CGS Serge Dual Universal Slope Generator

cgs-serge-usgThe Serge Dual Universal Slope Generator may have achieved more renown than almost any other 1970’s era Serge module.  Ken Stone of Catgirl Synth makes the PC board.  See his description.

Be sure to look at the CGS Serge VCS board as well.  The circuits are fundamentally identical.  The main difference is that the VCS is a single board and offers addition CV and trigger inputs and panel features.

This module is incredibly versatile.  It can perform as a

  • VC transient (envelope) generator
  • VC slewing (portamento)
  • VCO and VC LFO
  • VC low pass gate
  • VC pulse delay
  • VC pulse divider (subharmonic generator)

It’s amazing how compact it is in the banana jack format!

Construction

I tested batches of 2N3904 and 2N3906 transistors to find gain-matched pairs, using the method described in Electronotes #39, p. 6.  I also took particular care with resistor values, comparing with those used in my VCS circuits.  The stated 470K pull-down for offsetting the Bipolar output should be closer to 600K.  Note that the Bipolar output is also inverted from the normal Out.

There is a place on the board, not shown on the schematic, for another output, which is called “AC Out” on the VCS.  It puts a 47uf capacitor in series with the main out.  I did not use it.  Instead, I used the location of the capacitor to install a 1K series output resistor and take the normal Out from that, instead of the DCO output.  Reason is that I found with the VCS if I shorted the output to ground while patching, it stops oscillating, because the internal feedback is coming right off that external output.

Mechanical Details

Since discovering MTA-100 connectors, I’ve started using them in preference to direct wiring, so that a board can be completely removed for repair.  The DUSG board, however, doesn’t cooperate perfectly, emitting eight wires at random locations.  So I used ‘flying’ 3-pin connectors, Mouser 571-5-103946-2, and 571-5-103958-2.  These mate in the air.  The pins in these connectors are obviously meant for a specialized crimp tool, but I figured out how to solder #24 wires into them.  A bit finicky, but not too hard.

I tried 4-40 nylon mounting hardware for the first time.  It works great!  Mouser parts 534-4800 (1/4″ standoff),  534-4806 (1″ standoff), 561-P440.25 (1/4″ screw), 561-G440 (hex nut).  I wish I had used nylon before, it’s easy to work with and is non-conductive.

Photo of the wiring (click for more detail):

cgs-serge-dusg-wiring-1000px

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