Doepfer A-109 Voltage Controlled Audio Signal Processor - Manual

doepfer System A - 100 VC Signal Processor A-109
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1. Introduction
Module A-109 is a voltage controlled audio signal
processor containing the components VCF, VCA and
PAN (see fig. 1 on page 4).
The module is based on Doug Curtis' CEM3379 that
was used in many Sequential™, Ensoniq™ and
PPG™ synthesizers.
The audio in/outputs of the module are normalized,
e.g. the VCF output is fed into the VCA input privided
that no jack plug is inserted to the VCA audio input
socket.
The VCF is a 24 dB low pass filter with voltage
controlled resonance. The filter has a so-called
"constant amplitude versus resonance design", i.e.
the peak-to-peak output level remains within 6dB when
the output waveform rings from added resonance.
Manual controls for frequency and resonance are
available as well as 2 CV inputs for both (one with
attenuator). The frequency range is about 5 Hz ...
20kHz, resonance ranges from 0dB up to self oscilla-
tion.
The main VCA has a combined exponential/linear
control scale: exponential from about 0...+200mV
(corresponding to about -100dB ... -20dB attenuation),
and linear from about 200mV...+5V (corresponding to
about -20dB...0dB). The "rounded" knee at the scale
bottom allows an envelope to decay to zero with a
natural exponential sound. Manual control for ampli-
tude is available as well as 2 CV inputs (one with
attenuator).
The gains of the panning VCAs are complementary,
beeing equal and half of maximum at about +2.5V CV.
The control scales are linear between about +1 and
+3.5V CV, becoming logarithmic beyond these extre-
mes. Manual control for panning is available as well
as 2 CV inputs (one with attenuator).