[This is from something I posted to the synth-diy list a couple days ago.  There was a discussion about some variations on the Moog Ladder Filter, such as taps, and arithmetically combining taps to get other responses.  I had found some patents for much wilder variations on the Moog Ladder Filter.]

If you're interested in variations on the ladder filter, I'll suggest you go to the patent pdf tool ( http://www.pat2pdf.org) and take a look at this guy:

US 6,084,470
Filter Circuit Capable of Setting Various Filter Characteristics
Shiramatsu, Nakamura, Goto
Kabushiki Kaisha Toshiba
July 4, 2000

This patent is an insane collection of unusual ladder filters.

Also check out these:

US 5,912,587
Active Filter Stack
Mihailovits, Beatson
Mitel Semiconductor
June 15, 1999

US 5,225,790
Tunable Widband Active Filter
Noguchi, Rinaldis, Esling
Digital Equipment Corp.
July 6, 1993

US 3,983,512
Current Controlled Electrical Circuits
Lipscombe
Feedback Instruments Limited
September 28, 1976

I'm sure some of these have significant musical potential.

As an aside; I think the synth-diy community is not being as creative as we ought to be.  If we still had the wildly adventurous spirit that was thriving during the 60's and 70's, one of us would have published a really fine collection of ladder filter variations way before this patent was filed.

Or, in other words, "Crap! Why didn't *I* think of that?"