Yesterday, while in a fit of boredom, I was cruising the web looking for info about active filters. Stumbled upon a couple of programs for designing filters from two chip manufacturers: Burr-Brown (now part of TI) Filter Pro is a DOS program for designing filters based on the UAF42 universal active filter chip. Linear Technology FilterCAD is a Windows program that will design filters using any one of many LT chips. Both programs design filters for the chips available from the respective companies. But they were interesting to play with. Any of you know much about DIY electronics and/or building active filters?