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
Pure Subwoofer Front Plate Low Pass Filter High Level Input Diy Pre Tone Balanced Input / Output Board - Switch Caps - AliExpress
Low Pass Filter kit
circuit design - Standard ways to make a variable low pass filter - Electrical Engineering Stack Exchange