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AM2010 Module - Eľ Systems Dual VCA
Overview This
module is a clone of the Quad VCA module in the Eľ Systems Modular. Rather than having four VCA's I have gone with 2 VCA's to make construction easier. The module has two completely independent VCA's, and I have chosen not to include
the originals final summing stage of all outputs. The linear gain of the VCA can be controlled from the front panel as well as a control input.
Original Circuit
This module was originally designed way back in 1973 probably using Op Amps and a CA3080 - like the 2000 module. The Revision 2 design came out in early 1978 using an SSM2020 dual VCA chip, LM1458 and TL082 Op Amps. The replacement of the CA3080 improved the S/N ratio by 30dB and reduced audio distortion too.
1978 saw a resurgence in Modular sales with Eľ Systems having the funds to move into a house in Santa Cruz, 417 Broadway. Dave Rossum was busy designing a range of synthesizer chips with Ron Dow at SSMT. These new synth chips
replaced the ageing transistor and Op Amp designs in much of the Modular systems during the same year.
AM Circuit It is the SSM2020 design that I have cloned - including the unususal variable Lag control which is used
to eliminate clicking attacks when digital control signals are used. The lag time is variable from a few microseconds to one tenth of a second. A nice feature!
The effective control polarity can be selected by a switch for each
VCA. A single control voltage applied to both VCA's can therefore be used for mixing and panning applications. The specification of the module is 40dB of control rejection and 0.3mV of noise.
Specifications A SSM2020
based VCA typically has a THD figure below 0.1%, and s/n ratio of 84dB. Control rejection is up to 54dB, when trimmed. Later SSM VCA chips like the SSM2120 are significantly better, with THD down to 0.01% and a s/n ratio of 100dB.
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