hey friends,
I am working on a project (BD++) and am wanting to add CV to a parameter usually controlled by a potentiometer. here is the existing circuit:
I understand since this pot is acting as a variable resistor it can be replaced with a cv source via vactrol. Is it possible to just sum them together, pot and CV? something like:
is there an easier way to do this without using vactrol? opamps?
Maybe something similar too this would work with a transistor. I built up it once on breadboard and it worked decently well with controlling a CD40106 Schmitt trigger
http://electro-music.com/forum/topic-28799.html
just if you use a vactrol don’t forget to add a Resistor to the Led
Need to add this here: 
In the Sega MegaDrive Synth Sam uses switched jacks, that means when there is nothing in them, the potentiometer gets 5V as its input, but if you plug in a CV, it gets the CV voltage and then acts as an attenuator for the CV signal.
But, yeah, only very few circuits can be modded with this.
That’s a custom MIDI controller, though, so pretty far from the original “can I control how quickly this capacitor discharges with a control voltage?” question 
I was sure I was going to see a CV input added to a drill if I clicked this link. “I have to put a hole in steel-play C5! Play C5!”
@dud, we have your next project here…
this is kinda correct and you are almost there .
from @fredrik’s link:
"Although people try, it is not really possible to get good results from just plugging a control voltage directly into the vactrol LED through a current-limiting resistor. To make it work properly you need a constant-current drive circuit, which basically requires an op amp.
Because of all these considerations, it’s hard to recommend the use of vactrols on any basis that is intended to continue into the future. "
guess I got to go mod my stripboard =]
thats what I was relaying to him too
This part of the circuit isn’t very exact as it is, so no need to abandon the idea before you’ve played with it.
The decay pot controls how quickly C12 discharges after a trigger pulse, and there are several ways to control that electronically, either by limiting the discharge current (which is what the pot does), or possibly by scaling the voltage you’re charging it with. All these approaches require more complicated circuitry, especially if you want a solution that doesn’t depend too much on component tolerances. A JFET may be easiest, but then you have to figure out how to bias it correctly.
I am resurrecting this here, because I have found some new circuits. At first, I found this Kassutronics module, which is something similar to the Makenoise Maths (both are based on an old Serge design).
It uses transistors and an integrator (U1C):
I thought, maybe something like this could be used to add CV to a decay circuit? Maybe even simpler, because we don’t necessarily want to control the attack.
Then I found the Sonic Potions’ Descent, which is basically exactly what I want, a voltage controlled decay envelope generator:
This looks a bit similar to the Kassutronics module, but with a lot more opamps! So, I am wondering why… It should be simpler without the attack part, I thought, but obviously I don’t understand enough electronics 
Can anyone explain or knows if I could simply use the (modified) Kassutronics circuit to control a drum decay via CV?
I really enjoyed reading this article. Thanks for sharing! I’m definitely going to familiarize myself well with the LM13700 since it seems to be one of the best options to design CV controlled modules, besides going digital. 
Cool stuff! Can anyone explain how method 2 for current works? It looks a bit like a long tail pair, but with diodes and it’s AC coupled to the resistors? I don’t understand! XD
I got around to a blog post about one of my pet peeves: CV-driven vactrols.
Great post. Very instructive. 
