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Tweeter Protection Capacitor

Calculate the capacitor value for a first-order tweeter high-pass filter to protect tweeters from low-frequency damage.

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Formula

C = 1 / (2π × f_c × Z_t)

f_c— Crossover frequency (Hz)
Z_t— Tweeter impedance (Ω)

How It Works

A tweeter protection capacitor forms a first-order high-pass filter (6 dB/octave rolloff) that protects the tweeter from low-frequency signals that could damage its small voice coil. The crossover frequency is set by the interaction of the series capacitor and the tweeter's nominal impedance: f_c = 1 / (2π × C × Z_tweeter). Below f_c, the capacitor's reactance X_c = 1/(2πfC) is much higher than the tweeter impedance, reducing signal through the tweeter. This is the simplest possible crossover element — a single capacitor — as used in many bi-amp or add-on tweeter systems. More complex passive crossovers add inductors (second-order, 12 dB/octave) to the woofer section and may include compensation networks for tweeter impedance rise.

Worked Example

Tweeter: 8 Ω nominal impedance. Desired crossover frequency: 3000 Hz. Required capacitor: C = 1 / (2π × 3000 × 8) = 1 / (150,796) = 6.63 × 10⁻⁶ F = 6.63 μF Nearest standard value: 6.8 μF (actual f_c = 1/(2π × 6.8×10⁻⁶ × 8) ≈ 2930 Hz) Reactance at 1 kHz: Xc = 1 / (2π × 1000 × 6.63×10⁻⁶) = 24.0 Ω (Xc > Z_tweeter at 1 kHz — signal attenuated) Rolloff at 100 Hz: Ratio = 3000 / 100 = 30:1 in frequency Attenuation = −20·log₁₀(30) ≈ −30 dB

Practical Tips

  • ✓Use polypropylene or polyester film capacitors (MKP, MKT type) for best audio quality in tweeter crossovers. These have lower ESR and better high-frequency performance than electrolytic types.
  • ✓For a 2nd-order (12 dB/octave) high-pass, add a shunt inductor in parallel with the tweeter: L = Z / (2π × f_c). The combination of series C and shunt L gives Butterworth (Q = 0.707) alignment when both are designed to the same f_c.
  • ✓Verify the crossover frequency matches the tweeter's rated power handling — at 3 kHz with a 6.63 μF capacitor and 8 Ω tweeter, the tweeter receives the full amplifier power above 3 kHz. Ensure the tweeter is rated for the amplifier's power output in this frequency range.

Common Mistakes

  • ✗Using nominal impedance as a flat 8 Ω — tweeter impedance rises significantly above its rated frequency due to voice coil inductance. The actual filter frequency may shift higher than calculated. Zobel networks can flatten impedance for more accurate crossover behaviour.
  • ✗Selecting too low a crossover frequency — most dome tweeters should not be crossed below 2–2.5 kHz (some as high as 5 kHz). The resonant frequency of the tweeter (Fs) should be at least an octave below the crossover frequency. Check the tweeter datasheet.
  • ✗Using electrolytic (polarised) capacitors in crossover networks — non-polarised (NP or bipolar electrolytic, or film) capacitors are required for audio crossovers. Polarised electrolytic capacitors introduce distortion and can fail under AC signal conditions.

Frequently Asked Questions

A first-order (6 dB/octave) crossover has the gentlest rolloff and the best time-domain behaviour (minimum phase), but the slow rolloff means the tweeter receives significant energy below the crossover frequency. For drivers with narrow overlap bands, higher-order crossovers (12 or 18 dB/octave) provide better protection and reduced driver excitation outside their operating range.
A Zobel network (a series RC circuit in parallel with the tweeter) flattens the tweeter's rising impedance at high frequencies due to voice coil inductance. This makes the crossover filter behaviour more predictable. For simple first-order single-capacitor applications it is often omitted, but for higher-order networks it improves crossover accuracy significantly.
The equivalent formula for a first-order low-pass inductor in series with a woofer is L = Z / (2π × f_c). The woofer sees a −6 dB/octave rolloff above f_c. For 8 Ω and 3 kHz: L = 8 / (2π × 3000) ≈ 0.42 mH.

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