Sallen-Key High-Pass Filter Calculator

The Sallen-Key high-pass is the dual of the low-pass — swap R and C positions in the original 1955 Sallen-Key topology. Both versions appeared in the same Lincoln Lab paper that introduced the ladder-op-amp architecture.

Sallen-Key high-pass is widely used as the first stage of AC-coupled amplifiers, subsonic filters for audio (removing turntable rumble and amplifier DC drift before loudspeaker amps), and the HP section of bandpass filter designs. Cascaded HP sections create higher-order Butterworth/Bessel/Chebyshev HP filters.

Modern integrated filters (MAX7480 switched-cap, TI TLV320 audio codec decimators) and fully-digital filtering have reduced the need for discrete Sallen-Key designs, but it remains the simplest breadboard HPF and the first active-filter topology in most textbooks.

Enter cutoff frequency and Q. Enter capacitor value C (equal caps, C1 = C2). The tool solves R1 and R2 using the Q = ½√(R2/R1) and fc = 1/(2π·C·√(R1·R2)) equations. Round to nearest E24 or E96 standard values for build.

For higher order, cascade 2nd-order sections. For example, 4th-order Butterworth HPF: two Sallen-Key sections with Q = 0.541 and Q = 1.307. Everything runs client-side.