I haven't used the expression pedal to modulate the VCA yet, I'll try checking that out after work. I've used them on other stuff with no problems.
Are you using an expression pedal that has a TRS (stereo) output jack with a TRS cable to the expression pedal inputs? That's how the jacks are wired (they provide a 5V source which goes to the expression pedal pot on the ring connection, and is returned on the tip, IIRC). If you just use a mono cable to connect the pedal, you're basically just slowly shorting the 5v output which doesn't work nearly as well (weird pedal taper) and may not go to zero, depending on your pedal. I have a Moog expression pedal that works properly, and I also sometimes use a couple of old Ernie Ball volume pedals that I've wired to work with the ONE, and they work nicely as well.
A very useful trick for setting up stuff to work how you want it to is to apply the CONSTANT (last in the Mod Sources list) to your destination. Think of it as a fixed voltage that you can apply positively or negatively to that desination to fine tune where it sits. So if you assign the pedal, turn the VCA panel knob all the way down, pull the pedal all the way up and the VCA is still turned on, you can apply a negative CONSTANT amount until the VCA level is set where you want it. Then you can push the pedal all the way down and adjust the expression pedal amount in the Mod Matrix to set the volume level you want.
This works great for other stuff too, like setting filter amounts so that the Cutoff knob is centered for easy live tweaking, etc.
Edit:
I got a chance to try this tonight, and it does work, although Not Quite in the way I expected.
Here's how to do it:
Set the VCA level panel control to 12:00 (halfway up).
Assign the CONSTANT source to the VCA Level destination, and set the amount to about -90%, until the sound of a held note
just shuts off.
In the Mod Matrix, assign the expression pedal source to the VCA Level destination, at about 30%, or until the held note volume
just quits increasing with the pedal all the way down.
Your expression pedal should now control the VCA level!
It would seem that the pedal is modulating the total level amount dialed in with the front panel VCA level knob, which I didn't expect, but it appears to work quite well this way. Unexpected Grooviness!!
Depending how you adjust the CONSTANT modulation levels, you can also have the pedal just reduce, but not completely shut off the output level however you want (preset your pedal levels, anyone?) and increasing the pedal modulation will sort of affect the curve of the pedal, with it reaching full volume earlier in its travel.
Swap the modulation levels and polarities on a different layer, and you can continuously cross-fade between them with the pedal.
Keep in mind if you are using the master effects, (which are mixed in parallel with the dry signal) they will remain even when the VCA level is set to zero with the pedal, which can be a nice effect, but maybe not what you're after. The synth effect, which is in series with the voice will be turned down with the pedal.
I
LOVE how customizable this instument is! (Once you learn its hidden secrets, of course!)
Hope this helps!