Doepfer A110 Filtering Demo's with A-101-2 and A-120 by Raul Pena

Raul Pena ( from http://raulsworldofsynths.com/ ) made another excellent set of tutorial videos.
This time it is all about filtering; In a few episodes he explores all functions of the A-101-2 Vactrol Low Pass Gate filter, in comparison with the A-120 Low Pass Moog-style filter.
Very interesting stuff again...

Video 1: Doepfer A110 Filtering with A101-2 and A120 Intro
http://www.youtube.com/watch?v=QmrB9EOVgb8
" Short Introduction into Filtering the Doepfer A110 Standard VCO. Dicusses Features and Functions of the Doepfer A101-2 Low Pass Gate and A120 VCF Low Pass Filter.Sound and Video by Raul Pena. "



Video 2: Doepfer A110 Filtering with A101-2 Low Pass Gate
http://www.youtube.com/watch?v=qBt7wozgjjg
" Demonstration of the Filtering of the Doepfer A110 VCO with an A101-2 Low Pass Gate. Next More Filtering with the A101-2 Low Pass Gate."

More video after the break -

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

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 DatasheetCatalog.com