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Very Few Amplifiers Are Truly Class A

At least not up to their maximum rated output.

The long story is too long to tell.

The short story is that the best efficiency you can hope for out of a true Class A amplifier is about 50%, and that is when the amplifier is operating at maximum output power. At lower power levels the efficiency gets worse, since the "idling" power stays the same regardless of output power.

That means that an amplifier that can produce 50 watts of power into an 8 Ohm resistive load consumes 100 watts of power from the wall. Per channel. Into lower load impedances, the required quiescent current for Class A operation needs to go up, so the idle power does as well. Keep that in mind when you read claims of a "150 watt amplifier that doubles power down to 2 Ohms all in Class A!" An amplifier like that would idle at 1200 watts per channel, assuming no losses anywhere. A typical North American home AC mains outlet can provide 1950 watts (130 VAC times 15 Amperes) in total - can't very well power a stereo version of that amplifier or two monaural units from the same circuit breaker...

The point is that, as people have already pointed out, gen-u-ine class A amplifiers are necessarily heavy because of the power supply and required heat sinking and run very hot.

Single ended class A amplifiers are even less efficient, but most of these are relatively low power anyway so the heat and and heft aren't quite as evident.

Class D amplifiers in theory can be 100% efficient. In real life, with real components and some non-switching lower level stages, they still might be over 90% efficient. Hence less heat and heft.

So, yeah! Plug it in!

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