# Adding CV to existing circuits **URL:** \ **Category:** DIY STUFF\ **Created:** [June 25, 2020, 3:37am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305 "2020-06-25T03:37:25Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![CTorp](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/ctorp/32/15023_2.png) [@CTorp](https://lookmumnocomputer.discourse.group/u/CTorp)\ **Post date:** [June 25, 2020, 3:37am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/1 "2020-06-25T03:37:25Z")
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: ![vactolQ1](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/4/4a0a4a2ae4699a4b25eb2fee8c3789105f380505.png) 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: ![vactrolQ](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/4/4de4f071759f90d733c6d7cfbfe91d309ab45746.png) is there an easier way to do this without using vactrol? opamps? ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [June 25, 2020, 6:23am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/2 "2020-06-25T06:23:51Z")
Matthew Skala goes over some options here: > **[Voltage control modifications](https://northcoastsynthesis.com/news/voltage-control-modifications/)** > > Everybody wants to "mod" existing devices to add control voltage inputs. That usually doesn't work well, but let's look at why not, and some cases where it is possible. ---
**Author:** ![Circulartonic](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/circulartonic/32/10399_2.png) [@Circulartonic](https://lookmumnocomputer.discourse.group/u/Circulartonic)\ **Post date:** [June 25, 2020, 6:39am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/3 "2020-06-25T06:39:12Z")
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](http://electro-music.com/forum/topic-28799.html) ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [June 25, 2020, 9:35am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/4 "2020-06-25T09:35:35Z")
just if you use a vactrol don’t forget to add a Resistor to the Led ![4de4f071759f90d733c6d7cfbfe91d309ab45746](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/d/db9ccaf0ebfb66d0bbca0f7e5e39b3e44c9bf616.png) ---
**Author:** ![sebastian](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sebastian/32/39_2.png) [@sebastian](https://lookmumnocomputer.discourse.group/u/sebastian)\ **Post date:** [June 25, 2020, 10:41am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/5 "2020-06-25T10:41:51Z")
Need to add this here: 😉 [![](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/b/b4a3c0d910392f1ae679b3701949732c3475597b.jpeg "Sega Megadriver Synthesizer In Depth Look") ](https://www.youtube.com/watch?v=eAgwVvJwdnc) 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. ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [June 25, 2020, 10:55am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/6 "2020-06-25T10:55:49Z")
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 😃 ---
**Author:** ![Maxhirez](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/maxhirez/32/42_2.png) [@Maxhirez](https://lookmumnocomputer.discourse.group/u/Maxhirez)\ **Post date:** [June 25, 2020, 12:11pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/7 "2020-06-25T12:11:55Z")
> [@fredrik](#): > > Matthew Skala goes over some options here: 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!” ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [June 25, 2020, 12:17pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/8 "2020-06-25T12:17:56Z")
@dud, we have your next project here… ---
**Author:** ![Dud](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/dud/32/610_2.png) [@Dud](https://lookmumnocomputer.discourse.group/u/Dud)\ **Post date:** [June 25, 2020, 12:49pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/9 "2020-06-25T12:49:32Z")
![424px-akkubohrschrauber](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/0/0660815d7dbcb86cf7372ef6cc1e7c9e64143ee0.jpeg) ---
**Author:** ![SYNTHGUY57](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/synthguy57/32/14422_2.png) [@SYNTHGUY57](https://lookmumnocomputer.discourse.group/u/SYNTHGUY57)\ **Post date:** [June 25, 2020, 10:55pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/10 "2020-06-25T22:55:01Z")
this is kinda correct and you are almost there . ---
**Author:** ![CTorp](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/ctorp/32/15023_2.png) [@CTorp](https://lookmumnocomputer.discourse.group/u/CTorp)\ **Post date:** [June 25, 2020, 10:55pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/11 "2020-06-25T22:55:14Z")
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 =] ---
**Author:** ![SYNTHGUY57](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/synthguy57/32/14422_2.png) [@SYNTHGUY57](https://lookmumnocomputer.discourse.group/u/SYNTHGUY57)\ **Post date:** [June 25, 2020, 10:59pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/12 "2020-06-25T22:59:59Z")
thats what I was relaying to him too ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [June 25, 2020, 11:04pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/13 "2020-06-25T23:04:52Z")
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. ---
**Author:** ![sebastian](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sebastian/32/39_2.png) [@sebastian](https://lookmumnocomputer.discourse.group/u/sebastian)\ **Post date:** [February 25, 2021, 12:05pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/14 "2021-02-25T12:05:08Z")
I am resurrecting this here, because I have found some new circuits. At first, I found [this Kassutronics module](https://kassu2000.blogspot.com/2016/04/slope-generator.html), which is something similar to the Makenoise Maths (both are based on an old Serge design). It uses transistors and an integrator (U1C): ![](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/6/66191cc44161d3cc5c784b729f5f43c0d8706a9e.png) 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](https://www.sonic-potions.com/descent), which is basically exactly what I want, a voltage controlled decay envelope generator: ![Screenshot from 2021-02-25 12-57-57](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/4/4024cfa584f1f6b42ba63c472654e5f796673f97.png) 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? ---
**Author:** ![nschagen](https://avatars.discourse-cdn.com/v4/letter/n/94ad74/32.png) [@nschagen](https://lookmumnocomputer.discourse.group/u/nschagen)\ **Post date:** [February 26, 2021, 11:44am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/15 "2021-02-26T11:44:31Z")
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. 🙂 ---
**Author:** ![Plop](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/plop/32/11707_2.png) [@Plop](https://lookmumnocomputer.discourse.group/u/Plop)\ **Post date:** [March 6, 2022, 6:03pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/16 "2022-03-06T18:03:47Z")
I found those schem’ on a facebook page about synth 🙂 ![Add CV to any pot](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/3X/5/f/5f031d05603bccf9c979de8dfac4e62b4ec409ed.jpeg) ---
**Author:** ![sebastian](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/sebastian/32/39_2.png) [@sebastian](https://lookmumnocomputer.discourse.group/u/sebastian)\ **Post date:** [March 7, 2022, 11:30am UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/17 "2022-03-07T11:30:09Z")
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 ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [May 4, 2022, 2:22pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/18 "2022-05-04T14:22:33Z")
I got around to a blog post about one of my pet peeves: CV-driven vactrols. > **[How to vactrol](https://analogoutputblog.wordpress.com/2022/05/04/how-to-vactrol/)** > > Look around the synth DIY world and before long you’re likely to find a circuit containing something like this: Vactrol driven by CV through resistor This is an attempt to give voltage contro… ---
**Author:** ![peternoya](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/peternoya/32/13307_2.png) [@peternoya](https://lookmumnocomputer.discourse.group/u/peternoya)\ **Post date:** [May 9, 2022, 7:30pm UTC](https://lookmumnocomputer.discourse.group/t/adding-cv-to-existing-circuits/1305/19 "2022-05-09T19:30:41Z")
Great post. Very instructive. 👍🏼