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AM2010 Module - Eµ Dual VCA
Overview This module is a clone of the Quad VCA module in the Eµ 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 - as I have a separate final audio mixing module from Oakley.
The linear gain of the VCA can be controlled from the
front panel as well as a control input. There is no exponential control available, which is a shame. However the SSM2020 based VCA in the Audity does have exponential control, so I may be able to recreate a second VCA using this circuit.
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, 1458 and TL082 Op Amps. This improved the S/N ratio by 30dB and reduced audio distortion too.
1978 saw a resurgence in Modular sales with Eµ having the funds to move into a house in Santa Cruz, 417 Broadway. Dave Rossum was 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 - it will be interesting to see how well this design works in
practice.
Front Panel The front panel is a standard AM design, 90mm wide and 4U high, with black lettering. Controls knobs are standard Eµ Modular designs.
Build History
The schematic was transferred into Eagle CAD. in late 2003 and a 80mmx100mm PCB manufactured in January 2004. The board was easily populated (as parts are easy to find) and after correcting some minor errors the VCA works fine.
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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Resources
* Front Panel .fpd file