Class D amplifiers generally use a low-pass filter to attenuate
the switching noise in the output waveform while passing the
audio signal to the loudspeaker, but many engineers are not
familiar with the functions performed by the various components
in a Class-D amplifier filter or how to calculate the proper
values. This article explains the purpose of the filter
components and how to calculate their values. The heart of a
Class-D amplifier filter is a L-C low-pass filter. The corner
frequency of the filter is chosen so that the filter will have
minimal effect on the desired output frequency range while
attenuating the switching noise as much as possible. Figure 1: A
low pass filter for a Class-D amplifier The optimum value for
the filter inductor is L = RL/2πfC where fC is the desired
corner frequency of the filter and RL is the load (speaker)
resistance. Note that the inductor value is dependent on both
the desired corner frequency and the speaker impedance so the
inductor value will changes if the speaker