Doepfer A-101-3 Modular Vactrol Phase Filter


  • Feedback In 1-12
  • Stage Out 1-12
  • Polarizer 1/2 Out
  • Polarizer 1/2 In (2x)
  • Mixed Audio Out (2x: 1-6/1-7)
  • Shifted Audio In (2x: 1-6/1-7)
  • CV In (2x: 1-6/1-7)
  • Audio In (2x: 1-6/1-7)



Module A-101-3 is a 12 stage phase shifter with vactrols as phase shifting elements. Vactrols are known for their smooth sound behaviour. For more general details about vactrols please look at the Vactrol Basics page.

In contrast to other phaser designs the A-101-3 is much more flexible and offering a lot of new features not available from other phasers on the market (as far as we know, please tell us if we are wrong). The main difference is that our design offers access to each of the 12 input and output stages leading to a lot of new filters that cannot be obtained in other ways. Especially the free patchable feedback loops (yes, not only one feedback loop is possible) between each of the 12 stages, the separate phase shift control for the stages 1-6 and 7-12, and the 2 polarizers intended to control the feedback loops lead to completely new filter types (a polarizer is a circuit that is able to generate positive and negative amplifications in the range -1...0...+1 with -1 = inversion, 0 = full attenuation, +1 = unchanged signal, for details concerning the polarizer function please look at the A-133 VC Polarizer or A-138c Polarizing Mixer module).

The module sketch and the frequency response curves below will help to explain the outstanding functions of the module:

Internally the module is made of 2 independent 6 stage phase shifters (1-6 reps. 7-12) with separate audio inputs (with attenuators), audio outputs (with mix control), and phase shift control units. The phase shift control units feature both manual and voltage controlled phase shifting (e.g. from a LFO, ADSR, Random Voltage, Theremin CV, Foot Controller CV ...). For each sub-module a phase shift display (LED) is available. The LED shows the illumination state of the 6 vactrols of the sub-module in question as it is connected in series with the internal vactrol LEDs.

Each of the 12 phase shift states is equipped with an audio output socket and feedback input socket to obtain full flexibility to create a multitude of different filters. The audio input signal and the output signals of stage 6 resp. stage 12 are mixed with 2 manual controls to obtain effects at two audio outputs (for normal phase shifting effect this is 50% input signal and 50% phase shifted signal). The two submodules are internally connected via normalized sockets so that two 6 stage phase shifters can be obtained without external patches. Audio output of stage 6 is normalized to audio input of stage 7 and CV input 1-6 is normalized to CV input 7-12. But due to the open structure of the module even other stages than stage 6 and stage 12 can be used as outputs to generate different sounds (simply patch the desired stage output to the normalized mix input socket).

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