The project proposed here is an extension board to add 15 functions to the MOTM 440 VCF and more generaly to almost all 4 poles Low Pass VCFs with a similar topology ...
The idea of this extension come directly from the Oberheim Xpander/Matrix12 multi-function VCF schematic : the 4 poles of a classic low pass filter are mixed together with different coefficients to get multiple functions. The Multi-function VCF page contain math explanations and a copy of the original schematic, so i won't reproduce them here, just the list of the 16 VCFs functions as a reminder...
Schematics and pcbs proposed here have been designed for the MOTM440 VCF mainly... Pcbs match exactly the MOTM440 one for a piggy back assembly ( see pictures) . However it is quite simple to add this extension module to other VCFs. In most of the cases, if the signal can be picked up at the output ouf each of the 4 stages, there is no need to amplify it, just buffer & reverse ...
The modification is also not destructive and 100% reversible...At least until you do not drill new holes into your VCF front panel...
Schematics
I propose here 2 different versions and pcbs for this multimode extension. Both are doing the same thing but with different analog multiplex chips for performance and cost.
The first one is named Extrapole 15 : it is a true clone of the Matrix12/Xpander... Analog switches are CMOS 4051 and 4053. For the MOTM, the x100 amplifiers have been replaced by unity gain ones. Amplifiers are required with the CEM3372 to keep all stages at the same level... This is not needed for the MOTM440 : the signal is picked up directly at the buffer outputs of each filter stages.
Download Extrapole 15 schematic v1.0(pdf)
Extrapole 15 Prototype pictures - Clic to enlarge
Extrapole 15 schematic work fine but there are many drawbacks :
- The 405x have a limited range, a bit short to pass directly the levels produced by the MOTM440 discrete stages, so some clipping appear with high levels on inputs. a solution could be to attenuate levels before they reach the switch and amplify again on the output op-amp. It is not a problem if the Extrapole15 is plugged on a low level VCF ... With the MOTM440 who boost the signal at hi Q settings ( unlike the others LP4P) this oblige to take a lot of care of the inputs levels to avoid clipping....
- The 4051 is not silencious at all : It produce 'plops' at each mode change. This was not a problem into the Matrix12 because this parameter was not modulated in real time, just set once ...But 'plops' are not nice at all when modes are changed quickly by a modulation CV ( lfo, eg..)
The second version of this module, named Extrapole DG, is an attempt to build the perfect board.
So i have redesigned the thing with DG4xx circuits. DGs are very performant but much more pricey than 405x ...
The schematic has been simplified too. I have removed amplifiers stages so everything is now working at unity gain on inputs. No more auxiliary psu for the multpiplex chips, DGs work under +/-15V with a direct +5V logic. So the signal range is not limited here 
DG chips generate less 'plops' when switching than 405x, but they are not 100% silencious or may be it is the way the ADC drive them here? The control circuitry is identical to the previous version...
