What a passive crossover does
A two-way loudspeaker uses a woofer for bass and midrange and a tweeter for treble. Neither driver can reproduce the whole audio range well, and tweeters are easily damaged by low frequencies. A passive crossover – a network of capacitors, inductors and resistors between the amplifier and the drivers – sends each driver only the frequencies it handles best. The crossover frequency is where the two filters meet, typically 1.8 – 3.5 kHz in a 2-way speaker.
Filter order and slope
- 1st order (6 dB/octave): one component per driver. Simple and phase-coherent, but drivers overlap widely and the tweeter receives more low-frequency energy.
- 2nd order (12 dB/octave): the most common design. Butterworth gives a slight peak at the crossover when summed; Linkwitz-Riley sums flat with the tweeter wired in reverse polarity.
- 3rd order (18 dB/octave): steeper protection for the tweeter, more components and more sensitive to driver behavior.
Formulas used
| Filter | Capacitor | Inductor |
|---|
| 1st order | C = 0.1592 ÷ (R × f) | L = 0.1592 × R ÷ f |
| 2nd order Butterworth | C = 0.1125 ÷ (R × f) | L = 0.2251 × R ÷ f |
| 2nd order Linkwitz-Riley | C = 0.0796 ÷ (R × f) | L = 0.3183 × R ÷ f |
C is in farads and L in henries; the calculator converts them to microfarads (µF) and millihenries (mH) and suggests the nearest standard E12 values. Combine capacitors in parallel to reach exact values.
Level-matching with an L-pad
Tweeters are usually more sensitive than woofers. If the tweeter is 3 dB louder, the speaker will sound bright. An L-pad – a series resistor and a parallel resistor – reduces the tweeter's level while keeping the impedance the crossover sees close to nominal. Enter the difference in sensitivity between your drivers to get both resistor values; use non-inductive resistors rated for at least 10 W.
Woofer impedance compensation (Zobel)
A woofer's impedance rises with frequency because of its voice coil inductance, which shifts a low-pass filter away from the intended frequency. A Zobel network – a resistor and capacitor in series, connected across the woofer – flattens that rise. A common starting point is R = 1.25 × Re and C = Le ÷ R².
Choosing components
- Capacitors: polypropylene film for tweeter filters; bipolar electrolytics are acceptable for large woofer shunt values.
- Inductors: air-core for series woofer coils to avoid saturation; laminated or ferrite cores keep DC resistance low for large values.
- Resistors: wirewound or metal-oxide, 10 – 20 W.
- Mount inductors at right angles to each other to avoid magnetic coupling.
Measure, then refine
Textbook values are a starting point. Driver acoustic roll-offs, baffle step and the tweeter's resonance all influence the final response. Free tools like Room EQ Wizard and a calibrated measurement microphone let you verify the summed response and adjust component values.
Frequently asked questions
What crossover frequency should I use for a 2-way speaker?
Usually 1.8 – 3.5 kHz. Choose a point where the woofer is still smooth and at least about twice the tweeter's resonant frequency.
Linkwitz-Riley or Butterworth?
2nd-order Linkwitz-Riley sums flat with the tweeter in reverse polarity and is the most popular choice. Butterworth gives a small peak at the crossover.
Why reverse the tweeter polarity?
With 2nd-order filters, the woofer and tweeter outputs are 180° apart at the crossover frequency. Reversing the tweeter brings them back in phase.
Do I need an L-pad?
If the tweeter is more sensitive than the woofer – which is common – an L-pad lowers it to match and prevents a bright sound.
Are electrolytic capacitors OK in crossovers?
Bipolar electrolytics work in woofer shunt positions; film capacitors are preferred for tweeter series capacitors.
About XoverLab
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