The CGS Serge Wave Multipliers is the fourth and final module in my CGS Serge banana jack panel. Please read Ken Stone’s project notes.
The board contains three independent timber modifier circuits that function to add harmonic content to an audio signal, the opposite approach to filtering, which selectively removes harmonic content.
Each has a panel pot for the initial setting, plus a non-attenuated CV input. If you build this module, the addition of CV attenuator pots would be quite useful.
The first multiplier is an non-standard VCA. It can operate in two modes, Lo or Hi, as selected by a toggle switch. In the Lo mode it works much like a regular VCA, except that it can amplify the input to at least twice the incoming level. There is also a bit of low pass filtering, a rounding of sharp edges. In the Hi mode the signal is squared up, but still with rounded edges. In Hi mode the gain is less than one.
The second multiplier adds odd harmonics in a way that really benefits from sweeping the CV input. It has two signal inputs that mix. One is DC-coupled, the other AC-coupled and these are inverted in phase from each other.
The third multiplier also has two inputs that mix together. And it has two different, simultaneously available outputs. This multiplier doubles the frequency of a sine or triangle wave at its input, and also benefits from sweeping the CV. The second output squares up the signal, resulting in a kind of pulse wave modulation.
This was built using the version 1.0 CGS 113 PC board. It required close attention to resistor values, comparing the board against the schematic. Some capacitors need to be omitted from the board. The resistor on trimpot TR2 should be a 1K5. At least three, and as many as eight capacitors need to be installed on the back of the board. I started out with the three capacitors that Ken Stone installed. After scoping the second multiplier and discovering parasitic oscillations, I added five more 47pf caps. Click the image for greater detail.
I made a little demo recording of it processing a triangle wave of a VCO being stepped in a random fashion. It starts out showing the first multiplier in Hi mode. Then the second multiplier, followed by the third, and then I mixed all three together. It ends with the first multiplier again. I played with the initial pots and varied the CV levels, using an external attenuator. A Serge Negative Slew is used as an envelope generator for the CV inputs.