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one expression, 4 pots


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  • one expression, 4 pots

    ok, here's what I'd like to do:

    for a large device, I've got 3-4 different types of pots that I'd like to be able to assign to a foot control. The basic idea is, have a 4-position selector switch (ignore the 3T4P for now) and have it determine which function is on the foot control. My issue is: there are at least 3 different pot values involved: 100k-lin, 100k-log, 10k-log, iirc. Since I doubt I'm going to find a ganged, 3-value, mega-potentiometer...I was thinking of using a CV-type pedal to trigger some sort of programmable resitor  device... problems I've run into so far are:

    a) most programmable resistors (well, the one or two I've looked into so far) have 256 "positions" which might not be enough to allow for smooth operation, especially when emulating the "log" pots.

    b) keeping it simple....so far, the programmable resistors I've found operate on digital input. Almost midi-like Master Clocks and 4 bit control streams and....sheeesh...the support electronics gets ugly fast.

    c) My ideal would be to find a chip/system where I could set a couple bits, give it an analog (CV) voltage, and have it present an "analog resistance", i.e. have a smooth varience in resistance as my CV voltage varies.



    Is there any pre-designed, known, cookbook-type circuit that I could plagiarize incorporate into my project?


    oh, btw, the project is a Tau Pipe

    Blind Doc Jones' Pickles....Cures What Ails Ya

  • #2

    no one with any ideas yet?

    huh...kinda surprised. You'd think this kind of thing would have been done long ago.

    Anyway, I think I have my own answer, fairly elegant and once I've get the 'log' part sussed, it could be useful for a lot of effects. I'll let you know more once I get it on a breadboard and running.

    Blind Doc Jones' Pickles....Cures What Ails Ya


    • Kazinator
      Kazinator commented
      Editing a comment

      lefort_1 wrote:

      no one with any ideas yet?

      You only posted 9 hours ago, and this place is pretty dead. There are more busy electronics forums like diyaudio and whatnot.

      There indeed exist logarithmic digital pots. For instance Maxim makes a few.

      Shopping for pots is frustrating, if you want to deal with decent voltages. Most digital pots do not admit voltages on any of their inputs which are outside of the power supply range, and the power supply ranges tend to be limited (as little as 5V, single supply).

      If you are not picky about voltage headroom, then things look better.

      Some great parts have been long discontinued, like those nice Toshiba chips used in the ADA MP-1, that can handle +/-15V power rails.

      There is also consideration about the protocol. Some chips speak I2C, others just a serial two wire protocol. Some of the serial two-wire ones, however, can be hooked up in series.  They behave like shift registers: values that are shifted in are shifted out. So, say, if you need four digital pots (and they are 8 bits wide) you can hook up your board so that they behave like one 32 bit register value that you program by shifting in 32 bits at a time.  A separate signal indicates that the value is shifted in and should take effect, so everything changes simultaneously.

      One potential issue with digital pots is "zipper noise" when the values are changed, and one way to deal with it is to have a zero crossing detector, and update the values precisely when the signal is going through zero, so there is no step in the signal.  Some digital pots have an on-board zero crossing detector.