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Blue Box Quad VCA

Courtesy/Thanks to: René Schmitz

Description: Rene's latest VCA is the one in the picture VCA3. This is the VCA that I use inside my so called "Blue Box". It is based on VCA 2, but shows a number of minor improvements (which can be retrofitted to VCA2). He has also shown the external pot connections. These were was left out on the VCA2. He added a trimpot at the inverting input, so that the output offset can be eliminated. This makes it DC accurate. The adjustment is somewhat tricky, since it interacts with the CV rejection pot. You will have to adjust the trimmers alternatingly. Its well worth the effort, with careful adjustments you get it basically thump free. (But beware that your breath might have an impact on the setting, best is to insulate the pair with styrofoam or something.) Then there is the trimmer called "deaf-zone elimination" by that you can shift the point where the gain goes to 0. So that with 0V input (and the manual pot at the lowest setting) the VCA is just closed.


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   Schematics of the VCA-3
Rene Schmitz
Parts used
   PNP Epitaxial Silicon Transistor
   NPN Epitaxial Silicon Transistor
   Silicon NPN Epitaxial Type (Monolothic Dual Type)
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