[sdiy] 4-pole SVF theory

[Paul Perry]

As a general rule, when one tries to build a higher order filter with a 
single op amp, you run into two problems:
Frstly, the component values become more critical (that is, you need higher 
precision Rs and Cs to get the result you want.
Secondly, when you are actually varying Rs as you tune the filter, the 
signal level takes on even wilder excursions than usual. (I'm suprised more 
note isn't taken of the weird shit that happens when filters are actually 
being tuned.)

At least, these are my opinions - one hopes the engineers among us can say 
more.

paul perry Melbourne Australia