LFO based expression pedal

[chokeyou]

so I recently picked up one of those Boss/roland RE-20 (RE-201 emulation)

I'm aching for some enduced warble (this thing doesnt do enough)...on my old RE-201 I intentionally kinked the tape and stuff to get what i was after. the 201 is long gone, however

the RE-20 has an expression pedal input though, and you can control the delay speed, among other selectable things, with this.

I was wondering if you guys could point me in the direction of a resistance LFO (like the steady rocking of an expression pedal). I'm assuming this is going to be a regular LFO tied to an optocoupler, or is there an easier way? It only needs to be shallow (as i dont care for super crazy pitch shifting) and I'm unsure what shape will work best...maybe sine or square...i figure triangle doesnt spend enough time on the max/min to heed a consistent result. I should be able to put a pot after the LFO to center the LFO on the region of travel I want, right?

[chokeyou]

more info i dug up.

the recommended expression pedal is the roland ev-5, which is 50K linear (standard, i think?)

i saw some stuff about using FETs as variable resistors notably the H11F1, H11F2, and H11F3 available through smallbear, but it seemed to operate on a logarithmic scale, something i'm probably not after, which leads me to optocoupling with vactrols and such. I dug around the web, and found that there seems to be a shortage of info aimed at my use, but an abundance of info within other schematics and designs using optocouplers.

I read time and time again the difficulty people have getting the thing to follow quickly, which is probably something i'm after, even if I'm only going for a subtle chorus/vibrato on the repeats.

I'm not looking for matched (or even consistant) optocouplers, as I'm only tying one to the end of an LFO, +9V source. can anybody recommend one that reacts quickly and wont have much trouble responding to subtle changes in the internal LED brightness?

also, as far as my "floor" control goes, and I'm looking to set the "base brightness" of the LED above +0V input, would I be looking to add a pot bypassing the LFO circuit, adding voltage dropped across the LED?

I know the best way to learn is hook everything up and toy with it, but I'm looking for some guidance ahead of time so I can anticipate the parts I need to order...there isnt anything in town aside from a radioshack that only has an ill-stocked parts bin, its really sad. we all love smallbear, but they take time to get stuff to you, and as fast as mouser is, i cant devote multiple part orders and the shipping wait to something like this. I'm just looking for a bit of guidance is all. thanks.

[gez]

Just about all the (easy) methods of replicating variable resistance - FET, vactrol etc - will produce some distortion of the original waveform.  Whether it's apparent depends on the method used, and it may or may not matter at the end of the day.  It's one of the reasons I use OTAs so much as they are pretty linear, though that doesn't really help here.  Or perhaps it does?

There's a variable resistance circuit in the app notes of the 13700.  I've never tried it so can't vouch for it.  Plus it might be a little difficult to set up.

If you want cheap and cheerful, try something FET based.  The EA trem should give you a few ideas (nobody seems to complain about the waveform sounding crooked, and it has smooth/soft modulation from what I can gather).  Perhaps stick a trimmer in the resistance chain coming off the LFO?  It's pretty easy, so why not breadboard and see what happens?

[Processaurus]

Hi, it may be simpler to do what you want than messing with resistance, as most digital pedals with expression pedal ports just use the pot in the rocker pedal to make a variable voltage (like 0 to 5v, or 0 to 3.3v) and send that back to the processor.

If that is the case (as it is with my compact boss pedals that use the expression pedal) then all you have to do is mix in an lfo with the control voltage going back to the pedal.