Serge Ring Mod, Phaser, and Resonant Equalizer

res-eq-front-1One of a set of five panels, comprising a set of CGS modules, mostly those replicating 1970’s era Serge designs.  This panel contains:

Resonant Equalizer

The RESONANT EQUALIZER (EQ) is a unique ten-band filter designed specifically for electronic sound synthesis and processing. Except for the top and bottom frequency bands, all other bands are spaced at an interval of a major seventh. This non-standard spacing avoids the very common effect of an accentuated resonance in one key, as will be the effect from graphic equalizers with octave or third-octave spacing between bands. Spacing by octaves will reinforce a regular overtone structure for one musical key, thereby producing regularly spaced formants accenting a particular tonality. The Resonant Equalizer’s band spacing are much more interesting, producing formant peaks and valleys that are similar to those in acoustic instrument sounds.

There are three equalized outputs, two which mix the alternate filter bands, and one which is a mix of all filter bands. The upper (up arrow COMB) lets pass the outputs of frequency bands at 61 Hz, 218 Hz, 777 Hz, 2.8 kHz, and 11 kHz. The lower (down arrow COMB) mixes the other bands (29 Hz, 115 Hz, 411 Hz, 1.5 kHz, 5.2 kHz). This equalizer is different from other equalizers in that the bands can be set to be resonant. When the knobs are in the middle position, the response at the main EQ Output is flat. When the knobs are positioned between the 9 and 3 o’clock position, up to 12 db of boost or cut is set at the band. If the knob is set beyond the 3 o’clock position, the band will become resonant, simulating the natural resonance of acoustic instrument formant structures. Below the 9 o’clock position, increased band rejection is achieved.

Phaser

The VC Phaser (PHA) is perhaps the lowest noise and lowest distortion phase shifter available today. As an aid to recreating some of the subtle properties of phase delay in acoustic sounds, three separate outputs are provided with 360 degree, 720 degree, and 1080 degree of voltage controllable phase shift. A MIX output combines the 1080 degree phase shift with the input signal to produce the multiple notch filter effect that is usually associated with phase shifters. The VC Phaser’s log-conforming characteristics and the manual and voltage controls enable ultra-smooth, precisely centered sweeps of phase shift for both spatial effects and timbral modification.

AC Mixer

The AC Mixer is one of two provided on the Phaser PC board. It has three attenuated signal inputs, plus an auxiliary non-attenuated input that are all mixed at unity gain to one output.

Ring Modulator

The RING MODULATOR (MOD) is an AC or DC coupled Ring Modulator featuring superior audio processing capabilities. The Ring Modulator offers two VC inputs in addition to the two signal inputs which may be used to perform voltage controlled transitions between full ring modulation and amplitude modulation.

This modulator is interesting by virtue of having not only carrier and signal inputs, but also DC-coupled inputs. I was a confused at first, during calibration, because the VC X input works in the same direction as the Balance knob, i.e. a positive X CV input attenuates the X signal.  With the Balance knob fully counter-clockwise, the X signal comes through fully, and when clockwise the X signal is nulled. However, when the X signal is nulled by the balance knob, the VC X input has to go negative to bring the X signal back.  Having four inputs to play with on this modulator is cool.

Construction

There was little choice but to stack the three PC boards on top of one-another, mounted to a bracket secured under a row of jacks at the edge of the panel. This was because of the tight spacing of all the pots. I decided to put the Ring Mod board on the bottom, despite the lack of access to the trim pots after final assembly. This allowed the Resonant EQ to be on top and carry the power connector. I worked from bottom to top and performed the calibration of the Ring Mod before mounting the Phaser board on top of it.

res-eq-back-1Test Patch

I made the following test patch, using the ring mod, the phase shifter, the resonant equalizer, and a CGS VCA.  I created random envelopes using the BugBrand Noise Crusher S&H, a Serge Negative Slew and a Serge USG.  The two BugBrand VCOs are first ring modulated and then passed through the phaser and equalizer.  The phaser CV is driven at audio rates by a sine wave from the BugBrand Quadrature Sine VCO.

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