Control Freak

[Marshall Arts]

Do we need this? Cannot stop thinking about it!

[wgc]

Looks very interesting...

[micromegas]

Do we need this? Cannot stop thinking about it!

I have a similar scheme written in one of my notebooks

[drolo]

Do we need this?

What kind of a question is this .... :-)

[Leevibe]

Yeah, we need it

[m-Kresol]

need is such a strong word 

[Marshall Arts]

OK, after some research, here is what I want to do (in case the above graphic is not self-explanatory) and what I have learned so far: I want to build a digitally controlled pot that can be adjusted with selectable inputs (LFO, pedal, envelope) and that can be used for various effects (i.e. wah, univibe, volume etc.).

• My first approach was to take two LED/LDR pairs and control their resistance via PWM. Someone pointed me to this:
  http://www.madbeanpedals.com/forum/index.php?topic=18698.0
  I built something similar, but I found some drawbacks: LPF for the PWM is tricky (did not get the right op amp yet...), highly non-linear (can be calibrated, though), zero resistance is difficult/impossible (at least in my setup... I get down to like 10k, which is not small enough for e.g. a complete volume shutdown)
  
• With all the problems in the first approach, I tried digital pots (MCP41xxx) again. Someone said, that these make a "zipping" noise, when used (as they are only 8 bit = 256 steps). I therefore never tried it (until now): Now I programmed a small program, that actually swells the volume between zero and max back and forth: Guess what: No glitching, no zipping noise etc. I will further investigate, how that behaves with a pedal and if I want to create logarithmic tapers with it, but so far, this (plain and simple) approach looks promising. Various values are available as well (1k, 10k, 50k, 100k)
• If resolution with a single digital pot becomes a problem, I found a (more complex) solution using three digital pots (or a single quad-pot): Check figure 6 in this article: www.analog.com/media/en/technical-documentation/application-notes/an-582.pdf. It says 12 bit, with 3x8 bit potentiometers, it would be possible to create 16 bit resolution. Furthermore, the finetuning pot (the one on the right side) could be made a 1k digital pot to achieve even more precision/linearity for the inbetween-steps for the other two (100 k each) pots.

Comments/your experience welcome.

Matthias

[drolo]

Digital pots seem a good route to pursue. Although not having much experience myself, I was reading up on different possibilities a while ago looking for some way to build something in the lines of the Mission Engineering Expressionator (without as many bells and whistles).
But lacking time to invest into learning how to program etc, I settled on this for now ... It's ok though as the pedals I need to control at the moment work well with CV voltage and don't need an actual pot.

I am pretty certain that expressionator is using digital pots. The only gut shot I have been able to find is this one of a prototype:
https://www.talkbass.com/threads/coming-soon-the-expressionator.797290/

I thought this thread might be interesting for you as well:
http://www.diystompboxes.com/smfforum/index.php?topic=106448.0

[Marshall Arts]

Digital pots seem a good route to pursue. Although not having much experience myself, I was reading up on different possibilities a while ago looking for some way to build something in the lines of the Mission Engineering Expressionator (without as many bells and whistles).
But lacking time to invest into learning how to program etc, I settled on this for now ... It's ok though as the pedals I need to control at the moment work well with CV voltage and don't need an actual pot.

I am pretty certain that expressionator is using digital pots. The only gut shot I have been able to find is this one of a prototype:
https://www.talkbass.com/threads/coming-soon-the-expressionator.797290/

I thought this thread might be interesting for you as well:
http://www.diystompboxes.com/smfforum/index.php?topic=106448.0

Wow, thanks a lot! You seem to really have researched a lot on this topic! And others seem to have progressed quite a bit! Has the Expressionator ever been made available?

[drolo]

yep
http://missionengineering.com/?product=expressionator-2
a bit on the expensive side but it's a cool device

[Coda-effects]

With all the problems in the first approach, I tried digital pots (MCP41xxx) again. Someone said, that these make a "zipping" noise, when used (as they are only 8 bit = 256 steps). I therefore never tried it (until now): Now I programmed a small program, that actually swells the volume between zero and max back and forth: Guess what: No glitching, no zipping noise etc. I will further investigate, how that behaves with a pedal and if I want to create logarithmic tapers with it, but so far, this (plain and simple) approach looks promising. Various values are available as well (1k, 10k, 50k, 100k)

Are you working with PICs ?

I am currently trying to learn how to use digipots (in order to have presets by saving values in the PIC EEPROM), but it is not very easy for me to see how to implement it in the code.

Thank you !

By the way I think it could be very cool to have a tap tempo in order to synchronize several pedals together with multiple expression outputs to what you are currently playing... (currently imagining a mess caused by a synchronised delay time / tremolo rate / vibrato rate expression waveform haha)

[Marshall Arts]

yep
http://missionengineering.com/?product=expressionator-2
a bit on the expensive side but it's a cool device

Nice, indeed! I like the feature that it freezes pedal settings when I change the target-device: I see Stuck-Wah-Volume-Swells or Reduced-Volume-Wah-Usage ;-)
In the tech specs, it says that it has 20k Outputs, which is a downer for me: I want to control a wah (100k ICAR), volume in the fx loop (10k log) and a Voodoo Vibe (100k lin or log) with it.

If a single digital pot plays nicely as a volume control with a pedal (will find out soon), I might be able to build something with similar features. It might look like this, though: http://www.diystompboxes.com/smfforum/index.php?topic=113990.0 - Taprecise already contains two digital pots and an arduino Nano ;-)

[drolo]

those AD5206's used in the DIYSB link I posted earlier might be nice for that.
6 pots in a box and the datasheets says: Terminal resistance of 10 kΩ, 50 kΩ, 100 kΩ

[Marshall Arts]

Are you working with PICs ?
I am currently trying to learn how to use digipots (in order to have presets by saving values in the PIC EEPROM), but it is not very easy for me to see how to implement it in the code.
Thank you !
By the way I think it could be very cool to have a tap tempo in order to synchronize several pedals together with multiple expression outputs to what you are currently playing... (currently imagining a mess caused by a synchronised delay time / tremolo rate / vibrato rate expression waveform haha)

I am working with Arduino (see my last post from 5 mins ago). As I like to keep stuff open source, I usually take a nano and solder that on a custom PCB, so people can adopt the solutions more easily (comes with USB), tweak code, play around with it. Highly recommended to try Arduino!

[Marshall Arts]

those AD5206's used in the DIYSB link I posted earlier might be nice for that.
6 pots in a box and the datasheets says: Terminal resistance of 10 kΩ, 50 kΩ, 100 kΩ

Yes, but as far as I can see, you dont get different resistances in one package. So, I'd rather go with sockets and three individual digipots ;-)