CGS75 - Serge Voltage Controlled Slew (VCS) | Cat Girl Synth

 This is a licensed adaptation of the classic Serge DTG/DUSG

The Serge VCS module released in conjunction with from bananalogue.com is an extremely versatile control voltage generator and audio source. In the early 1970s, Serge Tcherepnin developed the Positive Slew and Negative Slew modules for the original Serge synthesizer. In time these merged into the classic Dual Universal Slope Generator. The Bananalogue VCS and CGS75 are an adaptation of Serge's original circuit with a few new features.

The VCS is a unity gain voltage follower. The rising and falling slopes are independently and jointly voltage controllable over a wide range.

VC Transient Envelope Generator

A pulse at the trigger input will start the envelope, or a gate input will sustain the level and the envelope will fall when the gate goes low. Rise and fall are independently and jointly voltage controllable, with variable linear and exponential wave shapes.

VC Portamento

Voltage is slewed according to the rise and fall times.

VC LFO

When the cycle switch is thrown, the trigger input is connected internally to the end trigger output, creating a VC clock with variable waveform and independent rise and fall times.

VC Oscillator

While not as wide ranged, or accurate as a dedicated oscillator module, the VCS is still an excellent audio source. The Exp CV input is scaled approximately to the 1v/oct standard. The Output wave can be swept from triangle to saw with linear and non-linear waveforms. End Out also produces a pulse waveform.

VC Non-Linear Audio Processor (Low-Pass Gate)

If an audio rate signal is slewed, the module responds like a VCF, and a rough VCA. The signal is low-pass filtered down to silence, similar to a low-pass gate.

Envelope Follower

Positive and negative peak detection envelope follower.

VC Pulse Delay

Trigger input starts the envelope and a trigger will be produced again at the End Out when the envelope completes its cycle.

Sub-Harmonic Generator

If a series of triggers are applied to the VCS faster than the total rise and fall times, the module will divide the incoming signal by a whole number. In the audio range the output will be the sub-harmonic series.

(Description taken from the former Bananalogue site)

A little on how it works.

Closing SWF or SWR feeds some of the output back into the voltage controlled inputs, changing the curve of the response - i.e. allowing for increasing or decreasing rates of change of the output, thus allowing the output waveshape to be non-linear.

Bipolar out gives an inverted signal.

AC out gives the non-inverted signal, but centered around 0V.

The schematic for the Euro version is different in the area of the switches SWF and SWR, though functionality remains the same.

For those interested in the theory of operation, I recommend Tim Stinchcombe's page on the subject.

The schematic of the VCS. Several suggested component changes are indicated on the diagram.