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. The resonance is also voltage controlled using vactrols, (filter will self-oscillatie).
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 greater with higher resonance. Out 2 compensates this behaviour by using an additional vactrol that is connected in series to the resonance vactrol. This additional vactrol forms an attenuator (or simple VCA) that approximately compensates the level increase for higher resonance settings. Two LEDs show the current frequency and resonance vactrol status.