AM3320 Module - Low Pass Filter (CEM3320)

Overview The CEM3320 was introduced in 1980 and it quickly established itself as the low pass filter in the Prophet 5 Rev 3, Oberheim OB8, 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 either Low Pass, High Pass or Band Pass.

Analog Metropolis Circuit The AM3320 module is largely 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. The design also has a CV for Resonance (Q).

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 be built only as a LPF, but you can vary the value of the four frequency capacitors to replicate the SCI or Digisound sound.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.

1. 300pF Oberheim OB8
2. 220pF Digisound
3. 150pF SCI Prophet 5

The original circuit has been upgraded with high quality LT1013 and OPA134 Op Amps.

Panel The front panel is a standard 5mm wide AM design with 3mm aluminium, 4U high, with black lettering. Controls knobs are standard Eµ Modular designs.

Build History The schematic has been transferred into Eagle CAD, and the board layout needs to be completed.

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This module PCB is for sale.

Sound Clip