Remote/Rack Wah – Getting Started

Posted: 2011/12/20 in DIY Guitar Effect Pedal Projects, Remote/Rack Wah

The basis for this project comes from R.G. Keen’s wonderful article The Technology of Wah Pedals found at his GEOFEX website. It the article he explains how the pedal for a wah circuit can be positioned remotely from the audio using an LED and Light Dependent Resistor:

So it’s a relatively simple matter to disconnect the pot from the circuit and implement the alternate circuit using a 3 conductor cable (XLR or TRS) and an expression pedal. I use the Boss FV300L since I have one sitting around (and b/c Edge has one too of course…) In place of the LED/LDR pair R.G. describes, I think it’s easier just to use an off-the-shelf NSL-32 optocoupler.

The first step was to build a very rough prototype with some spare jacks and scrap sheet metal parts, to make sure the concept would work. I usually do this for pedal experiments:

Edge’s rack wah is a Vox V847 but I happened to have an old Crybaby board lying around, so I used that instead. The circuits are virtually identical. Next was to attach the FV300L with a mic cable give the wah a test drive. (Note: This also required a cable splitter from TRS to two mono cables) Here I used it for a cover of “The Fly”:

Once I was confident the prototype worked as I hoped, it was time to design a nice box for it…

  1. rockhound252 says:

    what is the capacitor placer rear the xlr connector?

  2. trabantland says:

    It’s a 100uF capacitor that’s wired in parallel with the expression pedal pot. You can see it in R.G.’s schematic above.

    • Rockhound252 says:

      What’s that for? In parallel? Where connect that? Ground and +?

      • trabantland says:

        Yes, Ground and +9V. It’s right in R.G.’s schematic (see the dashed line surrounding it and the pot).

  3. Rockhound252 says:

    But i have the xrl and i have ground, send and return of the effect. Mmm the 3 cables of the old pot now are 1 groun and two… How i know what is the 9v? I dont understamd schematic sorry;(

  4. rockhound252 says:

    I’m looking the pciture and i see another little circuit. Could you explain me with any schematic??

    • trabantland says:

      It’s the satellite board which contains the optocoupler, as well as a couple of resistors and capacitors. The schematic is taken directly from R.G. Keen’s website which I put in the first post. If I have time I’ll put up my own schematic and maybe that will make it more clear.

      • monty jay says:

        Please do this when you have time, I would love it. For some reason, I keep trying to do things and end up scratching my head left to figure things out, lol.

  5. Chris Nichols says:

    That is without doubt the best cover of my favourite U2 song ever, the solo section just seems to go in so many different and fresh ways compared to Edges classic take on the solo, Superb Dave.

  6. monty jay says:

    Looks great and the video is very nice! I am wanting to do something similar too. I love rack wahs, haha.

  7. bob knarley says:

    this is a great project,
    is the LDR pcb available for sale?

    • trabantland says:

      Thanks. I could do that but the circuit is so simple it’s extremely to put it on veroboard or just wire it in a P2P-like fashion. I have more info on this project soon as I do a few more custom builds.

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