Sallen-Key 2nd-Order Active Low-Pass Filter

Two op-amps, four resistors, four caps — the textbook -40 dB/decade roll-off you can actually hear.

Difficulty: Breadboard. Estimated build time: about 50 minutes. Estimated parts cost: about US$2.20. 6-part bill of materials with live supplier pricing. A second-order active low-pass filter using the Sallen-Key topology, built around an LM358 (or any general-purpose op-amp).