CGS Serge Positive Slew, SSG, and Noise

serge-pos-slew-ssg-panelThis 5U size panel houses three CGS Serge PC boards:

Dual Positive Slew

Sending a gate or trigger into the START jack will cause the slew generator to lock into a rising waveform, regardless of what is happening at the other inputs, although the ramp is affected by them. The PULSE OUT remains on for this period. When the peak of the cycle is reached, both the OUTPUT and PULSE OUT outputs reset.

Sending a gate into the SUSTAIN jack will will start a cycle, much like above, with one difference. Once the cycle ends, and for as long as the gate signal remains, the OUTPUT will remain at its peak, and PULSE OUT will remain on.

The CYCLE OUT is derived from the analog OUTPUT. Connecting this output to START or SUSTAIN will cause the Positive Slew to operate as a VCO or LFO.

The CV input has a processor style level pot, allowing the response to the incoming control voltage to be tailored to respond directly to the signal, or to respond to it in reverse.

If not running a cycle as described above, the OUTPUT will follow any signal at the input, with any rising voltage subject to slewing as set by the RATE pot and any control voltage present at the CV input.


The SMOOTH & STEPPED FUNCTION GENERATOR (SSG) is s a complex multi-functional module to provide various slew and sample functions.

The Smooth section will place a positive and negative slew (glide) on a changing input voltage for lag effects, voltage controlled portamento, and non-linear, low frequency filtering. With the CYCLE jack patched to the input, the Unit will oscillate yielding a voltage controlled triangle wave LFO. A high level into the HOLD input will hold the current output level, whether the unit is oscillating or processing an external control voltage. This is identical to a track-and-hold function.

The Stepped function can be used as a sample-and-hold with voltage controlled slew rate limiting. Slew rate limiting limits the size of the step at the output. With the step size limited to a small value, if the input is a random voltage, the output is a random voltage also, but it will only vary slightly from step to step, gradually covering the entire range of the input random voltage. No large changes in the output will be allowed. With the Cycle jack patched to the input and a trigger applied to the Sample input, complex staircase waveforms are generated.

My modifications for this SSG were to omit the Coupler function and to add a three position toggle switch for the Trigger Input. The switch selects the signal sent to the Hold input (which is not on the panel). In the down position, the Hold input comes from the internal pulse circuit that is driven by the Trigger input. This is the normal Stepped function. The up position sends the Trigger input directly to Hold, turning it into a Smooth function. The center is off.

Serge Noise Source

I built only the noise generator portion of the PC board and left out the weird sample and hold, especially since the SSG has a sample and hold function.  The four outputs are White, Pink, AC Random and DC Random.  The AC Random is simply a capacitor-coupled variant of the DC Random, which is actually an oscillator with a random FM.

Construction Photos

This picture shows the mounting of the Noise Source and SSG PC boards behind the panel.  Note the opening in the bracket at the bottom center, which allows for wiring to go to the other side of the front panel.  Wires go both directions, because there is an SSG and a Positive Slew on either side on the front.


The second picture shows the side of the bracket with the Dual Positive Slew PC board.



3 Responses to CGS Serge Positive Slew, SSG, and Noise

  1. Fer says:

    Great combination!
    Are you using +-15V for the noise circuit?
    I assume that if the S&H is not built the 12V regulators are not needed, and that the noise circuit will work perfectly fine on 15V, am I right? Maybe a slightly higher amplitude at the outputs?
    (but in your picture I see one regulator installed)

  2. Richard says:

    I did install the +12 volt regulator, so the circuit would work as designed. It’s a really nice noise source, too. I especially like the DC random output. No need to bias up if you want a positive only random signal. It’s a noisy ramp wave.

  3. Fer says:

    thank you. I see, that’s why only one regulator is installed, the circuit only need – supply if the “RPD” resistor is installed.

    Still wonder if the noise alone would work with 15V, I’d say the warning is because of the ICs on the S&H part…

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