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AM2140 Module - Eµ Resonant Filter
Overview This module is a clone of the rather interesting Resonant Filter in the Eµ Modular. It has three simultaneous filter outputs at 12dB (2-pole) each, high, band and low pass
- along with Q and frequency control.
Original Circuit
Dave Rossum originally designed the Resonant Filter 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..
AM Circuit
It's the Revision 2 design that I have cloned precisely - including the use of the same rare AD820 matched transistor pair. The circuit is based around the SSM2020, TL082 Op Amps for the audio path, a CA3080 and a 741 in the control path. The AD820 is mounted on an 8-pin DIL header, so that different transistor pairs can be tested out later.
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 back in November 2002 and a prototype PCB ordered from Olimex. It was never populated as my design ideas moved on, and a second 80mmx100mm PCB was designed and manufactured in January 2004. This was populated and some basic errors corrected, but it is proving difficult to get working!
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Resources
* Front Panel .fpd file
* MP3 Demo