Sunday, February 27, 2011

A-101-2 Vactrol Low Pass Gate

I must say i am very happy with my latest module, the A-101-2 Vactrol Low Pass Gate.
This module is a combination of a 12dB Low-Pass Filter and a Voltage Controlled Amplifier (VCA), with 2 vactrols as controlling elements with very unique characteristics and is very popular amongst the members of the Doepfer A-100 Yahoo Users Group.

The module is based on the Buchla Model 292 Quad Voltage-Controlled Lopass Gate module. '
The term "Low Pass Gate" was first used by Don Buchla for a module that can be switched between Low-Pass and VCA.
Buchla's current (2004) update is the Model 292e Quad Dynamics Manager ,that is functionally identical to the older model, but includes some extra functions like velocity inputs, allowing controllers to control note volumes with performance gestures and preset storage.

Doepfer's LPG module can handle 2 different incoming control voltages that can be used to control the Low-Pass filter, or to open or close the VCA.
Additionally a combined mode LP + VCA is available which makes the sound more dull as the loudness decreases. This is characteristic of all acoustic instruments (the harder you strike, pluck, or bow, the richer the overtone structure), but had been lacking in electronic instrumentation before..

In contrast to the Buchla design the A-101-2 offers attenuators for both CV and audio input, a manual resonance control and two gate inputs to control the function of the module in addition to the manual toggle switch.
The filter is hard to describe, but vactrols are known for their smooth sound behavior. Compared to a 'normal' 12 dB filter like the Low-Pass filter from the A-121, they sound quite the same, but the Vactrol inside the A-101-2 does give it a smoother edge that can perhaps best be compared with a very subtle 'slew' effect. The VCA has that same 'slewy' characteristic, but doesn't sound dramatically different than a regular VCA to my ears.

The combination of the LPG and VCA, is (obviously) my favorite setting and with the resonance up this module really starts to shine. ( Resonance is adjustable all the way up to self-oscillation )
A nice extra are the 2 Gate inputs, that can make you ( rhythmically ) switch between the two functions of the module with an external sequencer or other triggers. I'm not sure if 2 Gate inputs was necessary though.., personally i had rather seen an extra CV input for the Resonance (QCV) for example.
(Perhaps this can be modified?)

Overall this is a very nice-sounding module, that i will use a lot in the near future. I'm not sure if i put it in my top-5 list of favorite modules, but it will at least be in my top 10 for the next few months.

- Check my earlier Vactrol Basics post for more general details about vactrols,
or take a  look at Doepfer's Vactrol Basics page.
More about the A-101-2 LPG  including an audio example at Andreas Krebs blog


  1. Replacing these vactrols with the originals used in the Buchla circuit will get you much nicer results. you should try it!

    best -Peter Grenader

  2. Thanks for the tip Peter.
    I will investigate on that, let me know if you have any links to more info.