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Quad VCA / CV-Mixer




This module is a hi-quality, all purpose Quad VCA/CV-Mixer. VCA are very important modules for all modular and synths in general...This module contain 4 VCAs with Exp/Lin modes and switchable summing stages on outputs. The Quad VCA module work with audio signals and/or CVs ( no capacitors on I/Os).

The core of the Quad VCA is a pair of SSM2164 from Analog Device. All 4 VCAs are independants and controled by 4 CV inputs and "level" potentiometers.

Three switchable summing stages are connected to VCAs outputs to allow any internalcombination between VCA and CV-mixers ... So it is possible to use this module as :

  • 4 independant VCAs
  • 1 VCA+ a 3 inputs cv-mixer
  • 2 VCAs + a 2 inputs cv--mixer
  • 4 input cv-mixer

by simply pushing few switches...

VCAs 1 and 3 are also fitted with reversible CV input attenuators. Combined with output summing stages this module can be used in a lot of various situations like :

  • cv-crossfaders
  • cv-panners
  • cv-mixer+vcas...
  • etc...

To simplify paching operation, signal inputs are normaled as follow IN1->IN2->IN3->IN4. All CV inputs are normaled the same way. So with only one signal on input 1 , the Quad VCA module become a :

  • CV controled distributor (1 ->4)
  • CV controled booster (x2, x3, x4 ) if mixer stages are activated



The schematic of the QuadVCA is directly inspired by the work of Phillip Gallo and the SSM2164 linearisation method proposed by Mike Irwin on EDN issue. But i have choosen to implement both CV circuits and to use a simple switch to select them to control the VCA, instead of a relay. This solution is finaly costless than to have to deal with 4 relays on the pcb. Another advantage, there is no need of an extra psu line for relays...The drawback it is this solution is using more parts and half of them on the CV path remain unused at any time ... It is a choice... The QuadVCA/CV-Mixer psu is +/-15V but it will work fine under +/-12V too


Pcb & assembly

The Quad VCA is made with a homemade double side pcb without metalized holes ( soldering both sides ... ). Op-amps for CV controls are TL074/84. Op-amps on the "audio" path should be good quality ones , unlike what i did here :the prototype is fitted with LM358 on pictures... .Something like OP275 , NE5532, OPA2604 will be good for this module ...

Note : to keep identical performances for all 4 channels/VCAs it is advised to use precision resistors ( 1% or better ) specialy for output opamps gain, and CV inputs ... There are a lot of wires to solder for pots, switches and jacks. They should be kept as short as possible. Shielded wire are used for signal I/Os.

Front panel

2x5U front panel in MOTM format fit with 4 pots brackets, suggested panel : Quad VCA 5x2U Front Panel (fpd)

Clic on pictures to view in big...


And the sound ?

No comment ... It sound good and work fine!



Quad VCA pcb + Construction manual
is available on order

© Marc Bareille 2005-2007