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.

>>05-22-12:Stanwal
In general efficiency is inversly related to sound quality.<<

That is simply not true.

Ignore the inexperienced post(er).

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.