Checking Class D Output Filter Response Across Speaker Loads

A class D output filter can produce a smooth response into a laboratory resistor and a different response into the intended loudspeaker. The load participates in the filter network. Review the amplifier, LC topology, and speaker impedance together before selecting component values from a convenient reference design.

Identify the supported topology

The TI LC filter design guide explains how load impedance changes filter damping and distinguishes filter arrangements for different class D modulation schemes. Match the chosen network to the amplifier’s actual output configuration and modulation.

Draw each series inductor and every capacitor, including capacitors from an output to ground. A differential capacitor and two ground connected capacitors cannot be exchanged by retaining the same printed capacitance without analyzing the equivalent circuit.

Use the speaker impedance range

Begin with a resistive model to understand the expected corner and damping, then add measured or manufacturer supplied speaker impedance versus frequency. Include the crossover network if it is part of the installed load.

The impedance marking on a speaker is a nominal classification. It does not describe every frequency near the filter transition. Inspect the upper audio band and the region approaching switching attenuation, where a change in load can expose a response peak.

Compare useful operating cases

Simulate minimum and maximum expected load conditions, component tolerances, and any supported disconnected load state. Compare passband gain, peak amplitude, capacitor current, and inductor current. Record which cases are permitted during normal operation and which require a separate fault response.

For example, changing a product from one speaker assembly to another while keeping the same amplifier board can change damping even if both speakers carry the same nominal impedance. Treat that substitution as an electrical response change until the installed load has been checked.

Measure the assembled response

Use a differential audio measurement across the load and start with low stimulus amplitude. Sweep the required band while monitoring unexpected peaking and amplifier protection behavior. Increase level only within the allowed operating envelope.

Repeat with the final cable and crossover. Compare the result with the model and investigate differences before retuning the filter. Probe loading, an incorrect component, or a different modulation setting can resemble a calculation error.

Save topology, load data, tolerance cases, response curves, and the amplifier configuration. Keep switching attenuation and audio passband acceptance as separate requirements. A useful filter selection preserves the intended response across the supported speaker set while providing the required switching suppression. A pair of LC values copied from a board with a different load does not establish either result.