# 1184 Quad vca Exponential? **URL:** \ **Category:** KOSMO MODULES\ **Created:** [January 15, 2021, 4:08am UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696 "2021-01-15T04:08:04Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![j.frad](https://avatars.discourse-cdn.com/v4/letter/j/3d9bf3/32.png) [@j.frad](https://lookmumnocomputer.discourse.group/u/j.frad)\ **Post date:** [January 15, 2021, 4:08am UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/1 "2021-01-15T04:08:04Z")
Hi there! I think I would like my quad vca mixer to have exponential vcas. Maybe mod 2 channels out of 4, something like that. Am I right to think I can just cut the trace from pin 3 to 4 and jump pint5 to 3 and the vca will behave exponentially? ![Screenshot 2021-01-15 at 05.05.58](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/8/881949fb874fe92d6bc06fce68a97791658101f3.png) ---
**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:** [January 15, 2021, 6:18am UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/2 "2021-01-15T06:18:59Z")
hmmmmmmmmmmm lets see , maybe? there could be something possibly missing . Let me look deeper and get back ---
**Author:** ![eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@eric](https://lookmumnocomputer.discourse.group/u/eric)\ **Post date:** [January 15, 2021, 6:40am UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/3 "2021-01-15T06:40:15Z")
Look at the datasheet here : [http://www.alfarzpp.lv/eng/sc/AS3360.pdf](http://www.alfarzpp.lv/eng/sc/AS3360.pdf) and compare with the schematics of the QuadVCA here : > **[Untitled.pdf](https://static1.squarespace.com/static/5912db7ee6f2e1435dbab4fd/t/5e7e013f5ae8b93895765662/1585316169621/Untitled.pdf)** > > 288.69 KB ---
**Author:** ![Rusty](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/rusty/32/7835_2.png) [@Rusty](https://lookmumnocomputer.discourse.group/u/Rusty)\ **Post date:** [January 15, 2021, 10:54am UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/4 "2021-01-15T10:54:21Z")
@eric This design seems a bit strange to me. Specifically the D1 on pin 6. Firstly CV needs to be conditioned to be 0 to 1.7 V, and you could cut off the negative side by putting D1 on pin 5. But now it seems to bias the signal side. Is there a mistake? Or I’m not seeing something? ---
**Author:** ![eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@eric](https://lookmumnocomputer.discourse.group/u/eric)\ **Post date:** [January 15, 2021, 1:05pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/5 "2021-01-15T13:05:10Z")
I didn’t analyse that schematics too closely as I’m building from Sam’s PCBs… But I agree, that diode looks suspicious on that pin ! ---
**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:** [January 15, 2021, 1:19pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/6 "2021-01-15T13:19:06Z")
That’s a 2 V Zener diode, it’ll conduct above +2 V and below -1 V. The CV on pin 5 is supposed to be 0 to +2 V (absolute maximum -2 V to 2.5 V). I agree, it looks like it’s on the wrong pin. Is it on pin 5 on the actual PCB? ---
**Author:** ![eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@eric](https://lookmumnocomputer.discourse.group/u/eric)\ **Post date:** [January 15, 2021, 5:36pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/7 "2021-01-15T17:36:57Z")
I had time to check the #1181 Dual VCA PCB, and… The zener diodes go to pins 6 and 9, the signal inputs of the 3360, not the CV inputs !!! ---
**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:** [January 15, 2021, 8:11pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/8 "2021-01-15T20:11:19Z")
So, @lookmumnocomputer, is that intentional? If so, how does it work? ---
**Author:** ![eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@eric](https://lookmumnocomputer.discourse.group/u/eric)\ **Post date:** [January 15, 2021, 8:53pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/9 "2021-01-15T20:53:50Z")
This came just in time, as #1181 is in my pipeline for this week-end ! I guess I’ll have to bodge the diodes in. Doesn’t look too complicated on #1181. I don’t know how easy it will be on #1183… ---
**Author:** ![Rusty](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/rusty/32/7835_2.png) [@Rusty](https://lookmumnocomputer.discourse.group/u/Rusty)\ **Post date:** [January 18, 2021, 9:30am UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/10 "2021-01-18T09:30:11Z")
Really 0 to +2? I was under the assumption that 1.7 is the maximum gain. ![image](https://global.discourse-cdn.com/free1/uploads/lookmumnocomputer/original/2X/c/c88ce8c223387b323f0a379994c1c20b4e604546.png) ---
**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:** [January 18, 2021, 1:13pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/11 "2021-01-18T13:13:42Z")
Between 1.6 V and 1.8 V. The block and connection diagram says 0 to +2 V on that pin. The absolute maximum rating is -2 V to +2.5 V. So the zener (when on the correct pin!) enforces a safe range; the useful range is smaller. Anyone heard from @lookmumnocomputer on this apparent error? ---
**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:** [January 18, 2021, 9:00pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/12 "2021-01-18T21:00:00Z")
Is this the same diode we were talking about here? > [@1184 quad mixer vca (AKA 1183)](https://lookmumnocomputer.discourse.group/t/1184-quad-mixer-vca-aka-1183/455/38): > > Yeah, the 2V Zener doesn’t seem to serve any purpose where it is. @lookmumnocomputer is it possible you meant to put it on the linear control voltage input instead of the signal input? Indeed. The datasheet that I have says “Current Outputs Capable of Swinging to Within 1,5V of Each Supply” so that first 100k resistor would limit the maximum output current to ±10.5V/100k = ±105µA so input signals above ±105µAx47k=±4.9V will get truncated at higher VCA gain settings. So as you say @fredrik … ---
**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:** [January 18, 2021, 9:31pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/13 "2021-01-18T21:31:53Z")
Yeah, looks like it is. > [@1184 quad mixer vca (AKA 1183)](https://lookmumnocomputer.discourse.group/t/1184-quad-mixer-vca-aka-1183/455/39): > > but yes the newest versions don t have these diodes and ill put an update on it right now as they can be a ball ache to find for something that serves no purpose! But I have trouble reconciling that with @eric’s seeing it on their PCB. Unless they got theirs about eight months ago. ---
**Author:** ![eric](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@eric](https://lookmumnocomputer.discourse.group/u/eric)\ **Post date:** [January 19, 2021, 6:31am UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/14 "2021-01-19T06:31:32Z")
Mine is a #1181 (Dual VCA), and I bought it end of november IIRC… ---
**Author:** ![twinturbo](https://avatars.discourse-cdn.com/v4/letter/t/8baadc/32.png) [@twinturbo](https://lookmumnocomputer.discourse.group/u/twinturbo)\ **Post date:** [January 23, 2021, 12:57pm UTC](https://lookmumnocomputer.discourse.group/t/1184-quad-vca-exponential/2696/15 "2021-01-23T12:57:47Z")
I have an early one, do i just clip out the diode? Only just got it working last week after discovering the CV input resistor was magnitude of 1000 out!