[sdiy] 4-pole SVF theory

[karl dalen]

> It's easy to run one 2-pole SVF after another and get 24dB
> HP/LP/BP responses, but the simplest way is to change the
> tap that you input into the second filter (e.g. feed the HP
> output from the first into the second, then take the HP
> output from the second for 24dB HP, for instance). But like
> this, you don't get simultaneous 4-pole LP/HP/BP outputs
> (like you do on the 2-pole version), since you have to
> switch things around to change the response.

Hmm, most pro analog desk EQ's i have seen uses a slightly altered
SVF and i have never seen a 3 or 4 pole, they where all classic
2poles, cost vs performance level i suppose, and diode limiters
for Q are banned for desk EQ's, obviously.

I wonder when a particular filter topology stops to be
what it's named to be?  Example Dixons orgasm S/K vs Korg
MS20 OTA S/K? Are they both really straight S/K or something
somewhat different? For instance what do one name the
Poly 61 filter topology? Modified S/K?

I mean SVF's goes under several names. 

Note: my  4-6pole SVF designs was with LM13700, that OTA
could be a reason for the performance, yet i took care
to match Gm, Iabc, matched caps etc, i dont think that 
caused any problem, but individual 3080 would not do,
matched quad like SSM2164 would be best.

KD