CGS voltage controlled slope

CGS75 the CGS voltage controlled slope is a licensed adaptation of the classic Serge DTG/DUSG.

Description
The Serge VCS module 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.

It will run on +/- 12 volts or +/-15 volts.

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/octave 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.

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 wave shape 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.

Construction


Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges. (With the boards supplied by me, the edges are already milled, and etching faults are very rare.)

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC socket if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components such as electrolytics, diodes, transistors and ICs.

When inserting ICs into sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

The holes without pads near the input and output pads are to allow a wire to be fed up from the bottom side, looped over and then soldered to the pad. The slack is then pulled back through the hole, creating an anchor that holds the wire by its insulation, reducing the chance of wire breakage. It is also helpful for a system such as Serge, where wires are traditionally connected from the solder side of the PCB.

A problem in the genuine Serge is that cycle locks up at various frequencies due to a charge in balance in the trigger circuit. If you experience this problem, it can be solved by changing the 10n capacitor to 1n. These changes are marked on the circuit diagram.

You may need to reverse the connections to some of the pots to get them to work in the direction you wish.

Setup
Adjustments on the VCS board are set to obtain a 0 to +5 volt level when the unit is cycling, producing a 100Hz triangle wave. An oscilloscope is required for this adjustment. In an oscilloscope is not available, adjust for the least distorted sounding waveshape.

Parts list
This is a guide only. Parts needed will vary with individual constructor's needs.

CC-BY-NC
Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work.

Suppliers

 * CGS75 Voltage Controlled Slope, revision 1, Elby Designs
 * Small Bear Electronics, for Davies 1900H style knobs