The advantages of Class D power amplifiers are well known. They are small and lightweight, produce virtually no heat, have very high output, and are relatively inexpensive. But just how do these miniature marvels deliver so much power from a brick-sized chassis that you can hold in the palm of your hand?
Class D power amplifiers operate in a completely different way than the traditional power amplifiers in use since the early 1900s. The conventional “linear” amplifier with which we’re all familiar has two “classes” of operation, A and B, with a third class (AB) a hybrid of the two.
Let’s take Class A first. In a Class A amplifier, a transistor (or tube) amplifies the entire musical waveform. The output transistor acts as a continuously variable valve that partially “opens” and “closes” to allow current to flow to the speaker. Specifically, a small, continuously variable signal at the input (the low-level audio signal) acts as the control on this valve that modulates a large current flow through the loudspeaker. Vacuum tubes are known as “valves” in other parts of the world for good reason.
A pure Class A power amplifier is large, heavy, and requires massive heat-sinking relative to its output power. It consumes almost as much power from the wall outlet at idle as it does at full power. A single-ended triode (SET) amp is an example of pure Class A operation. Some Class A amplifiers use two output devices, one of which amplifies the mirror image of the signal. This is called “push-pull” operation. In push-pull Class A, both output devices are active for the whole signal. You can think of a push-pull amplifier as a two-man saw. In Class A, while one is pulling, the other helps by pushing.
Class B operation uses “complementary pairs” of transistors or tubes, with one transistor handling one half of the musical waveform and the other transistor amplifying the other half of the waveform. Class B always works in push-pull mode because when one of the two transistors is off, the other must take over. This is like a two-man saw where, while one man is pulling, the other takes his hands off the saw. Another way to consider Class B operation is one transistor “handing off ” the audio signal to its partner at the waveform’s zero-crossing point.
Nearly all power-amplifier output stages operate as a hybrid of Class A and Class B, creating the familiar Class AB designation. A Class AB amplifier operates in Class A mode for the first few watts of its output power, and then switches to Class B mode. The more power the amplifier can produce in Class A before switching to Class B, the hotter it runs and the more heatsinking it needs.
Both Class A and Class B operation are extremely inefficient (especially Class A). Most of the power consumed is wasted as heat. In a typical Class AB amplifier, 80 percent of the power pulled from your wall outlet is turned into heat and only 20 percent into output power to drive the loudspeakers (when the amplifier is putting out 25 percent of its maximum power). Even when operating at maximum output power (the most efficient condition), a Class AB amplifier’s efficiency is less than 60 percent. A pure Class A design has an overall efficiency in practice of perhaps 10 percent.
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