Digipots and guitar pedals

kat 2017-10-08 13:55:16 UTC #1

This is relocating a reply to a question from @Gavin in this post.

For a while I have been in pursuit of "expression control for everything" with guitar pedals. One solution that I have been experimenting with is digipots controlled by a Teensy. In reply to @Gavin's question, yeah, this totally works. In my opinion, it doesn't make a lot of sense to use Axoloti for control, because Axoloti has so many other more powerful things it can do it seems a bit "wasteful" to me. Teensy is cheaper and smaller and can be built into a little self-contained controller or even fit within a standard guitar pedal enclosure.

I did write up a little how-to on getting the digipots to work here. What I haven't written up yet is the part of connecting this to MIDI control, which is also pretty easy with the Teensy, and then makes it possible to use an off-the-shelf MIDI foot pedal/expression pedal to control analog pedal parameters.

None of this is all that novel. Someone who does a much more sophisticated version is this guy, whom I have not seen live but have heard about from others.

My challenge, now that I have something that does what I thought I wanted, is figuring out what I can do with it that is actually musically interesting. Some of my initial experiments are pretty underwhelming, but I haven't pushed it very far yet. Would love to hear if anyone else has explored similar projects.

Starting points for guitarists

Gavin 2017-10-08 23:06:14 UTC #2

Actually, my ideas go beyond just replacing the pots in an effect pedal with digipots.
I am also looking to do a kind of Pedal Looper Board, instead of having switches for every connected pedal in the chain, you have a digipot and you control the amount of audio into each pedal, kind of like a fader for every pedal. The reason for this type of setup, it was the closest idea I could come up with instead of creating an Axo with an additional audio in and out for every external pedal you want to use. This way the Axo or any other dev board could easily be used to control the pedals parameters and how much of the pedal to use. There are even more complex methods like that of a "Matrix Mixer" that could be employed, but could be rather overkill.. But it does allow for your effects chain to vary between serial and parallel Audio chains.

In my own thoughts, the down side to digipots, is you are limited to how many can be controlled. If I have a pedal loop with say 4 pedals, each with 3 pots and a fader to control it, I would need 16 digipots, and I have not yet found a way of controlling 16 digipots with one dev board.

So I have also been looking at alternatives. There is even a post in here somewhere where I do a proof of concept test turning a pot on a pedal with a piece of cable that could be connected to a servo. I have even wondered if a servo could be modified to use the pot inside it to replace the pot in the pedal. Unfortunately nothing yet comes to mind how to do this.

Getting back to the digipots, you make the comment, that your initial experiments are underwhelming, and I have heard on other sites some people replacing pots in pedals with digipots, but no details as to how they control it etc, but they also did not like the results and ended up killing their projects.
So I do wonder if your underwhelming results maybe something similar ?
Or even if there is more of an expectation that it will be more than what it already is ?

kat 2017-10-08 23:58:09 UTC #3

I know (e.g. from posts like this) that there's lore about making sure the digipot works in the circuit properly. I have not paid any attention to this, and have just set up switching jacks so that when I plug in my digipot it substitutes in for one of the pots already in my circuit. I do not notice any difference in the sound between the digipot and the analog pot when I use them to do a simple sweep.

So I don't think my underwhelming results are because of not getting the circuit right... it sounds just fine, it just isn't that exciting. So far I have just been playing with modulating the tone and fuzz controls on fuzz pedals. Mostly what the ear picks out in this case is the changes in volume, even though the tone is changing. Thus, modulating the parameters ends up basically sounding like a tremolo on top of a fuzz, which is hardly a new sound. In principle playing around with analog presets is another nice thing I can try but honestly I have been more curious about coding up modulation patterns, so I haven't fully explored what presets would let me do.

I do have a song I am recording right now in which I might use a glitchy control sequence on the fuzz of my 'big cheese' clone. Definitely I have more control and more things I can do than without the digipot hack. It just isn't as profound as I might have hoped... and I think the reason is more about the way the ear responds to changes in tone rather than anything about the tech setup.

I've gotten a little bit cynical about what there could be that would be truly "new" in the world of guitar sounds... the field has been explored pretty intensely because it is so commercially profitable. Not saying I am giving up, I just think it's hard to find anything that is genuinely surprising in the world of guitar sounds...including digitally controlled analog effects.

