AM2140 Module - Eľ Systems Resonant Filter
This module is a clone of the interesting Resonant Filter in the Eľ Systems Modular Synthesizer. It has three simultaneous filter outputs at 12dB (2-pole); high pass, band pass and low pass. The filter also has Q and frequency front
panel controls, but no frequency or Q CV inputs.
Original Circuit Dave Rossum originally designed the Resonant Filter back in 1973, and released the module in January 1974. The filter was updated in 1976 using the
SSM2020 VCA chip, and released as Revision 2 design.
The frequency control determines filter centre frequency, and it can be varied from 20 - 20kHz. A resonant peak may be added to the response by the Q control. The height of
this peak is variable from 0 to +40dB. The filter has three outputs which are mixed together via an internal op amp stage. There is a simple audio in and mix output, and their are no individual ins for any voltage control. This can be
set up via the 2145 Filter Controller module, which provides voltage control over internal firmware. The 2145 wasn't released until August 1976, along with the Revision 2 Resonant Filter module.
AM2140 Module It's the
Revision 2 design that I have cloned - including the use of the same rare AD820 matched transistor pair, although an SSM2220 chip can be used instead. The circuit is based around the SSM2020, OPA2134 Op Amps for the audio path, a CA3080
and an OP177 in the control path. I have also added CV inputs for Q and frequency.
The module has the following front panel controls:
High Pass level
Band Pass level
Low Pass level
There are just two 3.5mm jack sockets mounted at the base of the panel, these are:
Front Panel The front panel is 2.5mm thick aluminium, 90mm wide
and 4U high, with black lettering. Controls knobs are from the E-mu Systems Modular.
I intend making a couple of AM8140 filters in FracRac format, as the filter sounds really good and I have a couple of SSM2020's spare..