Stereo Equalizer and Balance Schematics.
http://www.paia.com/duckerda.htm
http://www.ethanwiner.com/basstrap.html
http://www.zen22142.zen.co.uk/Circuits/Audio/3band.htm
http://www.web-ee.com/Electronic-Projects/projects/graphic/index.shtml
http://www.4qdtec.com/graph.html
http://www.jensen-transformers.com/as/as065.pdf