Digital pots?

gmeredith · November 11, 2009, 01:44:31 AM

I'm trying to find out where someone mentioned these - I think it was here. I need to get hold of some 4-gang pots for my Phat Philter Bank mod but I cannot locate some anywhere, so someone once mentioned you can make them as digitally controlled using a chip and a single analog pot to control all of the electronic "gangs" simultaneously. Does anyone know how I could do this?

Cheers, Graham

noystoise · November 11, 2009, 07:26:34 AM

maybe vactrols would work? i just bought a bunch of these: http://www.goldmine-elec-products.com/prodinfo.asp?number=G15396 i havent tested them yet. the rest of the store is pretty unbelievable! i think i found my new supplier

gmeredith · November 12, 2009, 12:17:11 AM

Hey, what a great idea!! You could control the cutoff frequency like a Roland D-beam thingy, and also the resonance.

I probably would still like "knobular" control over them though as well - like a switchable option for both - so you could have them preset at a static value. But you've though of a tremendous idea for my filter!

Cheers, Graham

noystoise · November 12, 2009, 02:42:19 AM

well actually i was thinking you could just use one potentiometer to drive all 4 vactrols in parallel. not sure if it would work though. right now im building a 4 channel resonant bandpass/mixer for a friend. he didnt want any envelope modulation but i think he might change his mind in the future so i was thinking of using vactrols for cv ins and just adding 4 mini jacks for future use.

noystoise · November 12, 2009, 02:02:37 AM

ok, i threw some vactrols on my cutoff pots. it seems to work alright. i guess the vactrols can only close to 400ohms.
4gangs

gmeredith · November 12, 2009, 05:28:25 AM

Quotevactrols

Ahh - it didn't quite click with me when you first mentioned it - for some reason I associated the term "vactrol" with just the photocell part, not the whole LED side. Now I see what you were getting at - yes, that seems a very simple and elegant way to make a multi-pot, so long as the min. resistance and max resistance are sutiable for the application.

I'll look into them!

Cheers, graham

SineHacker · November 21, 2009, 03:52:47 PM

why do people call them vactrols? aren't they just optocouplers??

Gordonjcp · November 21, 2009, 08:08:19 PM

Don't forget that digital pots are noisy as hell when adjusted, and can only be adjusted a handful of times.

They are completely the wrong thing to use for a filter. There are loads of VCF designs out there that you can rip off.

noystoise · November 22, 2009, 06:59:40 AM

Quote from: SineHacker on November 21, 2009, 03:52:47 PM
why do people call them vactrols? aren't they just optocouplers??

vactrols are analogue optocouplers or "audio optocouplers". they use light dependent resistors rather than light dependent transistors. transistors can be very noisy for passing audio. i dont recommend them. the only problem i have with vactrols are the characteristics of the ldr. from what ive heard, they can vary quite a bit from vactrol to vactrol. i need to look into it more.

Gordonjcp · November 22, 2009, 10:16:58 AM

Quote from: noystoise on November 22, 2009, 06:59:40 AM
transistors. transistors can be very noisy for passing audio. i dont recommend them.

I don't want to sound like an arse about it, but hang on - you don't recommend them? Because, eh, Bob Moog does, Tim Orr does, Don Buchla does...

noystoise · November 23, 2009, 06:47:52 AM

sorry, i was actually just talking about transistor optocouplers and my personal experiences with them.

hey graham, have you tried running all 4 cv ins to one pot instead of 4 separate ones? it might work...

Gordonjcp · November 23, 2009, 02:59:14 PM

Quote from: noystoise on November 23, 2009, 06:47:52 AM
sorry, i was actually just talking about transistor optocouplers and my personal experiences with them.

Ah right, not actually transistors themselves then - that makes a lot more sense.

Have a look at the circuit for the Gakken SX-150, that's got a nice simple filter in it. The TB303 Bassline has a pretty classic ladder filter, too.

noystoise · November 23, 2009, 08:33:01 PM

i'll definitely check them out. right now im putting together a synthacon filter for a friend.

gmeredith · November 25, 2009, 12:12:25 AM

Hi Noystoise,

I found that running all 4 CV lines together through a pot interfered with each individual CV envelope for the VCA unit on the keyboard in use - the Casio SK sampler. It was ok if you say, played a chord and each note was struck together, but if they were played like an arpeggiator, the CV lines combined never allowed any one line to decay to zero. So they all need to be independently adjusted, because each CV line enters a separate filter module to produce the 4-note polyphony of the filter.

Hi Gordon:

QuoteDon't forget that digital pots are noisy as hell when adjusted.....There are loads of VCF designs out there that you can rip off.

You mean like step noise as you adjust it?

Unfortunately all of the other VCF designs out there weren't independently polyphonic, weren't 9V battery powered and weren't small enough to fit into the casing of an SK-8. Hence my joy in finding this one. As well, any VCF unit cluster that would be driven off the CV lines of the SK would have the same issue.

Cheers, graham