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Free Class-D Audio Amplifier LC Filter Calculator Electronics & RF
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Free Class-D Audio Amplifier LC Filter Calculator

Calculate output low-pass filter inductance (µH), capacitance (µF), Butterworth cutoff frequency, and Q resonance across 4Ω and 8Ω speaker loads.

⚡ Amplifier & Filter Parameters

Carrier switching frequency
Typically 30 - 40 kHz

⚡ Inductor & Capacitor Components

Filter Inductor (L) -- --
Filter Capacitor (C) -- --
Actual -3dB Audio Cutoff --
Carrier PWM Attenuation --
Unloaded / 8Ω Light Load Q --
20 kHz Audio In-Band Loss --
Recommended Inductor Rating --

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The Physics of Class-D Reconstruction Filters

Class-D audio amplifiers do not amplify linear analog waveforms directly. Instead, they convert the incoming audio signal into high-frequency Pulse-Width Modulation (PWM) square waves switching at 350 kHz to 1.2 MHz. A passive second-order LC low-pass filter strips away the violent high-frequency PWM switching carrier, reconstructing the smooth analog audio waveform before it reaches the loudspeaker terminals.

The LC Filter Design Formulas

The low-pass filter consists of an Inductor ((L)) and Capacitor ((C)), with the speaker impedance ((R_{ ext{load}})) providing the necessary damping:

Cutoff Frequency: f_c = 1 / [ 2 × π × √(L × C) ]
Quality Factor: Q = R_load / (2 × π × f_c × L) = 2 × π × f_c × C × R_load
Inductance: L = R_load / (2 × π × f_c × Q)
Capacitance: C = Q / (2 × π × f_c × R_load)

The Load-Dependence Dilemma (4Ω vs. 8Ω)

Because the speaker impedance forms an integral part of the passive LC filter circuit, the filter's Q factor and frequency response change whenever a different speaker is connected:

  • An LC filter calculated for a 4(Omega) load will have its Q factor double to (Q approx 1.414) when an 8(Omega) speaker is plugged in, causing a prominent +3 dB to +5 dB resonant treble spike near 30-40 kHz.
  • If an amplifier runs with no speaker connected (open circuit), the Q factor approaches infinity, causing high-voltage ultrasonic ringing that can arc across PCB traces or destroy the amplifier.

Frequently Asked Questions

What type of inductors must be used in a Class-D output filter?

Use shielded drum-core or toroid power inductors specifically rated for Class-D audio (such as Coilcraft MSS1210 or SAGAMI 7G series). The inductor core MUST have a saturation current (Isat) well above the peak amplifier current to prevent core saturation, which creates severe harmonic distortion.

Why must the filter capacitor be a film capacitor (MKP) rather than ceramic?

Standard high-voltage ceramic capacitors (X7R, X5R) suffer from significant voltage coefficient piezoelectric distortion and non-linear capacitance shifts under AC voltage. Metalized polypropylene film (MKP) capacitors maintain perfectly linear capacitance, delivering ultra-low THD+N.

What is Post-Filter Feedback (PFFB)?

PFFB is an advanced circuit topology (used in high-end amplifiers like 3e Audio and Purifi) that takes the negative feedback loop from AFTER the LC filter (at the speaker binding posts) rather than before it. PFFB completely eliminates load-dependent frequency response variations and lowers distortion by 10-15 dB.

What is the difference between BTL and SE Class-D amplifiers?

Single-Ended (SE) amplifiers connect the speaker between the half-bridge switch output and ground, requiring one inductor and one capacitor (plus a DC blocking cap). Bridge-Tied Load (BTL) uses two half-bridges driving opposite sides of the speaker differentials, requiring two inductors (one per leg).

Why not set the cutoff frequency to 20 kHz instead of 35 kHz?

A 2nd-order Butterworth filter begins rolling off well before its -3 dB cutoff. Placing fc right at 20 kHz causes a noticeable -0.5 dB to -1.0 dB attenuation at 15-20 kHz in the audible human hearing range and introduces severe phase shift in the upper octave.