Using analogue synth VCA to control expression pedal inputs on stompboxes/multi-fx
Ok, this has been a thing many people have pondered. The code is now cracked.
Analogue synths use things called "VCAs" to control the output volume of the synth (among other things). The VCA (Voltage Controlled Amplifier) is essentially a volume knob that lets voltage turn it for you. That controlling voltage is typically called "CV" for "control voltage".
Many of us own FX toys such as Electro Harmonix, Boss, and Line 6 devices that have Expression Pedal ports on them. These ports are used to connect a basic expression pedal to the device and control some/many functions of the stompbox. These expression pedals are essentially just a control knob, just like a volume control or the like.
The problem has been how do we use these wonderful analogue and/or modular synth components (LFOs and VCOs and sequencers) to control our analogue stompboxes through their expression ports. The danger of using direct-cv from the analogue synth stuff is that some analogue synth CV is as high as 10 volts! Most of our stompboxes have roughly a 3 volt limit on the expression pedal inputs, go higher than that and you may let the smoke out of the stompbox and damage it beyond practical repair. At the very least, devices such as the Line 6 M5/9/13 reset their expression pedal parameters to zero when overvolted. In any case, not a fun thing.
The way that the typical expression pedal port operates on most stomp boxes and multi-Fx units (such as the Line 6 M5/9/13) is that the port sends out a small voltage to the expression pedal. When you move the pedal it attenuates (a big word for control or reduce) that little "send voltage" from the expression port to havee some affect on whatever the stompbox designer has connected it to.
So, here's how to marry the two technologies. Remember that VCA I spoke of? Remember how it is basically just a control knob that is "turned" by an incoming control voltage? We're going to use that as an ambassador between a modulated CV from an analogue synth and the expression pedal control voltage that is sent from the stompbox to the expression pedal.
Simply connect both the SIGNAL INPUT and SIGNAL OUTPUT of the synth's VCA to a multiple, then connect a third patchcord to that same multiple and connect the other end to the expression pedal input port on the stomp box. Done!
What's happening here is the VCA is being used to attenuate the outgoing control signal from the stompbox, excatly the same way the expression pedal would. Except we're using some sortof control voltage (CV) from some sort of CV source (like an LFO or a sequencer) to turn the VCA's knob for us. That way there is NO DANGER of over-volting the expression pedal input port on the stompbox, because it simply thinks it is connected to an expression pedal that is controlling it's own little "send voltage". In reality the analogue synth's VCA is what is controlling the expression voltage. So since the stompbox is not connected to any sort of voltage source, and it is simply having it's own control voltage adjusted (just like it does with an expression pedal) there is ZERO DANGER of hurting something.
MULTS? What the heck?
A multiple (aka "mult") is simply several jacks all connected together in parallel. All of the TIPS are soldered together, and all of the GROUNNDS are soldered together (like an elaborate Y-cord). So whatever is connected to one jack, is connected to all of the jacks. Anyone with basic soldering experience can construct a mult.
If you've been pondering the purchase of a few synth modules, or have the stuff but didn't know how to safely interface analogue synth CV with stompbox expression pedal inputs ... you are now armed with the knowledge.
The extra beauty of this concept is that with the proper stuffs, you can simultaneously control your expression pedal inputs with either CV, expression pedal, joystick, control knob, as well as a CV controlled VCA!
How about THAT shyte ... Huh?
I'll make up a block diagram of this VCA/mult/expression pedal input thingus in a day or so. I actually took time away from ~working~ to post this once I discovered that it worked on my own M13. So I really need to jet.
Think of it .... CV of x-port parameters! Gotta love it!
NOTE: Apologies to anyone that already knows about this, it was certainly news to me and I couldn't wait to share it.
Last edited by Flux Jetson : 03-24-2013 at 08:58 PM.