High-Pass Filter

How to use this calculator

A first-order high-pass filter passes frequencies above its cutoff and attenuates those below it. For an RC filter the cutoff is fc = 1/(2πRC); for an RL filter it's fc = R/(2πL). Enter R and C (or R and L), choose the filter type, and the calculator shows the cutoff frequency and the relevant impedance at the cutoff.

What it means in a real circuit

For R = 10 kΩ and C = 10 nF, fc ≈ 1.59 kHz — signals above 1.59 kHz pass, below it are rolled off at 20 dB/decade. High-pass filters are used to block DC, to remove low-frequency hum or drift, and in speaker crossovers to direct high frequencies to the tweeter. The RC high-pass has the capacitor in series (blocking DC) and the resistor to ground; the RL high-pass swaps the roles. Use this to design a DC-blocking stage, a rumble filter, or a crossover, and to verify the cutoff lands where you need it.

Definitions

Cutoff frequency (fc): the frequency at which the output rises to 1/√2 (−3 dB) of its high-frequency value. Roll-off: 20 dB per decade (6 dB per octave) below fc. DC blocking: the capacitor's impedance is infinite at 0 Hz, so a series capacitor passes AC but blocks DC. Time constant (τ): τ = RC; fc = 1/(2πτ).

Design tips

For DC blocking, size C so fc is well below your lowest signal frequency (e.g. fc = 1/10th of the lowest tone). Choose R as the bias or load resistance of the next stage — don't add an extra resistor if one already exists. For a steeper response, cascade two 1st-order high-pass sections. If the following stage is low-impedance, the effective R drops and fc rises — account for it in the calculation.

Common mistakes

Assuming the series capacitor's impedance is the only thing that sets fc — the shunt R (or the next stage's input impedance) is equally important. Using a coupling capacitor that's too small, which muddies low frequencies. Forgetting that the source resistance forms a low-pass with the coupling cap. Confusing the RC high-pass topology (C in series, R to ground) with the low-pass (R in series, C to ground).

Component-selection advice

Use C0G/NP0 or film capacitors for coupling when audio quality matters; electrolytics are fine for large DC-blocking values but have high ESR and age. Keep R in the 1 kΩ–100 kΩ range to balance noise and loading. Choose a standard value nearest the calculated R·C product and re-verify fc.

Limitations of this calculation

The formula assumes ideal components and infinite source impedance. It ignores the source resistance (which forms a low-pass with the coupling cap), parasitic inductance at high frequency, and any active stage's bandwidth. Real high-pass filters show a gentle DC roll-off that the ideal −3 dB point approximates. The RL variant assumes the inductor's series resistance is negligible compared with R.

Example use cases

DC-blocking between amplifier stages; a rumble filter to remove sub-bass; a tweeter path in a 2-way speaker crossover; removing 50/60 Hz hum and its low-order harmonics; the input of a line-level audio preamp.