Calculation and measurement of input and output impedances of a Class-D amplifier
Class D means that the output stage of these amplifiers are switched. The output stage consists of fast switches which connect the outputs to the positive and negative supply rails and so generating a PWM signal.

This PWM signal is then filtered to recover the wanted analog signal for the speaker.

As such a class D stage has a strongly non-linear behavior making it less trivial to determine the output impedance of such a stage.

As you should know, determining the output impedance of the classic (linear,non-switching) class A, AB, B and C stages involves choosing an operating point around which the amplifier stage is assumed to be linear. Then a small signal model can be made and the output impedance determined. There might be a feedback loop which can lower the output impedance as well.

For class D it is possible to determine a certain linearized behavior model but it is more cumbersome. Some clues might be found in this Thesis.