AM8320 Prophet Filter

AM8320 (2013) Module

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 4-pole Low Pass Filter configuration as used in the SCI Pro One.  I have owned a couple of Pro-Ones and always liked the filter sound. I initially built a CEM3320 LPF (AM3320) as part of my SCI Pro-One modular synthesizer in 2004 (which was eventually scrapped as I ditched the 4U panel format that it was made in). I initially used the Digisound LPF circuit as the basis for the design.  The Digisound resonance 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 and improvement in the sound. This design was sold as a PCB (AM3320) from 2004 – 08.

AM8320 (2008) The design was then upgraded to EuroRack format as the AM8320 in 2008, it 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  The AM8320 PCB was sold to a number of customers, around 2008 – 2010 but withdrawn once CEM3320 chips became very expensive and hard to locate.

AM8320 (2013) In July 2013 I built a small production run of AM8320 modules in EuroRack (just 10x) after locating some NOS CEM3320 at a reasonable price. The modules have the same electronic design but without the Frequency Fine control. This 2013 module sold out very quickly, check out the picture.

AM8320 (2017) Module

AM8320 (2017)  In the autumn of 2017 the AS3320 chip became available in large quantities and at a low price. This meant I could get the module back into production and I reworked the design to version 4 as a single PCB that mounts via pots and jack sockets to the panel. This makes for a faster and cheaper build. The control knobs have been upgraded from the mixer style to a much nicer vintage style.

The filter has; 3x Audio Signals inputs with level control, Frequency Cutoff, and CV1 and CV2 modify cut-off and the Q control adjusts the resonance of the filter. Higher settings of the Q control will take the filter into sine wave oscillation. There are six 3.5mm jack sockets mounted on the right  hand side of the panel; Signal Inputs (x3) CV Inputs (x2) Signal Output. The panels have red lettering and it has the official name of “Prophet Filter”.

Module Availability  The AM8320 V4 PCB and Euro Rack modules are available in our web store.

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