Explain Class D amps to "non audiophile friends"

You will find PWM modulation in other places where it has nothing to do with efficiency. For instance SACD is the same as class D before filtering (operating at 2.8MHz). Also Sigma Delta DAC operates on the same principle.

Kijanki, what you wrote is absolutely correct but what you'll find is that a class-D amplifier is a very minor tweak of a switched-mode power supply (SMPS) where the input (DC) voltage reference is replaced by a (modulating) music signal. Do this simple swap & you have a class-D power amplifier. Compare a class-D audio amp & a SMPS & see for yourself.....

STOP THE PRESSES: News flash...To the astonishment of the audio geek community it would appear that some Class D amps sound great, and others less so. Details to follow.

Bombaywalla - it is the other way around. Class D was invented tweaking SMPS. Engineers demoing SMPS showed that load regulation is so fast that SMPS can even play music.
Any linear power supply is in fact primitive switcher operating at 120Hz. Primitive because it requires huge transformers, big capacitors (to filter out 120Hz) has no line or load regulation, switches at max voltage, is susceptible to DC on line etc. That's why Rowland uses SMPS in preamps where efficiency is not important or power amps (model 625) that are class AB. Bad rap that SMPS has come mostly from primitive computer supplies.

Wolf_garcia, They sounded great from the beginning (at least to my ear). Second "version" was designed to make it easier for some critics to remove foot from the mouth.

I've read all the posts and I still don't understand how a class D amp works. What are the devices that do this "switching?" I get the difference between class A and class A/B, that thread was great, but for this one I think I need some kind of dumbed down explanation...more layman terms please?

Realremo, Imagine just one mechanical switch that switches between +10V and -10V. Output of this switch after averaging (filtering) will be zero volts if switch was the same amount of time (for instance 1s) at +10V and -10V positions (total cycle time of 2s). Manipulating ratio of times changes output voltage.

For instance:
1.5s at +10V and 0.5s at -10V will result in output of +5V because (1.5*10+0.5*-10)/2=+5
or
0.5s at +10V and 1.5s at -10V will result in output of -5V because (0.5*10+1.5*-10)/2=-5.

Real frequency of switching is very high (in order of 500kHz) where mechanical switches would not work - hence Mosfet transistors (fast switches). Mosfet transistors are even manufactured in two different technologies for use as switches or linear transistors.