Module A-123 (VCF 4) is a voltage-controlled high-pass filter, which filters out the lower parts of the sound spectrum, and lets higher frequencies pass through. The cut-off frequency determines the point at which filtering takes effect. You can control this manually, or by voltage control (filter modulation, for instance by an LFO). Two CV inputs are available. The cut-off slope is -24 dB/octave. The circuitry uses a Curtis CEM 3320 chip. Voltage controlled resonance: on the VCF 4, resonance can be controlled not just manually, but by voltages as well, right up to self-oscillation. In this case, the filter behaves like a sine wave oscillator.
As the special circuit CEM3320 used in this module is no longer available Doepfer stopped the production of this module. The synthesizer network have one filter in stock.
For more detailed information please check the English user manual below.