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Analogue  Modular  System  A-100







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Owners Manual

Steiner Vactrol Voltage Controlled Filter  (multi mode)


The A-101.1 is a multi mode filter based on an idea by Nyle A. Steiner from 1974. The outputs of the 3 filter types (Lowpass, Bandpass and Highpass) are summed to output 1 (A notch filter response is also available), and also to output 2 with level compensation (achieved by a further vactrol VCA that compensates for level shifts with high resonance). The filter types are achieved by where in the input signal goes, with separate inputs for Lowpass, Bandpass and Highpass. The input sockets are normalised, or 3 different signals can be input. The LED’s on the panel are connected to the LED’s internally which are part of the vactrol circuit, so the indications display how the vactrols are working. The filter resonance (emphasis) is also under vactrol control (voltage control).
In contrast to the original circuit vactrols are used instead of diodes as variable resistors for frequency control of the filter.  Even the voltage controlled resonance control is realized with vactrols, where the resonance can be increased up to self-oscillation. A special feature of the module are the two audio outputs. Out 1 is the output of the original circuit. In the original circuit by Nyle A. Steiner the output level was distinctly dependent upon the resonance (higher resonance = higher output level). Out 2 compensates this behaviour by the usage of an additional vactrol that is connected in series to the vactrol that controls the resonance. This additional vactrol forms an attenuator (or simple VCA) that approximately compensates the level increase for higher resonance settings. Two LED displays show the current frequency and resonance setting. These LEDs are connected in series to the LEDs inside the vactrols and show the LED illumination inside the vactrols

Doepfer A-101.1