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AM8140 Module - E-mu Systems Resonant Filter
Overview This module is a clone of the interesting Resonant Filter in the E-mu Systems Modular Synthesizer. The original E-mu Systems 2140 Resonant Filter has three simultaneous filter outputs at 12 dB (2-pole); high pass, band
pass and low pass. This State Variable Filter also has Q and Frequency front panel controls, but no CV inputs for modulating Frequency or Resonance.
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 dual VCA chip, and released as a Revision 2 design.
The frequency control determines filter center frequency, and it can be
varied from 20 - 20 kHz. 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 inputs 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.
AM8140 Module I originally cloned the design back in 2002 when I was building my E-mu Systems Modular - as the AM2140 module. In 2009 I
reworked the CAD design so it could be put in a FracRac as the AM8410. I still had a few SSM2020 chips left over so I have used them up in a couple of these modules. 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.
There are 4 trimmers to adjust output offset of LP, BP and HP modes , as well as a V/OCT trim which I have added to the original design.
The module has the following front panel controls:
Frequency Cutoff Resonance Frequency CV amount Q CV amount High Pass Output Level Band Pass Output Level Low Pass Output Levell
There are just five 3.5 mm jack sockets mounted at the base of the panel, these are:
Signal Input Frequency CV via CV pot Resonance CV via QCV pot Signal Output
Front Panel The front panel is a 3" wide FracRac with 16mm Alpha
potentiometers and push fit mixer style knobs.
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