Free Class-D Audio Amplifier LC Filter Calculator
Size output low-pass LC reconstruction filters, calculate Butterworth damping, prevent high-frequency speaker impedance peaking, and suppress PWM switching EMI.
🔊 Amplifier & Speaker Specifications
Recommended Capacitor (C): 1.0 μF Film / C0G
• If 8Ω Speaker Connected: +3.0 dB Peak at 35 kHz (Filter underdamped; add Zobel network!).
• Inductor Core Material: Use powdered iron (Kool Mμ or gapped ferrite) rated for peak audio currents without saturating into a dead short.
The Zobel Solution: Real-world loudspeakers exhibit rising inductive voice coil impedance at high frequencies. Always place an RC Zobel network ($R approx R_L$, $C approx 0.1,mu ext{F}$) in parallel with the speaker terminals to clamp ultrasonic peaking.
Recommended Tools & Equipment
Tested hardware and components for high reliability
Role of the LC Reconstruction Filter
A Class-D power stage switches high-voltage power rails (typically 12V to 60V) at rapid PWM rates (300 kHz to 600 kHz). The output 2nd-order low-pass LC filter fulfills two critical functions:
- Audio Reconstruction: Integrates high-frequency pulse width modulation into an analog audio sine wave by stripping the RF carrier.
- EMI / EMC Suppression: Prevents long speaker cables from acting as radio broadcast antennas radiating FCC/CE non-compliant electromagnetic interference.
Filter Sizing Equations
Butterworth Inductor: L = (√2 × R_L) / ω_c
Butterworth Capacitor: C = 1 / (√2 × R_L × ω_c)
Quality Factor (Q): Q = R_L × √(C / L)
Carrier Attenuation: A_dB = -40 × log10(f_sw / f_c)
BTL Implementation: Place L in each speaker leg (OUT+ and OUT-) with C across terminals
Why Inductor Linearity is Paramount
Unlike inductors in DC-DC power supplies that merely handle ripple currents, Class-D output inductors carry full audio signal currents. If the inductor core saturates during loud bass transients, its inductance collapses, resulting in massive Total Harmonic Distortion (THD) and risking catastrophic MOSFET overcurrent trip.
Frequently Asked Questions
Why does a Class-D amplifier filter peak when no speaker is connected?
The LC filter relies on the speaker voice coil resistance RL to provide mechanical damping. In an open-circuit (no speaker) or high-impedance condition, the filter Q spikes to 10 or 20, creating a huge resonance peak at fc. This can generate multi-hundred volt ringing that destroys the amplifier output transistors unless clamped by a snubber.
What capacitor dielectric is required for Class-D output filters?
Never use standard Class-2 ceramic capacitors (like X7R or X5R) because their capacitance drops precipitously with applied AC voltage (piezoelectric distortion). Always use metallized polypropylene (MKP) film capacitors or Class-1 C0G/NP0 ceramics for clean audiophile distortion figures.
Can modern Class-D amplifiers run filterless?
Low-power battery devices (smartphones, bluetooth speakers < 3W) can run filterless if speaker leads are shorter than 5-10 cm, using the speaker voice coil inductance alone to integrate current. However, any amplifier producing > 10W or driving long speaker cables requires a dedicated LC filter for FCC compliance.