Useful and fairly simple 2nd-order filters -- 12dB/octave, twice that of single-stage RC filters. The component values don't have to be quite so odd if you can accept a change in Q: softening the curve between pass-band and stop-band.
Not my work. Invented by Sallen and Key, apparently, and transcribed from:
http://sound.whsites.net/articles/active-filters.htm#s3
Sometimes these get adjusted as I find suitable values for other circuits.
