Explain Class D amps to "non audiophile friends"

Imagine switching two wires between +V and -V voltage at very high non-audible frequency. You will get high frequency square wave with average value of zero as long as this switch stays in each position 50% of the time. Change this percentage (ratio) and average value will change from +V to -V. Filtering this square wave (inductor and capacitor) will convert its average value to DC voltage that feeds speaker. Changing symmetry of this switch according to input voltage is what class D amp does. It is called PWM (pulse width modulation).

Efficiency of class D amp is less than 100% because ON resistance of this switch and transition time are not completely zero while output filtering inductor has also some resistance, plus some small power is used by modulator. Efficiency of my Rowland 102 class D amp is 90%.

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. In fact modulator used in modern class D amps is similar to Sigma Delta modulator. Main problem is to make switch (Mosfet transistor) that can switch extremely fast big current and has close to zero resistance. Faster switching allows to use higher switching frequency that can be filtered out better while bandwidth of amplifier can be extended further.

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.

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.

Mapman, I suspect that it will get much better because of semiconductor speed. Things based on timing scheme like Sigma Delta DACs are limited in resolution by timing accuracy while traditional DACs are limited by tolerance of components. Semiconductors are getting much faster every year including high current Mosfets.

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.

"Ok my brain just exploded all over my iPad."

Realremo, we are seriously trying to explain but I can see that making jokes is more important to you and Jeffatus (who finds it amusing). I won't make this mistake again.

Realremo, No harm done. I'm just overly sensitive today. Don't hesitate to ask if you want to go more into details. I'm not the best to explain but Al does it very well and he is forum Guru (in case you don't know it yet).