The VARIABLE Q VCF (VCFQ) is an excellent general-purpose VCF offering simultaneous low-pass, high-pass, band-pass and notch (band-reject) outputs. The resonance (Q) of this filter is dynamically variable by manual or voltage control. The VCFQ has two signal inputs. One incorporates an automatic gain control to prevent the filter from overloading at high Q settings. The second input has a level control so that the percussive effects of overloading the filter can be exploited. When a pulse is applied to the Trigger input, the filter will ring, producing a damped waveform similar to that produced by striking a resonant object. The nature of this ringing is controlled by the Q and the filter frequency. Percussive effects ranging from clicks to the sound of wood blocks and bell tones can be produced and controlled. This ringing effect can be used in conjunction with signals applied to either of the audio inputs to achieve highly controlled complex tonal qualities.
The VCFQ is one of the more elaborate CGS builds, due to the use of three small daughter boards, the Serge Gain Cell. The Gain Cells have to be separately built and fine tuned with a test circuit, before they can be added to the main PC board assembly.
The VCFQ will not oscillate by itself at high Q. It requires an input. However the high Q settings do result in very strong corner peaking in the pass band, which is where the AGC input comes in handy. A signal going to the AGC input will be attenuated as the Q increased.
These are the only true filters in my CGS Serge/Bugbrand banana jack cabinet. They are certainly unique. Listen to the demo recordings below.
This module is a variation on the 1973 Classic Serge Triple Wave Shaper module.
To quote the 1982 catalog: The Triple Wave Shaper (TWS) is a non-linear modifier which can transform a sawtooth wave into a sine wave. This module incorporates three independent wave shapers for modifying synthesizer waveforms or for processing signals from preamplified instruments. Although originally designed as a wave shaper for our early oscillators, this module has been found to be a excellent modifier of electronic and acoustic sounds, and is highly recommended for subtle timbrel modifications beyond the range of simple oscillator/filter patches.
Of course, it’s uses go beyond this. When used in series, they can be used to add folds to the wave shape. Each extra wave shaper you put in series adds a fold to the waveform. As such, quite a lot of tonal variety/harmonic enrichment can be achieve.
Since there was only room on the panel for two wave shaper modules, I decided to cascade two of them internally. The rightmost shaper is actually two placed in series with the two CV inputs wired to both. The panel input goes to the first, and the output is taken from the second.
The bracket is identical to the one used for the Serge Enveloper Generator and Triple Switch. (I fabricated the brackets from lightweight aluminum.) The bracket is secured under eight banana jacks that run in line down the panel.
Finally here are two VCFQ demo recordings. Each of them uses a sweep CV going into the 1V/octave input. In the first recording the first section sweeps over a square wave from a VCO on the input, and the second section sweeps the filter frequency, correlated with a sequence of triggers on the trigger input. The band pass output is heard. Various Q settings are demonstrated.
The second recording listens to a sweep of the band pass with the Pink Noise output from the Serge Noise module. The Q setting was increased manually from one sweep to the next.