Monday, March 10, 2014

Doepfer A-101-6 Six Stage VC Opto FET Filter/Phaser Announced

Another new module announcement from Doepfer.

" A-101-6 is a new filter module that uses so-called opto FETs to control the filter frequency.
A-101-6 Six Stage Opto FET VCF
Opto FETs are very similar to Vactrols but use light depending field effect transistors (FETs) instead of light depending resistors (LDRs).
A Opto FET is a combination of a light depending FET and a LED (light emitting diode) both put into a small light-proof case.

The advantage compared to vactrols is the much faster response of opto FETs compared to LDRs. 
This allows much faster attack/decay times and even FM effects. 
The disadvantage compared to vactrols is that the FET behaves as a normal Ohm resistor only for small levels. With higher levels the FET begins to distort.
Apart from this the notes mentioned on the Doepfer Vactrol Basics page are valid.

Module A-101-6 is made of six serial 6dB filter stages. The stages can be switched simultaneously to lowpass, highpass or one of two allpass types.
The following schematics show one of the six serial stages in the four possible modes:

A-101-6 Principle Schematics
The variable resistor shown in the schematics corresponds to the opto FET.
The type of filter is selected by two toggle switches or 2 control inputs.
The upper switch selects between lowpass and highpass provided that the lower switch is in the LP/HP position.
When the lower switch in the AP (allpass) position the upper switch is used to select between the two allpass types. Instead of the switches even external control voltages can be used (CV LP/HP, CV Allpass sockets).
For this the toggle switch in question has to be in the left position.

The brightness of the Opto FET LEDs and consequently the filter frequency can be adjusted manually (Frequ. control) and controlled by means of an external control voltage (CV) with attenuator.
The LED at the front panel reflects the LED brightness inside the opto FETs.

The resonance is controlled by the Feedback control up to self oscillation. By means of two trimming potentiometers the maximal feedback can be adjusted separately for LP/HP and allpass mode.
High feedback values can be used mainly in the allpass mode to obtain very extreme self oscillation sounds. Even an external feedback signal can be used instead of the internal feedback connection (FB In socket).

The Mix control is used to pan between the original signal (CCW position) and the effect signal (CW position).
In filter mode (LP/HP) this control is usually set fully CW.
In the allpass modes one obtains phasing sounds at center position or "pure" allpass sound in fully CW position."

Update May 6th 2014:
" thank you for your interest in the planned module.
Because of several suggestions by customers there will be probably some additions (like voltage
controlled dry/wet mix and maybe voltage controlled resonance/feedback).
So it will still take some time until the module is available.
An experienced A-100 user is currently about to try out the prototype (still without the additional voltage controlled features) and we wait for his assessment about the features of the module before we continue.
If the module is back I can do some demos." *source Dieter Doepfer

Release date and price for this module are not yet available.
Read more about Opto-Isolators at
and more about Vactrols at

The new Doepfer modules will be shown at #MusikMesse in the booth of their distributor Alex4 in hall 5.1, booth nr. C54

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