Please help me understand dual expression pedals wiring

[plague.doc]

Hello everyone. I'll explain my idea first. I'm using source audio C4 synth pedal for a while now and I always wanted to have more control over the settings in the field. Two knobs for this is just not enough imo. The manual says you can connect either 3rd party expression pedal via trs cable or some of their stuff (dual expression, hot hand) via trrs cable. The second option lets you control up to two parameters because of, quoting manual, "two separate axes of expression: x and y". That way you get: control signal on the tip, rings are x and y expression signals and ground on the sleeve

The idea is to build a DIY pedal with two knobs that will replicate this behaviour. It is connected to the synth via trrs cable, pots are wired as voltage dividers to change X and Y expression signals. This allows to get two more control knobs in every preset

Unfortunately this is where my understanding of the circuitry ends. I tried to connect both voltage divider inputs in parallel to the tip, outputs to the rings of trrs cable, trrs itself connected to the synth and the result was both controls interfering with each other. I'm thinking about using opamp voltage follower to isolate the voltages but I'm not sure how to do it properly

Maybe someone there knows something about how this source audio products (hot hand, dual expression) or even other dual expression pedals handle the signal or can help me with the schematic. Thanks in advance!

[plague.doc]

The part of the manual related to the question

[FiveseveN]

Show us exactly what you did with pictures please. The way you described it should work, barring interference or miswiring. What value(s) are the pots?

[ElectricDruid]

I'd have thought you want something like this:



The pots' exact value shouldn't matter too much (it'S a voltage divider), but I'd probably go with 10K, 50K, or 100K.

If that's what you think you did, but it still doesn't work, check the wiring carefully!

[Mark Hammer]

I have a Source Audio Hot Hand 3 controller.  This is a wireless unit one wears on a finger of your picking/strumming hand.  The base unit it transmits to provides two control outputs.  One conforms to the Line 6 standard, which is simply a 0-10k variable resistance to ground.  The other is a dual 0-3V3 output for controlling two parameters of a Source Audio pedal.  Since the "controller" is waving one's hand around in space, the unit permits the user to select which of the 3 dimensions one wants to use.  The corresponding output from the base uses a TRRS jack, which assumes a corresponding TRRS cable and jack on the pedal to be controlled.  I made myself a breakout box, such that the two control-voltage outputs could be routed separately, if desired, rather than constrained to the TRRS cable and plug.  Somewhat obligatory, since I own no other Source Audio products.

Tom's diagram is spot on.  The tricky part, for me at least, is soldering those bloody TRRS plugs.  They are not designed with human hands in mind.  That said, I would think that a TRRS-to-dual-1/4"-phone-jack junction box might be a handy thing to accompany your project, and offer maximum flexibility.  This would let you use standard expression pedals for your C4, or use only one and commit the other one to a different pedal that employs 1/4" expression-pedal jacks.  A control-flipper switch (DPDT toggle or slide switch) on the box would also be handy, permitting assignment of either control input to the X or Y parameter.  And since the junction box is entirely passive, there's no reason why you couldn't use one input for an expression pedal, and the other for some appropriate control-voltage source, be it a modulation source, envelope follower, etc.

[PRR]

[Kevin Mitchell]

From what I understand, the Source Audio Dual Expression Pedal is simply a dual pot set up as two isolated variable resistors - no active circuitry.

It's two pot wafers going to two independent TRS jacks - rather than two pots to one TRS jack.

Just think: two expression pedals under one foot.
What confuses this is the 4-conductor 3.5mm jack which does +V, GND, X & Y connections which I don't believe are applicable for the Dual Expression pedal, though I can't find a gut shot to investigate further.

[ElectricDruid]

What confuses this is the 4-conductor 3.5mm jack which does +V, GND, X & Y connections which I don't believe are applicable for the Dual Expression pedal, though I can't find a gut shot to investigate further.

If you've only got one pedal, there wouldn't be much point having "X" and "Y" outputs (and hence a TRRS) since you haven't actually got two dimensions, you've only got one. "X" and "X again" isn't as much fun!

I can see the point of splitting one pedal out so you can control two things simultaneously, but that's not really what we're on with here - this is trying to control two parameters in one place, rather than one parameter in two places!

As you say though, gutshots are the way to know for sure what's going with this sort of stuff.

[Kevin Mitchell]

I can see the point of splitting one pedal out so you can control two things simultaneously, but that's not really what we're on with here - this is trying to control two parameters in one place, rather than one parameter in two places!

I totally get it. I just don't understand how the suggested expression pedal can provide two axis's of expression. I figured it was a complimentary "mult" really. But, maybe there is a 2nd axis of control that I'm not seeing.

The best I could do was spot two wafers on the treadle pot in a photo. So, it's a reach for a impractical speculation

Actually, I might be right.
Got the manual. Two 50K passive expression outputs.
https://www.sourceaudio.net/uploads/1/1/5/1/115104065/dual_expression_pedal_manual.pdf
"The dual TRS expression outputs allows the SA161 to control two Soundblox Pro
(or other 3rd -party ) units simultaneously."

Edit:
Did I miss the boat where we weren't analyzing the Source Audio Dual Expression Pedal and onto another endeavor? My bad if so.
If we're doing 2 axis control, you'll need two TRS jacks to use with the synth pedal, correct?

[Mark Hammer]

If you can find one, many of the older PC-based car race games would come with a floor unit intended to serve as gas and brake pedal, and a separate "steering wheel".  That floor unit plugged into the parallel "games" port of one's PC.  The two pedals were spring-loaded units with 50k pots - the standard at the time for IBM games ports.  They are most definitely NOT as sturdy as the typical expression pedal, but if you can find one in a thrift shop or yard sale, it might be the most useful $5 you ever spent.  The return-to-heel-down, that the spring produces, can be handy.

[PRR]

If you can find one, many of the older PC-based car race games would come with a floor unit intended to serve as gas and brake pedal, and a separate "steering wheel".  That floor unit plugged into the parallel "games" port of one's PC.

About $70US from Florida now. (I remember when they were free for the takeaway.)

The 15-pin plug is unique to IBM joysticks and equivalent 'game' controllers.

[ElectricDruid]

Did I miss the boat where we weren't analyzing the Source Audio Dual Expression Pedal and onto another endeavor? My bad if so.
If we're doing 2 axis control, you'll need two TRS jacks to use with the synth pedal, correct?

OP says:

The manual says you can connect either 3rd party expression pedal via trs cable or some of their stuff (dual expression, hot hand) via trrs cable. The second option lets you control up to two parameters because of, quoting manual, "two separate axes of expression: x and y".

(My emphasis)

The TLDR is "Dual Expression provides two *outputs*, but Hot Hand provides two *axes*".

So yeah, you're right about the Dual Expression being a bit of a waste of spce in this application! However, the thing is so simple that sticking two pots in a box and wiring them to a TRRS plug to gain some more control over other parameters is a easy thing to do. Even wiring up a Dual-TRS-to-TRRS lead so you could plug two expression pedals into this thing wouldn't be beyond the wit of clever monkeys like us