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AM2100 Module - Eľ Systems Low Pass Filter
Overview
This module is based on a Dave Rossum filter design that never made it into the Eľ Modular - a SSM2040 Low Pass VCF. The stock Modular filter was a very clean transistor ladder dating from 1972/73, but whilst the HPF was updated to use
the SSM2040, the original low pass module escaped.
I have filled in some history, by taking the Dave's design from the SSM Voice Card and Eľ Systems Audity and produced a low pass 24dB VCF based on the SSM2040. It's a classic
slice of sonic engineering, and it is well worthwhile putting one of Dave's most famous designs into a module.
AM2101 Circuit The filter design is taken straight out of the SSM application notes, and is as used in
the SSM Voice Card and the awesome Audity. The SSM2040 is joined by a OP177 Op Amp on CV summing duties, and a couple of OPA2134 Op Amps to bring the signal down and then back up to normal modular signal path voltages.
The
circuit is temperature compensated with a 3600ppm 1K PTC resistor, as the chip will oscillate quite nicely at high Q settings.
Front Panel The front panel is a standard AM design, 90mm wide and 4U high,
with black lettering. Controls knobs are standard Eľ Systems Modular. There are 2 signal
inout to enabkle mixing of various sources, coarse and fine frequency controls, resonance and + and - CV controls.
Build History
The schematic was transferred into Eagle CAD in late 2002, and a 80mmx100mm board laid out in late 2003. The prototype module was built in February 2004, and it was operational immediately with no bugs.
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