AM8328 Mirage Filter

AM8328 VCF

Overview This module is based around the CEM3328 4-pole Low Pass Filter chip that was released in 1984, as one of the last filter chips and with some great features. It has differential signal inputs, an accurate exponential scale over 14 octaves and voltage controlled resonance. It has a unique feature, the signal level is automatically increased as the resonance goes up, thereby avoiding the usual signal level dropping off as resonance comes in.

The CEM3328 was late onto the analog stage, just before digital filters appeared, so it only made it into the BIT01 and Ensoniq Mirage (and a few drum machines). So this chip has never really seen action, and was never under analog pot control, just lonely digital parameter access! In the Mirage it was not even allowed to oscillate…..We just love giving it a new lease of life in a new analog module!


Detailed Description  The AM8328 uses the chip as per the CEM datasheet, and it’s a 4-pole OTA design. There are two signal inputs and the usual signal output, 2x Frequency Cutoff CV inputs with control pots, a Key Track pot which enables the note value to control filter cutoff, and a resonance CV input and control pot.  The Filter Q control will take the CEM3328 into full self-oscillation, beyond the 40 setting on the Ensoniq Mirage!

The CEM3328 has input and output buffer Op Amps to enable it to work in a modular synthesizer, and I have used high quality polystyrene capacitors in the filter, and they are very large and green…..K71-7 Soviet. The front panel has bright orange lettering, in keeping with its name…(a mirage in the desert…), and the filter has a very attractive sound, have a listen to the demo. The filter was designed in late 2012 and we manufactured 40 modules up until 2016.

EuroRack Module: 14HP wide
User Manual: Download PDF

Outcomes & Availability The AM8328 was redesigned in January 2019 to be skiff friendly with two PCB’s parallel to the front panel. The depth of the module is now 35 mm and its far easier to construct. A small run of 10x of completed modules sold out quickly in 2019. With the high cost of the NOS CEM3328 chip its unlikely we will make more modules.

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