So i've built my 2nd QUAD PHAT FILTER and installed it in a Casio SK5 (first one is an SK1)
A few things i did differently this time:
1. Swapped the audio input resistor to 370K. Probably still a little high, maybe 350K is ideal
2. Put a FILTER BYPASS switch in that switches between the pre-anti-aliasing filter output on the casio, and the post-PHAT FILTER output
3. Used just one single gang 100K pot for the filter cutoff. When I was testing it I used a 1M pot. I wonder if a 150K or 250K pot would be best, and drop the resistor on the 'B' lug of the resistor to only 70K or so.
4. Dropped the resonance input resistor to 820Ohms.
What I've noticed is:
1. Audio is still a little lower in volume than the non-PHAT Filter output
2. The Phat Filter output is less bright than the pre-Phat Fitler output. I really like the digital artefacts of the SK when you bypass the anti-aliasing.
3. The resonance is now too wild and crazy.
looking at the original -
http://s1107.photobucket.com/user/joer0952/media/qd.gif.html there is no input resistor on the resonance though. hm
I actually think ideally a few of the resistors should be soldered onto the lugs of the potentiometers, to make altering the circuit easier as you want to change up some of the resistance and get a better range.