CGS Serge resonant equalizer

CGS202 the CGS Serge resonant equalizer module is based on the Serge Resonant Equalizer.

It will work on either +/- 12 volts or +/-15 volts without modification, though in the case of the latter, all input voltage sensitivities, and output voltages are proportionally increased.

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.

Note from Aaron Lanterman (19 Dec 2022)
I wanted to share some research I've been doing into the Serge Resonant Equalizer, with the help of Serge Tcherepnin, Yves Chartier, and many folks on the SDIY mailing list.

The top three filters on the CGS schematic are incorrect. The input of the 2nd op amp should be taken from the + input of the 1st op amp. The resulting filter is a "ideal current-inversion negative-immittance converter (INIC) bandpass filter" per a "Handbook Of Operational Amplifier Active RC Networks" TI (actually Burr Brown) app note by Carter and Huelsman. Note that the mixing op amp has slightly different resistors from the higher bands to the virtual ground than the lower bands; this is needed to get the volumes of the different filters to match up.

Construction
Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges.

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC sockets if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components, such as electrolytics, diodes, transistors and ICs.

When inserting the ICs in their sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

Traditionally, polystyrene capacitors are used for all of the smaller value capacitors in this module. I have not tried using other types an cannot say whether using the polystyrene capacitors makes any audible difference. From 10n, up greencaps were used. In modern terms these would be MKT or MKS. Note that there are also ten 10n decoupling capacitors. These decoupling capacitors can be MKT, MKS, monolithic ceramic, or anything else that will physically fit.



Setup
There is no setup required.

Parts list
This is a guide only. Parts needed will vary with individual constructor's needs. Alternative part numbers are provided in brackets.

CC-BY-NC
Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work. Serge Tcherepnin retains all rights to his work.

Suppliers

 * CGS202 Resonant Equalizer, revision 1.1, Elby Designs