Explain Class D amps to "non audiophile friends"

In general efficiency is inversly related to sound quality. Class D gives you a lot of power in a lighter, cheaper, more efficient package. Good for sub use. High quality Class D amps are not cheap so there is no free lunch.

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.....