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AM3320 - VC Low Pass Filter (CEM3320)
Overview The CEM3320 was introduced in 1980 and it quickly established itself as a smooth sounding low pass filter, the basis of the SCI Prophet 5 Rev 3, Oberheim OBXA, PPG Wave
2.0 and SCI Pro One. Each of the internal 4 filter sections is based on a variable gain cell that Doug Curtis patented in 1977. This enables internal temperature compensation and made the design attractive to polyphonic synth
designers in the early 1980's. The cells can be configured as LP, HP or BP, as well as 2-pole and 4-pole slopes.
AM Circuit The AM3320 module is based around the Digisound modular design, which is a bit more sophisticated
than some of the basic SCI implementations which miss out the CV rejection trimmer.
A FET Op Amp is used to mix the audio signals and bring them down to the low level needed by the CEM3320. A similar Op Amp buffers the output and
another Op Amp sums the CV's. CV/Octave and CV rejection trimming are on board.
The circuit can only be built as a LPF, but you can vary the value of the four frequency capacitors to replicate the OB8, SCI or Digisound sound.
- 300pF Oberheim OB8
- 220pF Digisound
- 150pF SCI Prophet 5
The original circuit has been upgraded with high quality LT1013 and OPA134 Op Amps.
Front Panel
The front panel is an AM High Density design, 90mm wide, 3mm thick aluminium, 4U high, with black lettering. Controls pots are 3.85mm diameter, knobs are 19mm and 13mm diameter - black and chrome switches are sub-minature toggles and 3mm LED's are used. An audio signal LED monitor is ditted with a 10 LED bar graph.
Build History The schematic was ransferred into Eagle CAD in July 2005, and the prototype tested successfully in December 2005.
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Resources
* CEM3320 Datasheet 
* AM3320 Project Notes
* AM3320 Front Panel