Fun CV control "kludge" for WTPA1 users

Went up to my studio during my lunch hour today - had time to open up a new shipment of VTL5C3 vactrols and alligator clipped one to the center pin of the course pitch control. Hooked up my modular synth and there you go, CV control kludge! Not linear at all but very controllable. I've been adding these to various pieces of gear where voltage controlled resistance can be used to do some elementary "modularizing."

This CV control could never match the accuracy you get with MIDI but it is fun for adding strangeness and modulation.

Will post diagram/photo/audio when I get it all soldered, fitted etc.



  • edited February 2012
    I did a more "serious" CV in a while ago using a CD4046. I told Todd I'd post it a while ago and never got around to it.

    Cleaned up the schematic a bit today, so here it is

    The SW- connectors are for attaching an SPST switch to turn the CV effect on/off (by killing power to the 4046). The Power should route to pin 16 on the 4046 (via SW1-2, if you're using the switch). The other connectors should be pretty self-explanatory as to where they connect on the WTPA PCB.

    I built the circuit on a little prototyping board.

    You can hear it in use on this track:

    Hopefully this makes sense.

  • Great work, guys.
    WTPA2's VCO is done with a 4046, although the topology is a bit different.

    One question re: the 4046 VCO -- I assume the diode is in the VCOOUT path to allow the 4046 to be turned off without the WTPA's square wave reverse biasing that pin on the 4046.
    But, do you not get a condition where the 4046 can send the clock pin HIGH but not LOW? Check it:

    You might be able to buffer the output with a complimentary MOSFET pair. When the power went off, neither one would be on, and when it was on, you'd drive the clock pin strongly. You could use a 2N7000 and a ZVP2106A.

    Let me know what you think,
  • Yeah, the diode was pretty much intended as a protection mechanism. The MOSFET pair is a good idea- if I revisit the circuit I'll give that a try.
  • edited June 2012
    Interesting effect going on in the audio demo!
    Reminds me alot of the sound of Tom Whitwell's (tombola on muffs)
    project for a cv controlled radio here:

    Tom has a coupla demo's of the results on Soundcloud.
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