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Can I drive a Vactrol directly from CV?
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Can I drive a Vactrol directly from CV?
glennfin
Wondering if I can drive the input of a vactrol (LED) directly from a typical CV output or do I need to buffer it first..

Thanks
ersatzplanet
All you need is a resistor to light the LED section of it. Think of it like a standard "jacklight" or LED patch cord. A positive Cv will light the led. The restive side of the Vactrol will need more circuitry depending on what you want to do with it.

I designed a module based on a signature somebody used on this forum that said something like Vactrols being like drunk CV. The module was just a standard resistor to the LED on the vactrol, and the resistive side of the Vactrol was part of a standard resistive voltage divider circuit. Basically the CV IN would be replicated on the output, but with all the "slop" that a vactrol would add, the slew up and down, the lag in general - "The Drunken CV Processor" was born.

Since LEDs only really respond to positive voltage, two vactrols could be used with the LED polarities reversed and one would output the positive side of the CV in and the other would do the negative side. It would be sort of dead in the middle close to zero, but might make a fun effect.

I have a bag of unmatched cheap vactrols from a surplus site and they would be perfect for this since they are not matched and are wildly different.
mcbinc
glennfin wrote:
Wondering if I can drive the input of a vactrol (LED) directly from a typical CV output or do I need to buffer it first..


The LED needs milliamps to operate meaning ~1k resistance. That's low enough to cause significant loading and CV drop. You'll want a buffer if you care about the CV value in any way.
ersatzplanet
mcbinc wrote:
glennfin wrote:
Wondering if I can drive the input of a vactrol (LED) directly from a typical CV output or do I need to buffer it first..


The LED needs milliamps to operate meaning ~1k resistance. That's low enough to cause significant loading and CV drop. You'll want a buffer if you care about the CV value in any way.


The CV droop is only a problem if it is also going somewhere else. if the destination is the Vactrol itself, then there is no problem, just like a Jacklight, it will work with just the resistor. Now if it is pass through to something else, like in a LED patch cord, it will drop the voltage also. Typically this is only a problem with pitch accurate CVs but I think the original poster was thinking the Vactrol was the target and nothing else.
windspirit
Yeah your 1v/o sequences which arent using a buffered multiple might droop but if you are just using the vactrol with a gate or an envelope as is normally the case then it doesnt really matter.

If building your own vactrols then red LEDs have the lowest forward voltage.
glennfin
Yea, it looks like I should go with a buffered CV to the vactrol since that same CV may be multipled to several locations.
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