I’ve been trying to find a decent diy vca for the lm13700. I’ve done the differential pair transistor vca (tough for me to match the pairs). I like the n-channel jfet “I am o” and the vactrol vca too (both passive). I just got into the lm13700 and I’ve been trying to hunt down a useful diy vca for it. This is super simple and not a lot of components. I left the cv attenuate pot out only because I have a panel already set up for a dual vca circuit, and it just didn’t have room for the extra pots. Both circuits work great on breadboard. Just duplicate the circuit on the other half of the 13700 and the tlo74 (or 2 other opamps you might have if you don’t use quad opamp. I used two tlo72 to get my four total) and it’s a dual vca! Thanks to Juanito Moore!
There’s a nice article here explaining the design of an LM13700 VCA, starting with the basics and then working up to a practical design:
It’s a similar design, but a little more elaborate, adding a resistor to protect the OTA against overcurrent and a diode to protect the transistor against negative CV inputs. Also, as explained there, the small CV input resistors are problematic with the 100k pots (and you do want the pots to be 100k), so it scales them up and then adds a couple resistors on the transistor collector to compensate.
Right on! I saw this one too. I just hadn’t got around to building it yet. I set out to build at least one good one today and juanito’s did the job pretty nice!
Both approaches are used. YuSynth uses 1 cap + to - instead of 2 caps to ground, as does Kassutronics. But LMNC, MFOS, AI Synthesis, Befaco, Erica Synths, Ken Stone, Music Thing, Mutable Instruments, North Coast Synthesis, and Thomas Henry all use 2 caps. So does Barton, though for some reason he insists on specifying 10 nF instead of 100 nF. I don’t know for sure which is better but I recall @fredrik used to insist 2 caps is better, and I’ve kludged at least one YuSynth PCB to make it that way.
I’ve been looking more closely at the electric druid schematic. I hadn’t realized just how similar they are. I’m going to make the changes since it is all still on breadboard. I haven’t started soldering yet…
I made the changes to half the circuit so I could test them simultaneously. I just plugged what I already had patched with my two envelope generators. It was two different audios and envelopes. I wouldn’t know what the difference is but they both work side by side independently as they should. I’m gonna build the elec druid model just because you say it has better protection. I will likely accidentally abuse it. Also, I don’t think I have any electric druid models in my rack. I’ve got some of Juanitos stuff in there already. I wanna be fair.
Also, I don’t think I have any electric druid models in my rack. I’ve got some of Juanitos stuff in there already. I wanna be fair. 
