[sdiy] ladder filter questions?

[dan snazelle]

I was wondering if anyone here could help me understand how a transistor/diode ladder filter works...my math is poor so Tim's excellent paper on the moog filter isnt much help!!!

I was experimenting all week with some diode filters on the breadboard.

the odd thing was that it was working when i had nothing more than a pot to +v and -v hooked up with a 1k resistor to the emitters (for cutoff)
but when i switched to a real cutoff circuit the filter stopped working and I had to change all my resistor values.



i fear the thing i dont understand well is how the  gain structure/feedback works. it seems if i put too much feedback into the - side of the ladder, i get no or little sound, and if not enough, i get thin resonance.


I am also just sort of randomly deciding where to put coupling caps (do they have to be on the inputs? do they have to be on the outputs into the differential opamp, etc)

it sure seems more confusing than OTA filters but i had trouble with those in the beginning too!

thanks for any help