The output from a Class-D amplifier consists of a PWM waveform that is typically resolved into a clean audio waveform by passing the signal through an LC filter to remove the high frequency portion of the output signal. Modern H-bridge amplifiers can operate without this filter by using the inductance to replace this filter. However, given the power levels typical to audio applications and the output switching frequencies required for this type of operation, a filter is often still necessary to meet electromagnetic emission standards. Many designers wish to use capacitive-only output filters given the small size and low cost required for these components, but this approach involves significant risk and it is not recommended for use on Class-D outputs.

One challenge commonly seen when introducting a filter to a PWM waveform is an increase in the noise floor of the signal. If the filter is not carefully designed, this can increase to levels easily detected by the end user. Generally, it is safe to set the filter corner frequency at or above 2x the effective maximum switching frequency with minimal impact to the noise floor. When designing for devices which utilize post filter feedback (TAS2560, TAS2557, and TAS2559) please refer to Post FilterFeedback Class-D Amplifier Benefits and Design Considerations application report.