What is a typical amp's efficiency rating? TECHNICAL


Bronze Member
Username: Cheapskate

Post Number: 96
Registered: Mar-04
i am considering building a "hi-end" boombox amp using class-d components which are rated at 90% efficiency. the amp is supposed to be so efficient that it doesn't require a heat sink.

if 90% efficiency is considered "exceptional" then just how efficient is an average class a/b amplifier? i have no idea where to find such specific information and am hoping that some sort of electrical engineer or tech type visits this thread with an answer.

class-d seems like an obvious choice for extending battery life, but how much better is it than conventionl technologies?

thank you in advance to anyone who can elaborate.

J. Vigne
Unregistered guest

What you're asking is a question whose answer will depend very much on the topography and bias setting of a particular amplifier. Most amlpifiers that are used in home audio run in a Class AB mode of operation. This class of operation was designed to increase the efficiency of amplifiers and results in about a 25-30% efficiency, the rest of the energy coming into the amplifier is wasted as mostly heat. If an amplifier is run in a less efficient Class A mode the number plummets to about 12-15%. Class B is normally found in the least expensive sound reinforncement amps. Class C, used for PA systems in stores and auditoriums, is the most efficient of the common modes of operation. Class C can reach about 40-50% efficiency if I remember correctly. Of course the high levels of notch distortion make this class unacceptable for home audio. There is an earlier Class D that has all but disappeared from the planet that is more efficient still that was never, to my knowledge, used for audio purposes. There are early switching amps that are uncommon and being replaced by the digital amplifiers.
Class D amps are quite efficient but not large enough for most home audio applications at this point. Other drawbacks are the need for heatsinking despite their efficiencies since they are chip based designs. The lack of current supplied by most common Class D amplifiers presents a problem in home audio. HK seems to be addressing these problems and others will follow, but, for now Class D is still not a truly useful design for consumer audio from what I've seen.

If an engineer wishes to correct my numbers they are more than welcome. What I've stated is to the best of my memory.


J. Vigne
Unregistered guest

Actually the efficiency of an amplifier is quite impressive when you consider most loudspeakers are beneath 5-10% efficiency.


Bronze Member
Username: Cheapskate

Post Number: 99
Registered: Mar-04
wow! so a class d amp is roughly in the neighborhood of twice as efficient as a/b! that isn't shabby at all. it would be worth the extra expense to invest in class d then to conserve batteries.

the amp i was looking at was 25wpc which has been done before in boomboxes, but with twice the thermal efficiency and 92dB speakers, batteries would last alot longer with a class d amp or allow the amp to play louder for the same amount of time.

i just saw a smaller "ipod amp" that has class d amps. it might have been an advent.

thanks for the info. i was guessing that conventional a/b amps were 70-80% efficient, but 50% or less isn't a small difference.

i know that class a amps are power hogs as they amplify the negative as well as positive waveform and run hot. i remember seeing a class h amp once too. most amps are a/b though including the really cheap amp kits i was considering i bet. $30 for a/b or $90 for d of the same rating... hmmm...

for a high end boombox, class d is a no brainer then.

thanks a bunch people. :-)
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