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AM2101 Module - Eµ Low Pass VCF
Overview This module is based on a Dave Rossum filter design that never made it into the Eµ 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 Eµ Audity and produced a low
pass 24dB VCF based on the SSM2040. Its 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 741 Op Amp on CV summing duties, and a couple of TL082 Op Amps to bring the signal down and then back up to normal modular signal path voltages.
The circuit is temperature compensated with a 3000ppm 1K PTC resistor, as the chip will oscillate quite nicely at high Q settings. The harder to find 3600ppm resistor is not needed in a VCF application.
Front Panel
The front panel is a standard AM design, 90mm wide and 4U high, with black lettering. Controls knobs are standard Eµ 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 laided out in late 2003. The prototype module was built in February 2004, and it was operational immediately with no bugs. Unfortunately the front panel DID have some bugs, the hole spacing for the pots is wrong, however it was easy to work around.
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Resources
* Front Panel .fpd file
* MP3 Demo