The Strange Case Of The A Class

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The Strange Case Of The A Class

If you’ve been keeping up with my rant on amplifier classes you’ll recall that in yesterday’s post I explained class AB operation.

One of the takeaways from that explanation would be that without an input signal, a Class AB amplifier constantly draws a small amount of power out of the wall—enough to warm its heatsinks. Not until there is an input signal does the amplifier start to draw much power.

Remember back to the beginning post on efficiency where I pointed out that in a Class AB amplifier, for 100 watts delivered to the loudspeaker another 100 watts would be converted to heat? The net result of that is an increase in heat for every watt delivered to the speaker. More power delivered to the speaker equals more heat generated by the amplifier. Makes perfect sense, right? The harder the amp works the hotter it gets.

Class A amplifiers are the opposite.

In a Class A amplifier, the more watts delivered to the speaker the cooler the amplifier gets! In fact, the point of a Class A amplifier’s maximum power output happens also to be the point of maximum efficiency.

Weird, right?

A 100 watt Class A amplifier with no input signal draws 200 watts out of the AC wall receptacle. All 200 watts are converted to heat. That same 100 watt class A amplifier delivering 100 watts of audio power to the speaker still consumes the exact same 200 watts from the AC wall socket, only this time, half of the 200 watts consumed goes to heat while the other half goes to making music.

Thus, Class A amps are strange beasts indeed.

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Paul McGowan

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