Döpfer A101-2 - Low Pass Gate

A 12 dB low pass filter that can be switched to VCA or a combination of Low Pass and VCA. The controlling elements for frequency (LP mode) resp. amplitude (VCA mode) are so-called vactrols. Because of the vactrol circuit the audio signal is not fully attenuated in the VCA mode at the minimum setting of the F/A control.

The frequency (in LP mode) resp. the amplitude (in VCA mode) is controlled manually (F/A) and by the 2 control inputs CV1 (without attenuator) and CV2 (with attenuator). The audio input is equipped with an attenuator to enable distortion too (above position 5 distortion is obtained with standard A-100 audio levels, e.g. VCO). The resonance function "colors" the sound and is adjustable all the way up to self-oscillation. Resonance and consequently self-oscillation may vary with the filter frequency because of vactrol tolerances. Due to the circuit the resonance has a little bit of influence on the audio level (increasing resonance = increasing audio level). To obtain the original Buchla sound the resonance control has to be set fully counterclockwise.

The function of the module is controlled by a manual switch. The left and right positions of the switch correspond to LP resp. VCA mode. In the middle position one obtains the combination of Low Pass and VCA. In this position it is also possible to control the function of the module by the two Gate inputs G1 and G2. The table printed at the front panel shows the connection between the gate levels (L = low, H = high) and the module function.