How do my expression CV pedals work?

quicksnap · Post by **quicksnap** » Sat Sep 20, 2008 1:02 am

I'm starting to work with circuitry a little more and was wonder how the expression CV pedals work. When I plug a guitar cord into my filter CV port on the phatty and apply a +/- Voltage, it changes the filter accordingly. How do my expression pedals accomplish this when they appear non-powered. Is there a battery tucked away, or is it operating differently?

Voltor07 · Post by **Voltor07** » Sat Sep 20, 2008 3:13 am

The expression pedal has a 50K-ohm potentiometer inside that adjusts the voltage slightly enough to cause a decrease in voltage, which in turn changes whatever parameter you have the cable plugged into. That's all. Hope this explains it.

quicksnap · Post by **quicksnap** » Sat Sep 20, 2008 5:57 am

Yup! I was messing with the cable applying resistors to it, but I suppose I was using too tiny of one to notice any voltage change.

Does this mean that the CV inputs of the LP generate a voltage? I ran a patch cable from it and tested with a multimeter, but I couldn't pick anything up.

narrowcaster · Post by **narrowcaster** » Sat Sep 20, 2008 8:48 am

AFAIK, expression pedals require a voltage provided by the jack you plug them into. As to why your multimeter isn't showing it, that's an interesting question. Maybe I'm wrong, or maybe someone can give you a tip about the right way to measure.

Voltor07 · Post by **Voltor07** » Sat Sep 20, 2008 10:12 am

Were you using a TRS cable? The voltage is applied to the ring, so a standard guitar cable would short out the voltage. BTW, if measured correctly, you should get a reading of 4.98V DC.

quicksnap · Post by **quicksnap** » Sat Sep 20, 2008 3:56 pm

I am using a phone TS cable and, for the life of me, cannot get a reading. When I apply a voltage to it I can still get expected +/- changed to any CV, but when I just plug in a TS cable and try to test it, I cannot get the multimeter to budge.

I both unsleeved the wire and then clipped off one end and stripped it down to layers. No readings on either.

What the heck am I doing wrong?

Consider that I just bought a breadboard and made a light bulb circuit. I'm still kind of new to this area.

Voltor07 · Post by **Voltor07** » Sat Sep 20, 2008 4:51 pm

As I said...a TS cable shorts out the voltage! Only a TRS cable with the multimeter's red lead on the ring and the black lead on the sleeve will give you a voltage reading.

quicksnap · Post by **quicksnap** » Sat Sep 20, 2008 9:56 pm

Thanks for your help! Mystery solved.

Voltor07 · Post by **Voltor07** » Sat Sep 20, 2008 10:29 pm

No problem! Welcome to the wonderful worlds of Moog and electronic circuits!