All about it
The MM314 VCF is a great-sounding two-pole state-variable filter with a few offbeat features.
Firstly, it is DC-coupled so not only are bass frequencies reproduced with clarity but it can also process control voltages-both as a slew limiter at low resonance, and as a "woggle" source at higher resonances.
Secondly, it has an unusual architecture that results in the TEST output morphing from a 1 pole lowpass at zero RESONANCE setting to a 2-pole bandpass above the midpoint of the RESONANCE control
Thirdly, it has a VCO-grade exponential converter and the response of the two poles is fine-tuned, giving it accurate tracking and rich resonance. It will not self-oscillate at high resonance but will squelch beautifully when tickled by a signal.
Fourthly, unlike most other 2-pole VCFs there is no soft clipping circuit in the resonance path. This means that at low levels it sounds very clean, but at high resonance with high input levels the output levels can get very hot and can hard-clip distort when the filter locks onto a signal harmonic. This of course can be put to good musical use!
The three 2-pole lowpass outputs (OUT 1, OUT 2 and OUT 3) are identical and are independently connected to the output signal via 1k protection resistors.
We have added a switch that drops the range of the TUNE pot by 5 octaves without messing with the filter architecture-ideal for being able to attain manual cutoff while you have low-pH levels of envelope modulation!
Just like the original, we have used mean Mica mofos as filter caps. An LM13700 substitutes for the original's two CA3080 OTA chips.
Also like the original, the expo converter (the MM-1748) is an epoxy-potted module. There is no official documentation of the circuit and indeed it was blocked out on schematics, presumably because it is almost identical to the ARP 2500 VCA's expo converter. I managed to successfully reverse engineer it by deduction from the AR317 schematic, and Dave Brown independently reached the same conclusion by testing circuit connections on an actual module. Yay!!!! The current limit resistors between the converter and the OTA cells have been increased in value as the original design's 4.7k resistors could drive both the CA3080 and LM13700 past safe operating limits.
There is variable DC offset as the cutoff frequency is changed. This means that the unit may not be the best choice for dynamically varying slew of 1V/oct control sources.
+5V: Not required
Eurorack standard, 10HP width
40mm depth below mounting rails with connected power cable
Queued for expected production in 2017
Watch this space