Long ago I set out to find a way to combine a volume/wah/expression pedal into one single/compact/easy-to-operate pedal…
Oh, and without using rotating pots to control it.
I went through numerous iterations, using LED/LDR’s Morley style, Hall sensor devices, several rocker pedals (including a vintage Colorsound Wah/Fuzz that I had once butchered before these things were considered worth more the stamps used to ship them)
Finally I found that the DOD FX17 had the form factor I was after. I actually sold the first FX17 I had and some years later bought it again, having forgotten why I had sold it. In fact I hated the volume pedal part (noisy 3080) and the switching logic. You cannot switch from Wah to Volume with the foot switch. You can only bypass the wah..
Also it had 2 trimpots that affected at the same time the Wah, the volume and the CV output. So it was impossible to find a setting to make all 3 work as they should.
What I really liked about it is the variable capacitor and oscillator that provided the control voltage. So I decided to keep only that part and to replace all the rest.
The CV signal is optimized through U1a, where you can amplify and offset it.
It then goes to the various control elements.
After trying out several kinds of VCA’s, using LDR’s FET’s etc I ended up using a TDA7052 amplifier. It’s not really made to operate as a VCA but works pretty fine and most importantly, I had some around.
I kept the same wah circuit as the FX17, because I quite liked it. (it’s the same circuit as the FX25 auto wah).
There are also two CV outputs whose voltage can be set independently. (For example, I am using one with 0-5V for the Pitchfork and one with 0-3.3V for FV-1 based pedals).
For switching I am using a HEF4013 (the HEF refers to the model with a built-in schmitt-trigger, good for debouncing the switches a bit)
There are two momentary switches. The original one on the heel and another one I added on the toe side. One switches from Vol to Wah. The other one switches to bypass and engages the CV outputs.
For switching from Vol to Wah I am using a switched dual OP amp NJM2120. I could have gone with Fets but I had these around and they use less components.
The switching part was the most fun as I had not worked with flip flops or logic chips before so that was good learning.
I guess all of the control/switch stuff could have easily been reduced to one microchip but … I have yet to start with that stuff ..
The bypassing of the volume and CV’s works like this:
The CV (0.6-6V)is fed to T1, which is set as a voltage follower. The 2 trimpots optimize the voltage swing to control the gain of the TDA7052 chip (0.4-0.8V). When it is in Vol mode, Q2 is low and T2 is off. When you press the Vol/Wah switch, Q2 goes high and turns on T2, thus presenting the maximum voltage at the common emitter. Any variation of the CV no longer has an effect. At the same time
At the same time Q1 changes state also and switches the NJM2120 chip.
More or less the same principles apply when you press the FX/EXP switch. Vol is bypassed, NJM2120 switched to Vol, T4 and T6 are turned off, allowing for the CV outputs to swing.
The whole thing might not make real sense to everybody as it really is tailored to my particular needs but I am pretty happy with how it works.
Just posting it here in case some of the circuit bits might be of use to someone.