AM8372 Ambient Filter

AM8372 Mock Up

Description This module is designed around the CEM3372 VCF chip which featured in the incredible Oberheim Matrix 12 and Expander synthesizers in the mid 1980’s. The filter can be configured in 15 different ways; providing a nice variety of Low, Band and High Pass filters at various poles.

The idea of basing a module around the CEM3372 Matrix 12 VCF implementation is not new. Jesper Erwik Johansson in Sweden developed such a module in 1998, and sold a limited edition of 12. Jesper added some interesting features, such as a front panel display of filter type and the ability to clock through the filter modes. He also made use of the three VCA’s on the input side of the filter core.

Doepfer make a version of this filter, using Jespers ideas, plus the ability to morph between 32 filter types. This is a micro-controller based implementation with a 7-segment LED display.

AM8372 Module I originally designed the filter back in 2006, but it wasn’t until the release of the Rpar AS3372 chip that the project became viable, as I did not want to use expensive NOS CEM3372’s. The analog filter is based on the Matrix 12/Expander implementation with 15 filter modes and a miniature 8×1 LCD displays the filter type in text, just like the original Oberheim.

The AS3372 is a 24 pin chip with the additional of a 5th and 6th VCA, which are not used in this module. The input VCA’s are used (INA and INB), along with the resonance VCA (RES) and final VCA (VOLUME). High quality polypropylene filter capacitors have been used.

An 18F1220 PIC microcontrollers reads the 9mm 10 step encoder, sets the filter type via analog switches and displays the filter type on the LCD. The PIC remembers the filter type last used when powered off and back on. The assembler code was written in 2004 and only needed some minor port changes to fit into an 18-pin PIC.

Outcome & Availability The module is currently in development. A complete implementation of the AS3372 chip as a 24dB VCF, a linear VCA and VC Panning is available as the AM8123 module here.

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