The basic circuit layout is the same as Class B for all intensive purposes. What is different is the signal that is fed into the output devices.
Instead of feeding these devices an analog musical waveform, the audio is fed into a circuit called a Pulse Width Modulator.
The whole theory behind this topology relies on how slow a speaker reacts to a signal. You see, if you send a very
brief voltage spike to a sub, the weigh of the cone will not allow it to move very far, if at all.
If you send a series of these pulses together, the speaker will move just a bit.
If the pulses are a bit wider, we get more motion from the cone...
You can see that if we combine different pulse widths, we can recreate a waveform of almost any shape. Bingo!
If we switch the output devices all the way and back off again very quickly, and do so at different rates, we can get an audio waveform to be produced by the speaker.
The incredible advantage to this topology is that the output devices are either all the way on, or all the way off. They are never half way on, and acting like resistors. When in the resistive region (anywhere between completely off or on), they heat up. Anything that heat up wastes energy in the form of heat.
In application, these amplifiers have the highest overall efficient. Varying between 70% and 80% in 1/3 power tests. Class B amps come in around 30% to 40% efficient. Class A amps are the worst of the bunch, coming in at 10-15% efficient...
|Topology||Topology efficiency||Amp Efficiency||Benefit||Drawback|
|Class A||25%||15%||Low Distortion||Inefficient|
|Class B||75%||35%||Efficient||Crossover Distortion|
|Class D||95+%||75%||Most Efficient||Noisy, for subs only.|
Hopefully that helps to demystify what the different topologies mean.