Monday, February 21, 2011

Vactrol Basics

A Vactol is an opto-isolator device that is often used for channel-switching in many modern amplifiers, but these electronic parts can be used in various other ways too, for example in synth-modules like Doepfer's A-101 Series and the A-102 Low Pass Gate module.
There are many kinds of optical isolators, but the most common is the LED/photo-transistor type.

A vactrol is a combination of a light depending resistor (LDR) and light source (LED) both put into a small 100% light-proof case.
The principle of a Vactrol is very simple.
When an input current is applied to the LED, the output phototransistor turns on.
The only connection between the LED and phototransistor is through light -not electricity, thus the term optical isolator. 
These optical isolators are primarily digital in nature with fast response times suitable for interfacing with logic gates. Rise and fall times of a few microseconds, faster for some isolators, are typical.

So together, the coupled pair act as an electrically variable potentiometer.
Since the output element of the AOI is a resistor, the voltage applied to this output resistor may be DC and/or AC and the magnitude may be as low as zero or as high as the maximum voltage rating.
Because the input will control the magnitude of a complex waveform in a proportional manner, this type of isolator can be used as an analog control element in all kind of ways in analog synthesizers.

Find a review of my A-101-2 Vactol Low Pass Gate module  HERE

More info on Vactrols:   Doepfer Vactrol Basics page
                                      Vactrols on Wikipedia
                                      PDF via


  1. Interesting stuff; can you describe a few examples of how you'd use it in a specific patch?

    Marc JX8P

  2. @Marc Vactrols introduce voltage control to any function that is controlled by a fixed or variable resistor (e.g. a two-terminal normal or trimming potentiometer). Typical examples of vactol use within the A-100 are:
    Attack, Decay, Release of all envelope generators (A-140, A-143-1, A-143-2)
    Slew time of slew limiter (A-170)
    Delay time and duration of trigger delay (A-162)
    Clipping functions of the extreme filter (A-106-1)
    Clock rate of digital noise (A-117)
    Random rate and level of analog noise (A-118)
    Frequency of Trautonium filter (A-104)

    The A-101-2 Low Pass Gate module that i bought has 2 Vactrols inside, 1 for it's low-pass filter and 1 for the VCA.

    An A-101-9 (Dual) Universal Vactrol Module can be used to control other modules. (after some modification of these modules)
    Also see