A-101-3 Vactrol Modular Phaser

Module A-101-3 is a 12 stage phase shifter with vactrols as phase shifting elements. Vactrols are known for their smooth sound behaviour.

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).

The first 12 frequency response curves show the behaviour of the module when stages 1...12 are used as outputs for the final mixer (no feedback, no additional patching). This is the standard phaser application with a different number of phase shift stages. The frequency response curves of the higher stages show the typical comb filters of a phaser. The notches move through the audio spectrum as the manual phase shift control is operated or a control voltage is applied (for a standard phase shifter this is normally the triangle or sine wave from a LFO). The number of notches increases with the number of stages: number of notches = integer of number of stages/2. Odd stage numbers lead to different behaviours in the higher and lower frequencies (low end: high pass behaviour, high end: passage). Even stage numbers show the same response in the higher and lower frequencies (passage for both). Stage 1 is nothing but a high pass filter, stage 2 is the standard notch filter.

The second 12 frequency response curves show the behaviour when an inverter is inserted between the stage output in question and the final mixer (one of the polarizers can be used for this job). The result is the inverse frequency response compared to the output without inverter: e.g. low pass for stage 1,  band pass for stage 2. The resulting frequency response curve is simply obtained by vertical mirroring the first 12 curves.

Additional feedback colors the sound. The third 12 frequency response curves show the behaviour of the filters with one feedback loop. Feedback comes from the stage used as output back to stage 1 (e.g. if stage 11 is used, feedback from stage 11 to stage 1).

But it is not imperative to use the same stage for feedback and audio output. Groups 4, 5 and 6 of frequency response curves show the behaviour with different feedback loops. In group 4 the same output 12 is used for all graphs but but the feedback goes from stage 12, 11, 10, 9 ... and so on back to stage 1. Group 5 is nearly the same but output 6 is used for all graphs. In group 6 the output stage is the varying parameter and the feedback goes from stage 8 to 1 for all filters.