Gavin 2017-10-09 04:31:48 UTC #4

Doing something new, has got to be almost impossible...
But being exposed to the Synth way of doing things through the Axo community, (which I am sure I still have not grasped the basics), I do believe there are ways to use the techniques as more of an entire performance rather than a specific effect, but I am also looking at it in terms of being more dynamic and responsive to what I am playing the time. I am probably getting over my head a bit here, but when I watch video's of some of the members controlling their Axo with some kind of a device with pads, they tap the pad and something changes etc, I want to do this with the switches on my build, and if a digipot can make a relative adjustment to an external pedal as part of all this, all the better. And instead creating triggers with notes like you would with midi or something else, things could be triggered by the shape of an envelope you record into a loop, no 2 recorded envelopes will ever be exactly the same, so their will always be an element of adaptive improvisation.

Back to digipots, (I do go on a bit..)
The other thing I have heard of people using instead of a digipot is a DAC ic. But very little stated about how it went. I know both digipots and DAC's aim to achieve similar results so there is some crossover.
Is this something you have looked into ?

kat 2017-10-09 14:09:23 UTC #5

My guess is that Axoloti on its own is the tool for most of what you are describing... at least that is the approach I would personally take (and do personally take). The one thing that is still limiting my own experiments with more dynamic sound processing is not being able to do reliable real-time (and, in a dream world, polyphonic) pitch tracking, but that's a topic to explore in another thread.

I do think the digipot approach will help you get to that kind of dynamic control if you need external effects for things that Axo can't do... and it is pretty easy to set up, so it is at least worth a try. But my own opinion would be to do this on a trial/demo level before designing a whole system around it. Again, because the things that seem really cool intellectually don't always sound as cool. You may find only one or two pedal parameters that are worth dynamically changing, and maybe it doesn't require having dozens of digipots...maybe just two.

I have also set up a DAC with a Teensy and it is also really easy. I have been collaborating with some sound artists who work with modular synths to make a little Teensy-based MIDI-to-CV and CV-to-MIDI controller and we are using MCP4922 DACs for this. But I don't see what it would buy you in the guitar world since guitar effects are not typically controlled by input voltages.

One of these days I plan to write up a little how-to for getting the DACs wired up with a Teensy, and getting the MIDI part of this set up with the Teensy... but probably won't prioritize either of those in the short term unless somebody would find it especially helpful and gives me a push to do it!

orbitbot 2017-10-09 20:06:21 UTC #6

The i2c bus should be able to do this, if I've understood things correctly. You need to configure an address for each individual potentiometer (see relevant datasheets for the digipots you might be looking at), and there's an upper limit to how many you can control / bus (might have been 16?).

That'd be either midi CC messages in the digital domain, or Control Voltages in the analog.

Digital to analog converters are used to f.e. convert PWM (eg full and low voltages oscillating at a set frequency) to a continuous analog signal (such as 2.4 volts in a range of 0-3.3V). So that's how you'd get a control voltage, some microcontrollers have actual DAC chips or otherwise breakout boards are quite cheap (around 1$ / piece from China or 5x from somewhere closer). The Axo itself should have some analog out pins, you can probably search the forum.

Digipots are variable resistors, DACs give you voltage. Looking around the guitar pedal world, there's a bunch of stuff that's using LED/LDR to convert voltage to resistance, which is also available as ready-made components in the form of so called vactrols. F.e. the Chase Bliss line of pedals are completely digitally controlled using vactrols.

There's a brief comparison of approaches in this thread http://www.madbeanpedals.com/forum/index.php?topic=26504.msg256920#msg256920 , and f.e. https://www.maximintegrated.com/en/app-notes/index.mvp/id/1828 discusses issues with gain scaling.

Take the above with the appropriate grain of salt, I've yet to actually implement some of my master plans, which in the guitar pedal domain mainly involves presets or adding oscillators to control pots As for novelty, there's little new under the sun, but a lot of less-trodden paths.

