not built this yet, I've no idea at all what the optimal number of switches/buttons/control knobs should be. Any thoughts from those who have already built a pi-stomp greatfully received!  I'm unlikely to be constrined by physical space of layout in the same way as the stomp is.

I also intend to build it with balanced, probably XLR i/o connections.  I don't know what the input impedance designs are for the stomp, I'm assuming they are hi-Z for magnetic pickups, if not, that's another thing for my design. 

It's my intention to document all aspects of my Pi-Rack build over on my website - www.fuaim.co.uk   ("Fuaim" is the Scottish Gaelic word which translates as "the place of noise, the place of sound".  Undoubtedly I'll be asking questions through this forum as this is the first thing I've ever built using a Raspberry Pi.  Thanks in advance for your help! 

cheers!!

[Randall (Admin)]

Yeah, love the idea Andrew!

pi-Stomp includes a unity buffer on each input with impedance of 1Meg ohm.  2.8Vpp.  So unbalanced hi-Z inputs that will accept up to "Consumer Line Level".

Since you want XLR (balanced) ins and outs, you'd need to add a simple balanced <=> unbalanced converters.  You could use something like these:
https://www.sparkfun.com/products/14002
https://www.sparkfun.com/products/14003

Or roll your own, the schematics and IC datasheet tell you just about everything you'd need.  There are of course others on the interwebs as well.

You'll probably need a bi-polar supply add a phantom power supply too, right?  So then you need to decide where you're going to get that power.  Unfortunately, the 5v rail on a RPi is very noisy.  And ideally, you need at least 10v for +/- 5v.  I suppose you could use a boost converter.  Seems kinda inefficient and noisy to go AC > 5VDC > 10VDC > +/- 5VDC.  If it was me, I'd bring your AC mains into your rack unit, include the pi-Stomp supply inside, and have a separate bi-polar supply also connected to mains.  You might want to put all the noisy power electronics toward one end of the rack unit so their EMI doesn't bleed into your audio.

Regarding controls, you can have up to 8 assignable analog inputs and 5 switch inputs.  That's also the maximum that the current pi-Stomp software is able to display on the LCD.  Then of course, you can have as many MIDI controller inputs as you want.  BTW, this week I'm making available a simple breakout board that easily adds serial MIDI in/out jacks plus an expression pedal input to pi-Stomp Core.

I enjoyed poking around your website.  Good stuff in there.  As a hobby recorder/mixer, I appreciate that you document your whole mix process for your songs.  I've long said I was going to do that for my mixes just so I could remember what I flippin' did

 

[Randall (Admin)]

Another thought regarding bipolar power supplies...

Eurorack (synth) supplies are +/- 12v and meant for audio, so you could get your balanced to unbalanced conversion power plus have 24v (rail to rail) for your phantom power.  I've heard 24v is the recommendation for new designs anyway.

You probably need only about 50mA (10mA for each module + 10mA for phantom) for all of that so the smallest of supplies (often called "Lunchbox" supplies) would probably work great.

Search google, tindie, etc. for: eurorack power   and maybe add "lunchbox"

It seems most of them are super simple. 12v wall wart or 5v USB as input with a 47uF (or larger) electrolytic across the lines into a DC/DC converter (Mean Well brand is common).  If you picked one with 5V input, then you could likely just use the pi-Stomp supply as input.

From my quick morning "research", this converter plus an electrolytic, might do the trick:
https://www.mouser.com/ProductDetail/MEAN-WELL/DPAN02E-12?qs=5aG0NVq1C4xa85tADbZA0A%3D%3D

Hmmm, this has me interested in trying it myself since it would be awesome to provide balanced ins or outs if it was fairly cheap and easy to do so.

[Andrew_S]

Hi there, 

I'm still doing this.  :-)  Just slowly..... 

I'm looking at this - https://www.hifiberry.com/shop/development-kits/dac-adc-stage-development-kit/   I've spoken to HiFiBerry and they'll sell me a single unit, despite saying they've got a minimum order quantity of 100.   

It's not exactly cheap, but it covers all the features for audio i/o I'm wanting to use. If I bought 4 of the SparkFun i/o boards (2x in, 2x out) that would add up anyway.  The HiFiBerry card also allows for phantom powering of condenser mics.  Do I need this?  Not really, but as I'm putting this together with the intention of it being for studio use it might be a neat feature to have if I was micing up a guitar cabinet for example. 

My project will move on next month...  after this next payday...   :-)  :-)

[Randall (Admin)]

Wow, that's just about everything you need.  Presumably they've figured out how to power it all from the pi (or USB) too, unless that terminal block is for external power.  I'd like to know what voltage the preamp board is running at.  Too bad they don't have more specs listed, but I guess half the fun could be reverse-engineering it.

