Controling the Kemper with a theremin (or any CV source)

    My Moog Etherwave Plus theremin has two Control Voltage outputs (one for each antenna). I hadn't really thought about this before and I may be confusing things here, but aren't expression pedals based the same principle?


    On this assumption, I thought it would be fun to connect one of the theremin's CV outputs to a pedal input on the Kemper. Unfortunately, nothing happened (I tried both pedal modes).


    Was my assumption wrong? Are modern expression pedals fundamentally different from the old CV standard? Or is it worth giving it another try (using a different TRS cable, trying different settings on the theremin...)? It would be fun to get it to work!


    Edit (two years later):

    it verks.png

    See below.

    Quote from Michael_dk

    I don't know what Control Voltage outputs are :) but the pedals are basically just presenting a resistance to the kemper, going from eg. 0 to 25 kV. Sounds from the name like there is voltage going on with the theremin, which is quite different. But I may be wrong as to what the CV actually does :)

    Oh, ok. But varying the resistance would (assuming constant current) result in a varying voltage, right? Might be different standards altogether though.


    Control Voltage is kind of the predecessor to midi. It was (and is) used to make modular analog synths talk to each other using patch cables. An LFO or envelope generator would output a fluctuating voltage, for instance, which could be used to control another module, like a filter. In the case of my theremin, the voltage is controlled by the player's proximity to the antennae, just like the instrument's own volume and pitch.


    One of the things that got me thinking my theremin could function like a standard pedal is this page, where it says "Expression pedals work by feeding a control voltage to a device."


    Edit: ahhh, ok. This clarifies things. Thanks @paults!

    Quote from paults

    The CV output is an active device, produces 0-5volts output, and can be used as an input to a CV-compatible input.


    An expression pedal is a passive device that is placed within the expression pedal circuit.

    So I've thought about this again recently and I think I finally get it, and also how I could make this work.

    Quote from paults nails it:

    The CV output is an active device, produces 0-5volts output, and can be used as an input to a CV-compatible input.


    An expression pedal is a passive device that is placed within the expression pedal circuit.

    So in the case of modular synth CV control, the CV modulator actively sends a variable voltage (typically 0 to 5V, but often up to 10V, or bipolar, down to -5) to the device being controlled (one-way trip over TS cable).


    In the case of an expression pedal, it's the device being controlled itself that sends out a reference voltage, which is attenuated by the expression pedal (using variable resistance) and then sent back (return trip over TRS cable).


    What I want to try is this. I'll take a splitter cable that is TRS on one side and has two mono TS ends on the other (i.e., an insert cable). The TRS side goes into the Kemper. On the split side, the send jack goes into the input of a VCA (voltage-controlled attenuator*) and the return jack goes into its output. That way, the VCA will be in the return-trip circuit and play the part of the expression pedal.


    Then, in turn, I'll control the VCA with an active Control Voltage from anywhere in my modular rack.


    That should work, right? I'm only attenuating the reference voltage coming from the Kemper, not amplifying, so I'll never send a higher voltage back. The modular CV controls the VCA (i.e., plays the role of your foot on the expression pedal) but isn't fed to the Kemper itself, so it doesn't matter if its too high.


    I should be able to get this set up next week. Hopefully, it will make the Kemper a real part of my modular system, so I can morph my guitar sound using any voltage source: clocked LFO's, sequencers, noise through sample-and-hold, ADSR envelopes, or indeed my theremin. Fun times ahead! I'll post a video if I don't blow myself up.


    * Usually called "voltage-controlled amplifier", but "attenuator" is more accurate in many cases.

    I've barely had a few minutes to experiment, but I've completed the setup and it's working! =O


    That means I can now do anything that an expression pedal can do -- pitch, volume, wah, morph -- using a control voltage from my modular system. I can have a clocked LFO modulate the delay time on an analog-style delay to create doppler effects. I can make the stereo width expand in time with the beat, or the amount of overdrive, or maybe even "play" a wah like a resonant filter using a stepped CV from a sequencer or a sample-and-hold circuit... So many things to try!


    Just as a proof of concept in the little time I've had, I've recorded a short video. The Kemper's pedal input that I'm controlling using the setup described above is set to Pitch; I'm controlling the VCA with a CV signal from my theremin. My E-bow is on my guitar to create a constant tone that I can turn into a melody theremin-style, using the Pedal Pitch effect. For some harmonic context, there's a random sequence looping through a sound source in my modular synth rack.


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    (PS -- paging DonPetersen :))

    Okay @Robrecht - I can't possibly, for the life of me, think of how I could use that live but..


    ...holy CRAP that's cool!


    Good work!


    KPA Unpowered Rack, Kemper Remote, Headrush FRFR108s, BC Rich Mockingbird(s), and a nasty attitude.

    Like 1

    Hehe, it's definitely very "Why? Because you can!" ^^^^^^


    I won't be using it live either, between regular expression pedals and midi control the Kemper has that base covered already. But when jamming at home I sometimes route my synths through my Kemper for its effects, and it'll be great to be able to animate those as well, to really make them part of the whole patch.


    I love the Kemper's delay in the video I posted. Too bad I've patched volume control (left hand on the theremin) so crudely. I just routed the Kemper's output through another VCA, so it's entirely post-everything, cutting off the delay tails. Next time I'll run the guitar signal through a VCA before it goes to the Kemper, or add another Y-splitter cable to simulate a second expression pedal for volume.


    In any case, it's cool to see the expression pedal inputs are responsive enough for this kind of thing. I suppose there'll be a limit to that (the pedal signal may be run through a low-pass filter to suppress jitters) so audio-rate modulation is probably not an option, but I'll need to experiment further to find out.