Gavin 2017-10-11 00:20:23 UTC #7

Thanks All, yes indeed, I need to plunge into the deep end with a small test, being able to control at least two pedals at a time is imperative. I think I will start very simple with only controlling the amount of Audio passing into the pedals in the pedal chain, this will only require 2 digipots, 1 for each pedal, but may also require an op-amp setup of sorts, but I will work all that out as I go along.

kat 2017-10-11 11:07:58 UTC #8

Thanks for the links to add to the discussion, @orbitbot. That gain scaling discussion is a useful reference and I suspect I'll be back to look at it again in the future also.

If you have ever tried the Chase Bliss pedals, I'd be curious to hear reactions. I've never played one but I've been impressed by what I could tell from the standard youtube demo videos and whatnot.

And @Gavin, keep us posted on your project... I also have an op-amp "splitter-blender" (routes guitar signal to two separate loops with independent gain control) sitting on a breadboard semi-abandoned because I didn't end up creating any sounds with it that were all that exciting. I liked the idea of blending together two effects in a weird way, but then everything I initially tried just sounded like audio mush. I have somewhat the same experience trying blending together various processing paths with Axoloti. The ear (or at least my ear) does not seem to be as responsive to the complexities of sound processing that my brain finds appealing! But maybe I just have not been creative or smart enough to come up with the right ideas yet...

orbitbot 2017-10-11 19:05:15 UTC #9

I haven't had the pleasure, and though I think they're cool, for that money (at least herewards) I'm more looking towards even more comprehensive effects . Likely the Source Audio Nemesis and upcoming Ventris as of today, probably buying these within a few months since they seem like definite keepers and value for money.

Gavin 2017-10-11 21:32:52 UTC #10

I've done the splitter blender thing before, well before my Axo adventures, I actually had 3 parallel effects loops of pedals chained together, I had a case as big as a bass guitar case to hold it all together, quite cumbersome. I know as a bass player it definitely turned heads, I had far more effects then the guitarist but there was something about it that worked well. But it was difficult to use, having to change levels in loops, I tried different setups with volume pedals etc. I think it can still be achieved effectively, but would want it setup in a matrix mixer controlled by a microcontroller of sorts.
I never chase specific model or pedal types, too expensive for me, I just by cheap second hand ones from pawn shops, its like a lucky dip, you never know quite what you are going to end up with. I did specifically buy a new zoom B3 though.

timeorspace 2018-02-21 23:13:09 UTC #11

The beauty I see with digipots inline with regular pots and an arduino is the ability to save parameters into groups of presets that can later be recalled and used or tweaked with the manual knobs. I've worked on a mock up of this approach which I plan to take further incorporating an lcd screen and LEDs on each knob so you know whether the current setting is above below or on the preset value. Also, no change to the preset value until you go past its set value. I hope I explained this in a way so it is understandable.

Gavin 2018-02-22 03:16:38 UTC #12

Yep, I thought about similar as well, in my setup, I have 64 preset values for each patch managed through and Arduino, but as I have access to 127 of these, I was thinking of adding extra to control level on the pots of the effect pedal, not only this however, to also incorporate how much of the effect to apply to the audio signal, so the signal will split, one portion to the effect, the other without, by controlling the % split, I control the amount of the effect.
Controlling the amount of effect seems very workable, controlling the pot level on the pedal is different because my preference would be a kind of retro fit system, so it could easily be adapted to other pedals. Although I am always thinking about it, it is low priority at the moment, heavily absorbed in making expression pedals, hope to have some pictures soon.

kat 2018-02-22 16:15:04 UTC #13

Hi @timeorspace and @Gavin ... glad this continues to be an active area of development for you both. I will share that I ended up going in a rather different direction with my digipot explorations, but one that turned out to be exciting (to me anyway). I have an analog fuzz pedal that I made that is built with switching jacks so that I can swap in a digipot instead of the fuzz knob. What I discovered after playing with it for a while was (1) modulating it with choppy rhythm sequences made something really interesting and new to my ears, and (2) modulating it at audio rates makes the circuit do some strange synthy stuff that was a complete surprise. So rather than using the digipot for saving settings, I am using it to push an analog fuzz circuit into weirder behaviors that for me have musical qualities.

