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AM8320 Module - Low Pass Filter (CEM3320)
Overview The
CEM3320 filter chip 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, I have chosen to use the LPF configuration as used in the SCI Pro One.
AM4320 Circuit I initially built a CEM3320 LPF as part of my SCI Pro-One modular synthesizer, using the Digisound circuit as the
basis for the design. This is a 4U high module - 90mm wide. The design was not as impressive as I hoped, and it was clear that the original SCI Pro-One resonance feedback circuit was different and was making a substantial difference in
sound.
AM8320 Circuit The AM8320 module uses some of the more sophisticated features of a Digisound modular design (CV rejection trimmer) along with the core design of the SCI Pro-One low pass filter. The SCI
resonance path was accurately replicated, as the Digisound circuit does not sound as effective as the Pro-One.
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 original circuit has been upgraded with high quality LT1013 and OPA134 Op Amps.
The filter frequency has two front
panel controls; FREQ for coarse adjustment and FINE for fine adjustment. The filter has a Q control to adjust the resonance of the filter. Higher settings of the Q control will take the filter into sine wave oscillation.The module has the following front panel controls:
There are six 3.5mm jack sockets mounted on the left hand side of the panel, these are:
Signal Inputs (x3) CV Inputs (x2) Signal Output
Front Panel The front panel is a 3" wide FracRac with Alpha potentiometers and push fit mixer style knobs.
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