Explain Class D amps to "non audiophile friends"

is that a class-D amplifier is a very minor tweak of a switched-mode power supply (SMPS)

05-22-12: Kijanki
Bombaywalla - it is the other way around. Class D was invented tweaking SMPS.

Kijanki, I think that we are saying the same thing.... :-)

Jeff, this Wikipedia writeup should help to clarify what has been said. The entire article is well worth reading, but note particularly the block diagram, and the paragraph headed "Signal Modulation" that describes it.

As noted in the article there are ways of implementing a Class D design other than the one depicted in the block diagram, but it illustrates the basic concept.

Regards,
-- Al

Thanks for the info, guys. Us non-engineer types need to read and re-read the technical info a few times before we start to pick it up. 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?

Thanks!
Jeff

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

OK, light analogy. In order to achieve certain brightness of 100W light we can either adjust brightness in linear fashion by increasing bulb supply voltage or we can apply full voltage and blink it very fast. Continuous brightness that our eyes see depends on amount of time the light is on to amount of time it is off (average brightness). If we pulse it symmetrically (50/50) ON and OFF our eyes will perceive half of the brightness. That's how light dimmers work.

First method of achieving desired brightness corresponds to traditional class A or AB amps while the second method is class D. Dimmer is class D regulator. Thermal inertia of the filament is equivalent of output filtering.