Speaker Crossover Calculator: Design Passive Crossovers for Hi-Fi Speakers
This speaker crossover calculator helps you design passive crossover networks for 2-way speaker systems. Enter your tweeter and woofer impedances along with the desired crossover frequency to get the exact capacitor and inductor values you need. The calculator also includes a Zobel impedance-stabilization circuit and an L-pad attenuator.
Why You Need a Crossover
A single speaker cannot reproduce the full range of audible frequencies (20 Hz – 20 kHz) with equal efficiency. A woofer handles low frequencies well but rolls off at high frequencies, while a tweeter handles high frequencies but cannot handle bass. A passive crossover network splits the amplifier signal so each driver receives only the frequencies it is designed to reproduce.
- Tweeter (high-pass filter) — receives frequencies above the crossover point.
- Woofer (low-pass filter) — receives frequencies below the crossover point.
Filter Orders
- 1st Order (6 dB/octave): One capacitor in series with the tweeter, one inductor in series with the woofer. Simple design with minimal power loss, but limited attenuation of out-of-band frequencies.
- 2nd Order (12 dB/octave): Two components per driver — series C + shunt L for the tweeter (high-pass), series L + shunt C for the woofer (low-pass). Good balance between complexity and protection.
Filter Characteristics (2nd Order)
- Butterworth (Q = 0.707): Maximally flat amplitude response. Sums to +3 dB at the crossover frequency. The most popular choice.
- Bessel (Q = 0.577): Maximally flat group delay, meaning excellent transient accuracy. Softer slope near the crossover point.
- Chebyshev 0.5 dB (Q = 0.864): Steeper initial roll-off than Butterworth at the cost of a 0.5 dB passband ripple.
Crossover Frequency
The crossover frequency should fall within the overlap of the frequency responses of both drivers. Typical values:
- 2-way speaker: 2,000 – 3,500 Hz (check driver datasheets for recommended overlap region).
- For a tweeter rated down to 2 kHz and a woofer rated up to 4 kHz, a crossover near 2.5–3 kHz is a safe choice.
Formulas
1st Order (K = 0.15915 = 1/2π):
- C (µF) = K × 10⁶ / (Z × f)
- L (mH) = K × 10³ × Z / f
2nd Order Butterworth (K_C = 0.11254, K_L = 0.22508):
- HP tweeter: C₁ = K_C × 10⁶ / (Z_tw × f), L₁ = K_L × 10³ × Z_tw / f
- LP woofer: L₂ = K_L × 10³ × Z_wo / f, C₂ = K_C × 10⁶ / (Z_wo × f)
Zobel Circuit (Impedance Equalization)
Speaker impedance rises with frequency because of the voice coil inductance (Le). This rise can shift the actual crossover frequency upward. A Zobel (Boucherot) circuit — a resistor R_z and capacitor C_z in series, connected in parallel with the speaker — flattens the impedance curve:
- R_z = Re (DC resistance of the voice coil, Ω)
- C_z (µF) = Le(mH) × 1000 / Re²
L-Pad Attenuator
An L-pad reduces the sensitivity of a driver (typically the tweeter) while maintaining a constant impedance load for the crossover. It consists of a series resistor R_s and a shunt resistor R_p:
- R_s = Z × (a − 1) / a
- R_p = Z × a / (a² − 1)
- where a = 10^(dB/20)
Measurement Systems
All electrical quantities in crossover design (impedance in Ω, frequency in Hz, capacitance in µF, inductance in mH) use SI (metric) units universally in both American and international engineering practice. There is no distinction between metric and imperial units for passive electronic components.
Practical Tips
- Use non-polarized (bipolar) electrolytic or film capacitors rated for audio use.
- Air-core inductors are preferred over iron-core for low distortion, especially for tweeters.
- Choose the nearest standard E24 capacitor and E12 inductor values available from your supplier.
- Always verify the final crossover frequency with a measurement microphone and audio analyzer after assembly.