All about it
The MM3328 VCF is a bright, beefy, brassy 4-pole Lowpass filter based around the vintage Curtis Electromusic Specialties CEM3328 chip found in the Ensoniq Mirage and other synths of a certain age.
Apart from its relevance to those looking to add some mid-80s polysynth vibe to their modular, this filter also features two antiphase input signal paths. One of these paths is mixed in with the resonant feedback amplifier to counteract the "passband signal drop" which affects 4-pole filters, the other is not. This makes it possible to avoid the "passband drop" phenomenon, and to crossfade between input signals in the two paths using the external resonance control.
The inspiration for this was buying an unbuilt set of DIY boards to make a kit filter based on this chip, being less than thrilled with the design quality and thinking even I could do better. It also seemed like a great idea to add a 4-pole filter option to the Capricorn Series.
Due to its dependence on old chips this filter will be a limited edition available in batches as parts become available.
This has been based on the CEM3328 datasheet reference design, but certain component values from the datasheet reference design have been recomputed to make them better match the datasheet specs for the chip, and also so the circuit can better fit with modular signal levels.
Inputs 1 and 2 are permanently assigned to the "uncorrected" signal path whereas 3 and 4 are assigned to the "corrected" signal path.
Also, the need to invert the CV summing mixer to drive the CEM332 has given us the opportunity to make CTRL 2 an inverting input, ideal for negative envelope modulation.
In case you're wondering, RES is a resonance CV that sweeps the entire resonance range within 0V to 5V of input signal when the RESONANCE control is at MIN. Use an external linear attenuator with signal sources that exceed this range, or to shape this to taste.
+5V: Not required
Eurorack standard, 10HP width
38mm depth below mounting rails with connected power cable
In prototyping; expected production in late 2016
Watch this space