Expression Pedal Question

[Anonymous]

I have ended up with an extra wah pedal shell and figured I would wire it up as an expression pedal to work with some of my “commercial pedals”.  Looking at the Moog, Roland and M-Audio expression pedals that are listed as compatible with these pedals it seems they are all pretty much the same internally.  The expression pedal input sends a voltage out to the expression pedal and the pot in the pedal is wired as a voltage divider and sends the voltage back to the expression input.  All of these pedals use a 10k pot.  The Moog and Roland have an additional 50k pot coming off of the 10k pot as a variable resistor to ground (sleeve on the TRS).  The expression jacks on the pedals also accept a CV of either 5v or 3.3v so I am guessing that is what is being sent out to the voltage divider when using an expression pedal instead of CV.

My question is whether the value of the pot being used as the voltage divider would be critical.  I ask because I already have a 100k wah pot that I would like to use if possible…

 

[Cybercow]

Brooks – kinda, sorta, maybe could use a schematic to see exactly what’s happening with the expression INPUT on that circuit. If it’s expecting a CV and not just an expression pot insertion point, then a simple pot is unlikely to work. If the expression INPUT is supplying any CV voltage, then the 100K pot in your wah shell should work fine.

[Anonymous]

Below is what I found for the Moog EP-3 Expression Pedal…which I have been told will work with the expression input on the pedals I have (EQD and Chase Bliss).  All others, beside anything designed to work with Line-6, seem to be either exactly the same or very similar with the differences being in the DPDT option to change the toe up / down reaction of the pot.  All show a 10k pot.

I have a cable I made to use CV with these pedals and I just checked it.  The side that goes in the pedal is a TRS with the ring floating / unconnected and on the other side that goes to the CV source it is T to voltage and S to ground.  So, I guess that would indicate when attaching TRS to TRS using an expression pedal the voltage is coming through R and hitting the pot and getting sent back through the T?  Moving the pot varies the voltage being returned.

This is what EQD says about their pedal…need to find the instructions from CB pedal but I remember it being the same (except 5v)…and is what I used to make the cable to connect to CV source.

Expression Jack
Use any TRS expression pedal to take control over the Length, Depth, Rate, Tone, or Mix panel controls! You can also use Control Voltage with the EXP Jack. The CV range is 0 – 3.3v.

EXP Jack Internal Connections:
Sleeve= Ground
Ring= 3.3v
Tip= Wiper (CV)

[Cybercow]

Brooks – that should work. That circuit does not provide a CV. Rather it divides what it is given. Roland uses the same topology for some of their expression pedals – except for the switch, and they use a 50K pot on the treadle.

[Anonymous]

Yeah…that is how I understood it…but, probably didn’t explain it well.  The effect pedal is providing a voltage and the expression pedal is dividing and returning it.  When a CV input is used instead of expression pedal the voltage is not sent from the effect pedal (ring on TRS cable is not connected) and the CV is just sent through the tip to the effect pedal input.

Because the treadle pot is acting as a voltage divider the 100k should work the same as a 10k?

In hindsight, I guess the original question should have been posed as: when used as a voltage divider does the pot value matter?  I just tried to research that a little and found that the value could impact current flow but I am guessing in this application that does not matter.

Thanks for the help.

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