Adding voltage control to Digisound 80-23 LFOs

[creatorlars]

Hello --
I am working on a multipurpose joystick controller console and I wanted four simple, low-parts count LFOs with several selectable waveshapes. It looks like the Digisound 80-13 fits the bill exactly -- can't imagine it being much simpler:
http://www.digisound80.co.uk/digisound/modules/80-23/construction/80-23.pdf

Only problem is I'd like to have a VC input for each LFO as well. It seems like a simple single vactrol using the Bridechamber PCB would work to replace the 1M Lin pot, but I wonder if there's an even more elegant/cost effective solution. I do not need perfect linearity or anything.

On another note, I'm looking for a very simple Quad VCA circuit for this same controller.

[Tim Servo]

Hey Lars,

There are a couple of LFO designs on another thread:
http://electro-music.com/forum/topic-32372.html

A gent named Nikolas has a very simple 324 based VCLFO, so that might do the trick for you without any Vactrols. I also have an LFO design there with a very wide frequency range (better than 8,000:1) and a couple of waveshapers. I'm working on a VC version, but the existing one could be modded with a Vactrol.

The Digisound design could also be VC'd with a vactrol. It looks like an interesting design. I'm wondering if the Saw out is a true Saw, or if it's half of the "double edged saw" that you'll see at the input of the comparator (if this is the case, you won't get a negative excursion in the wave). Still, might be worth breadboarding to play with. Anyway, check the other thread for some interesting stuff.

As far as the quad VCA, you might want to look at the 2164 quad VCA chip. If you can work with expo response VCAs, you can four out of a single 2164 and some op amps. If you need linear VCAs, you can get two out of a 2164, so it's still a pretty cost effective solution. It's also a quiet chip and has monster dynamic range.

Tim (at home sick and breadboarding today) Servo

[creatorlars]

Great advice! Thank you. My initial intrigue about the Digisound design is because it already has an etchable layout for a quad module, and has 6 different selectable outputs (for this purpose I will only need one at a time.) But then again, for this purpose I may only need two LFOs, really...

Planning ahead on this one though, so I'm going to "shop around" for what I need.

I am wondering about the exp/linear question for the VCAs... I want to easily be able to use them internally with the joysticks and LFOs in order to get joystick control over LFO Rate & Amplitude on a single joystick... but also want to be able to use the VCAs with any external audio/CV signal, so an expo/linear design like the MFOS one might be best.