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.
Overview The CEM3328 was late onto the analog stage, just before digital filters appeared, so it only made it into the Crumar BIT01/99 and Ensoniq Mirage (and Oberheim Prommer and DX-Stretch). 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 originally I used high quality polystyrene capacitors in the filter, and they are very large and green…..K71-7 Soviet. The front panel had 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 AMSynths manufactured 40 modules up until 2016.
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 completed modules sold out quickly in 2019. With the rare NOS CEM3328 chip its unlikely we will make more modules (but…).
2025 Update In October a customer contacted us to see if we had any PCB’s left over. We don’t (sorry!), but we went out and bought the last CEM3328 stock available, and redesigned the filter once more, with one PCB and SMD. We used the latest 2500 style control layout, PPS capacitors replaced the K71-7’s, and we ensured the filter was totally accurate.
We built a final run of 10x AM8328 Mirage filters, which were put on sale. We hope you enjoy it!
did i miss out on this run?
No, the first run is still available. Just email me