This is my take on the legendary Serge Smooth and Stepped Generator (SSG). I received the SSG PC board as a compliment from Ken Stone, who designed the board. The circuit is of course by Serge Tcherepnin. Ken has an excellent writeup of the SSG.
The banana jack color coding used on this panel is as follows:
- Yellow: signal input
- Blue: control voltage input
- Black: gate or trigger input
- Green: signal output
- Red: gate or trigger output.
The SSG contains two almost identical voltage-controlled circuits that place a slew (a limit how fast the voltage can change) on an incoming signal. The effect is a lag, that, when applied to a VCO pitch, will produce a portamento. That is just the beginning. Each slew has a Cycle output, which when patched to the input turns the module into a triangle wave VCO. There is also a Hold input, which takes a gate signal. When Hold is held high, the slew rate becomes so low as to stop the output from changing at all.
The board also contains two ‘one-shot’ generators that emit a short negative pulse when triggered at the input. The one-shot output normally sits high, around +15V. Patching it to a Hold input turns the Smooth function into a Stepped function, a type of sample and hold. The Serge panel did not provide the outputs of these one-shots, but internally jumpered one of them to the Hold input of one slew, thus making it into the Stepped generator. I put the inputs and outputs of both one-shots onto panel jacks, so I can have one or two Stepped Generators, by patching to Hold inputs. I added a 1K resistor in series with each one-shot output, for current limiting.
The last circuit on the SSG board is called the Coupler. It’s a comparator that, in the original design, looked at outputs of the two slews and put out a gate indicating that one is higher than the other. I decided to make this into a separate module on the panel, and I modified the circuit just a little. I disconnected the inputs from the slews on the PC board, added 100K terminating resistors on the inputs, replaced the small feedback capacitor with a 4M7 resistor for DC hysteresis, and brought the 0 to 6V output to the panel.
The LED driver board is powered by a short cable coming over from the SSG board itself. As with the other CGS Serge boards in this panel, I used a combination of MTA 100 and flying wire connections to insure that the board can be completely removed from the panel without unsoldering. (Click this pic for more detail.)
Each slew has a trimming pot that sets the initial slew rate. I somewhat arbitrarily set the faster slew (the one using the 100nf timing cap) to max out at about 1Khz when the knob is turned all the way up. I set the slower one for about 500Hz max. When oscillating, these circuits are not very stable. But they seem to settle down and stop wavering after a while.