05-22-12: Jeffatus
Question: it appears to me that class A, B or A/B share some similarities, yet class D seems worlds apart from the rest. Is this correct? Could you compare the differences between class D and class A or B amplification?
Yes, it is a fundamentally different approach.
Put simply, at any instant of time each point in the signal path between the input and the output of the traditional amplifier classes has a voltage that is (to a close approximation) proportional to the voltage that is present at the input of the amplifier at (approximately) that instant of time.
A Class D amplifier does not work that way. At some of the points in its signal path, including the high power stage, what is proportional to the voltage of the input signal is the width or timing of a series of pulses. A "pulse" in this context refers to a voltage that (to a close approximation) switches between two (and only two) possible values.
An analog output signal whose instantaneous voltage is proportional to the instantaneous voltage of the input signal can be (and is) recovered from that width or timing by simply filtering out frequency components of the power stage's output signal that are far above the audible frequency range, and that are unrelated to the audio information itself. As indicated in the Wikipedia writeup I linked to, those high frequency components "serve no purpose other than to make the wave-form binary so it can be amplified by switching the power devices."
Regards,
-- Al
