Mixing/Blending/Opposite of Panning with Foot Treadle Control

[Chrisfromiowa]

I'm trying to figure out how to build a pedal to blend to signals with a foot controlled variable. I mean, I could do regular mixing/blending pedal, but then I'd need two expression pedals hooked back to front, or two volume pedals set back to front with an ABY on the outputs.
Has anyone ever seen or made anything like this, I've done a bunch of searching and either I don't know how to describe it, or almost no one has ever needed it.
I'm using a Roland Guitar Synth and it allows either Full Synth/No Synth All Dry/50-50 Synth Dry and I want something a little more subtle and controllable.
Thanks

[ElectricDruid]

This will do what you want:

http://electro-music.com/forum/topic-54633.html

It's a pair of LM13700 VCAs being used to control two different inputs, and a differential pair used to generate inverse currents for each of the control inputs. The circuit and shown is for synths and uses a bipolar supply, but LM13700 works happily at 9V and you could reduce the circuit a bit by simplifying the control voltage circuitry.

HTH,
Tom

[ElectricDruid]

In fact, having a bit of a look, there's a lot you can strip out. There's inverting stages ahead of each VCA to avoid signal inversion overall (the output I-to-V mixer stage is inverting). If you could cope with the 13700's impedance directly, you could drop those, and feed the inverting input instead of the non-inverting inputs.

Also, on the control voltage page, R11 22K can be fed directly from the Initial control and all the rest of that branch can go. You could do something similar with the Crossfade side too, but you'd have to work out what range of voltage that Q1 transistor base needs to see.

HTH,
Tom