About 3 weeks I have been working on a Voice board for this platform. The computer could produce sound w/o those boards, but with the boards the sound quality should go to semipro level. 

I am doing this task now to confirm the voice board connector pinout before I will produce one unit.  

The voice board is basically VCF. Each board has two of them. So as result 3 boards will have 6  separated channels. The board's output will goes to the mixer and also to the ADC for additional post processing (such as delay, chorus, reverb etc). 

Currently I tried the next VCF circuits:

1 The state variable filter
2 Steiner Parker 
3 Moog's ladder

The first two give a best result and I am deciding now -- should I put bought of them on the voice board. But in case if not, which one of them should I use.