[benh]

I don't know if you guys saw it but there's also https://www.raspberrypi.com/products/iqaudio-dac-pro/ which is rather cheap and has differentials in/out

It's apparently meant to go with a separate XLR board which is sadly nowhere to be found.

[Randall (Admin)]

I wish, but unless you sent the wrong link, that's just a DAC, without an ADC.  Near certain their only board with an ADC is the Codec Zero.  See the comparison table on page 4:
https://datasheets.raspberrypi.com/iqaudio/iqaudio-product-brief.pdf

I received a Codec Zero a couple of weeks ago and plan to try it out this week.  I'll post back here with notes on how it performs.

[benh]

You are right :-( I got confused by the fact that the Zero had inputs and incorrectly assumed they all did

[Randall (Admin)]

I got my IQAudio Codec Zero working today.  The board itself looks the same size as a Audio Injector Zero, but when plugged into the Hat Hacker, it extends almost 1mm further towards the pi-Stomp board with enough overlap between the two boards that I worry about shorting something.  I put a strip of electrical tape between for now.  Might need to make the next round of pi-Stomp PCB's a bit narrower.

Also a bummer that the input and output are on opposite sides of the board compared to the AudioInjector, requiring longer input wires. 

Software wise, all it took was the following in /boot/config.txt:
dtoverlay=iqaudio-codec

I replaced the equivalent audioinjector line, but you might be able to have both.

Then ran:
git clone https://github.com/iqaudio/Pi-Codec.git
sudo alsactl restore -f ./Pi-Codec/IQaudIO_Codec_AUXIN_record_and_HP_playback.state
sudo systemctl restart jack

And it was good to go.

My initial reaction was that it has a bit more clarity (highs) and less noise.  But I'll have to do some side-by-side tests and recordings to confirm.

[Andrew_S]

Still moving on - but very slowly.  I need to get some pace about though.....  (my fault for not cracking on with it)

I've been given an audioinjector stereo soundcard, which I thought I'd use just as a kind of proof of concept for myself, to help me figure out how many buttons, controls, etc I'll need to design in for functionality in my finished Pi Rack.

BUT...... and it's a king sized but... I cannot get the drivers for the audioinjectr soundcard to install. Most probably because I'm a total newbie with the RPi and have made some rookie mistake> I thought I'd get it done when I found this youtube of a guy doing almost exactly what I'm trying to do, using exactly the same audioinjector soundcard (https://www.youtube.com/watch?v=0stTmBmTyHk).  But while I can download the .deb package, whenever I right click on it, it opens rather than installing. If anyone's got advice on this - please let me know!!   

Now, I don't know if I'm doing the right thing or not by trying to prototype my idea using non-standard parts.  Happy to be given advice and guidance  :-) 

In other news, I also picked up (for other reasons) an M-Vave Chocolate bluetooth midi 4 way footswitch over the past month. While it was bought for use with another device I thought it might be useful for controlling the Pi Rack too.  (There's a thread about them over on the MOD devices forum)

[Randall (Admin)]

Don't know what hardware or OS you're working with but assuming you have some Raspbian based OS which includes jack, then your AudioInjector setup should be possible as the github README suggests:
https://github.com/Audio-Injector/stereo-and-zero

If you'd like to use the pi-Stomp tried and true setup script, you can do this:

git clone https://github.com/TreeFallSound/pi-stomp.git
pi-stomp/setup/audio/audioinjector-setup.sh
sudo reboot


If that doesn't work, then describe what exactly is not working. 

[Andrew_S]

Thnks Randall, I'm using the Patchbox OS and have had MODEP running on it.  Using a web browser I could build pedalboards, albeit without any way of actually using them. The Audioinjector stereo sound card came from a fiend wh'd had it kicking around for ages and wasn't using.   

I followed your instructions earlier.  I've downloaded the audio.injector.scripts_0.1-1_all.deb.  The next instruction is to install  the attached deb file.  This is where I'm failing as I can't seem to install the deb scripts from the audio.injector.scripts_0.1-1_all.deb which I downloaded from Audioinjector's git hub.  The youtube video shows that this should be a simple matter of right clicking on the .deb file.  When I do that it doesn't autoinstall, it opens a dialog box which offers me the options of:
Open
Archiver
Open with....
Extract to...
Extract here
Cut
Copy
Move to wastebasket
Copy path(s)
Rename...
Properties


That's where I'm currently stuck .  Thanks for your help. 

[Randall (Admin)]

There's no manual steps to do using the pi-Stomp setup script, but I did miss a step before.  Do this:

git clone https://github.com/TreeFallSound/pi-stomp.git
cd pi-stomp
setup/audio/audioinjector-setup.sh


Hopefully you see the message "The audio injector card is now setup."

So now reboot:
sudo reboot

When you're back try these:
arecord -l
aplay -l

If they list your audioinjector card, then it should be all set up.