CGS VCA

cgs-vcas-front-1This panel is one of five that comprise a set of modules from CGS.  It contains three different functions:

The panel format is a new one that I designed.  It uses MOTM form factor (5U high by 2U wide), so that it can be mounted on standard MOTM rack rails.  The vertical spacing of four columns is also the same as MOTM, but horizontally there are ten rows, spaced at 0.8 inches apart.  These accommodate the banana jack format like I used on my previous CGS Serge panel.  The panel is blue to match BugBrand.  I ran into one hitch, not with this panel, but with others in the group that have pots in the top row.  The pots protrude into the space needed by the rail, necessitating filing the rails a little.

Jack Colors

  • Yellow – Signal Input
  • Green – Output
  • Blue – CV Input

 

Knob Colors

  • Yellow – Signal attenuator
  • Blue – CV Attenuator
  • Red – Initial setting or Offset

 

Passive Pots

As simple as it gets, these are just four independent passive attenuators.  The pots get a ground wire to the power supply.

CGS VCA

I wanted a few more DC-coupled VCAs and chose the CGS VCA, which is based on the VCA3+ by René Schmitz.  Ken Stone added op amp buffers to the Schmitz circuit.  I had expected to see the standard unity gain happening at around +5 volts of CV at the CV input, but it was only about 0.2.  After some discussion with Ken on the CGS email list, I changed two resistors in the CV section to get the unity gain.  My panel design has two signal inputs, which mix.  These are easily patched from the Pots just above on the panel, if attenuation or input mixing is wanted.  The red knob is the initial gain, and the blue knob attenuates the CV.  The response is linear, and the signal quality is excellent.

 Voltage Processors

Built with CGS81 PC boards, these are similar to other processors I’ve made with the board, but yet a bit different.  There is one normal attenuated input, one bipolar attenuated input, an unattenuated auxiliary input, a bias knob, and normal plus inverted outputs.  These are DC-coupled unity gain mixers, but of course will pass any audio signal, as well.

Construction

The three PC boards are mounted on two sides of an aluminum panel that fastens under a center row of banana jacks.  Once again I used 4-40 nylon hardware to mount the boards to the panel.  The power is distributed by jumpers, so that only one header is needed for the whole panel.

cgs-vcas-back

cgs-vcas-back-2

And here’s another view of the front.

cgs-vcas-front-2

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