I am going crazy at the moment with a big project at work that will be done March 1. But after that, I am planning to pull together some audio clips and post some updates to the little write-up I have on my website. The digipot stuff has turned out to be more interesting than I was expecting, and I hope to keep experimenting.

orbitbot 2018-02-25 23:42:06 UTC #14

@kat That sounds interesting, do you have a recording setup to share what it sounds like?

Wrt to the other things, I'm probably going to completely separate the control of my axoloti with an ESP-32-based board firing MIDI controls at the patches and hook most things up to the ESP-32 (mainly to try to have access through wifi/bluetooth to see if it's actually possible...) Got most of the parts, but need to get cracking on soldering and programming at some point.

Blindsmyth 2018-02-26 00:28:51 UTC #15

He folks really interesting topic! I use the axoloti very often as guitar effects unit, usualy in combination with some kind of preamp and other pedals.
I would love to have some axoloti based digital controll over my palmer pocket amp (and maybe more pedals). More concrete I would love to change the drive setting switch (wich has 3 different settings) and the on/off from my axoloti. Since this is all a combination of on/offs I think I could do this with a relais.

Do any of you have recomendations for a relais that I could use for this kind of purpose? Preferably it shouldn't be too pricey.

Also this matrix mixer Idea of @Gavin sounds really intriguing. At the moment I am using my rme ucx in standalone and midi controlled by axoloti for this kind of purpose but having several chains always involves going through the dac/adc so I would love to outsource part of the routing process into the analog world.

Gavin 2018-02-26 01:50:07 UTC #16

The main idea comes from what I think would be an improvement on what we call at the moment, pedal loopers / switches / routers etc.. they are quite pricy, you can setup presets of what pedals are on or off etc.. but to me this is just a convenience, not something that improves what you are doing. Then when I discovered matrix mixers, I thought this would be pretty cool for pedals, some people have already done this, but there would be an awful lot of adjustments for every setup. So then it dawned on me, something in between both systems, a pedal looper like setup, with digipots or some other IC like the PT2257 that can split the audio by % to the effect and or away from the effect, controlled by midi from the Axo. The PT2257 IC could actually be the icing on the cake for this sort of setup. If possible to add the ability to switch the effect to either serial or parallel would also be a target.

lokki 2018-02-26 08:03:33 UTC #17

these pop up infrequently on eBay:

the shit if you want to mix audio/midi.

or:

http://www.deepsonic.ch/deep/htm/360_systems_audio_matrix_am16.php

also there was a schematic online for a matrix chip solution with osc control, but i cannot find it right now.

as for controlling pedals digitally there are multiple ways, depending on the use case:

if you replace a switch, a relay works, but is expensive and often a simple 4066 solution can work:

http://www.thetonegod.com/tech/switches/switches.html

for pots the situation is more complex.

as soon as audio passes through the pot (which happens a lot in guitar pedals) you have to be very careful. digipots can usually only control signals in the supply voltage range, so you have to make sure your signals stays within bounds. sometimes a vactrol based approach is better suited than. basically a led and a phototransistor acting as a variable resistance... this was and is used in a lot of tremolos and compressors.

Blindsmyth 2018-02-27 01:16:28 UTC #18

cool thanks for the article! 4066 sounds indeed good and is cheap. Would be awesome if I could hack my pocket amp into a pseudeo 3 channel amp, digitally controlled.

All these matrix solutions look very capable but are waaay to big and heavy for the 21.century imho.

In that sense I'm really happy about using the RME UCX. It's software even looks like a digital matrix mixer and it can be controlled via midi. It does have eq+compression on each channel and I can record each channel either on my laptop or my phone (when it's in class compliant mode). Only downside is that each loop runs through dac/adc, allthough that one is really high quality in that case.

Gavin 2018-02-27 04:59:48 UTC #19

4066 solution is a little week for my goals, and the ready made audio patch bays seem to a bit of overkill.
Further reading I have found IC PT2258, 6 ins, 6 outs, runs off as little as 3 volts, communicates via I2C. Although I think it has 2 fixed address options, I can run two chips which will allow for audio volume level routed either to a stomp or bypass, this should allow for 5 stomps to be controlled. Should be able to control with Arduino receiving Midi signals.

Gavin 2018-02-27 05:54:02 UTC #20

Correction, operates at 5 volts and up, and has 4 fixed address options.