circuit design - Standard ways to make a variable low pass filter - Electrical Engineering Stack Exchange
Low-pass Filters | Filters | Electronics Textbook
Low Pass Filter Construction
Passive Low Pass Filter Pedal Wiring – This would also be great as an addon for … | Guitar Gear Geek
Simple d.i.y. Low-Pass Filter for Interfacing PMT Amplifier to PC Sound Card (Used with Free “Pulse Recorder and Analyser” Software MCA) – diy Physics Blog