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-line bill of materials. Compare supplier offers when available. A second-order active low-pass filter using the Sallen-Key topology, built around an LM358 (or any general-purpose op-amp).
Wire the LM358 with a stable supply
Use a single-supply rail of 9 V or 12 V (a 9 V battery works fine). The LM358 will not swing to either rail, so create a virtual ground at half-supply with two 10 kΩ resistors and a 10 µF capacitor to ground. Connect the op-amp's non-inverting input bias network to this virtual ground when you want to filter AC signals around it.
Build the input RC stage
From the input through-line, place R1 = 16 kΩ to a node we'll call N1. From N1 place C1 = 10 nF to the output rail (the op-amp output, not to ground — this is the positive feedback path that makes the Sallen-Key sharper than just two cascaded RCs).
Build the second RC stage and the op-amp
From N1 place R2 = 16 kΩ to the op-amp's non-inverting input (+). From the non-inverting input place C2 = 10 nF to ground (this one DOES go to ground). Wire the op-amp as a unity-gain follower: tie its output directly to the inverting input (-). The output is the op-amp's output pin.
Drive it with a frequency-swept signal
Connect a function generator (or another op-amp wired as a sine oscillator) to the input. Set the amplitude to 1 V peak-to-peak and start at 100 Hz. Sweep slowly up to 10 kHz while watching the output amplitude on a scope. You should see the output stay at 1 Vpp until ~700 Hz, drop to 0.707 Vpp (-3 dB) at exactly 1 kHz, and fall away rapidly above — by 10 kHz the output should be roughly 1/100 of the input, i.e. 10 mVpp.
Compare the knee to a first-order RC
Replace the Sallen-Key with a single RC stage (just R1 + C2, no op-amp). At 1 kHz both filters give -3 dB. But at 10 kHz the simple RC is only down 20 dB (factor of 10), while the Sallen-Key is down 40 dB (factor of 100). This is what "second-order" buys you: same -3 dB point, much sharper attenuation above it.