This little monster is a Caur (or elliptic) filter. It has a bizarre-looking frequency response, and is commonly used as an anti-aliasing filter for analogue-digital conversion. I think I might have got the idea back to front by driving it from a DAC, but it does much a better job than the little RC filter I also tried.
(The DAC is driven by an ADC to give an interesting output.)
It's a big filter, by my standards. I feel rather thankful I understand enough to make useful adjustments, and even that took a few practice sessions. It's another one I transcribed from Rod Elliott's excellent filters page:
http://sound.whsites.net/articles/active-filters.htm#s73
AC simulation showing the dive-into-notch frequency response:
http://everycircuit.com/circuit/6581583703441408
It normally amplifies a little. I adjusted the mixer to make the traces line up for comparison, and added the VCVS for the same reason. The mixer output is the weird one. The VCVS input is a more normal 24dB/octave slope.
I think the weird output does a slightly better job on the DAC signal. The 24dB/octave output lets peaks through a bit more, which you can see if you select the sawtooth input.
