AM8320 Prophet Filter

AM8320 Prophet Filter (2008-2013)

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.

AM8320 (2008) The original PCB kit design from 2004 was 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 The AM8320 PCB was sold to a number of customers, around 2008 – 2010 but withdrawn once CEM3320 chips became very expensive and hard to locate.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 AMSynths built a small production run of AM8320 modules in EuroRack 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.

AM8320 (2017) With the availability of AS3320 filter chips the AM8320 was redesigned into a skiff friendly format and there is a separate module entry for this new filter.

http://www.amsynthstore.co.uk/AM8320_Prophet_Filter/p1461448_10872995.aspx