Amplifier efficiency

June 3, 2021
 by Paul McGowan

There seems to be a lot of misinformation swirling around an amplifier’s efficiency. Maybe I can help by explaining a few things.

First off, what do we mean by the term efficiency? It refers to the production of heat vs. the production of sound. Think of it in the same way you might think of fuel efficiency. How many miles per gallon does a car get? The more efficient the car, the more miles per gallon. The fuel consumed in a car has an energy potential. If all of its potential is converted into the forward motion of the car we can say it is 100% efficient.

If an amplifier were 100% efficient it would generate no heat and whatever wattage it pulled from the wall would be delivered to the speakers.

The typical class A/B power amplifier is about 50% efficient. This means that for a given number of output watts, let’s say 100, the amplifier will produce 100 watts of heat.

I am asked all the time if this means a loss of power. To some, the idea that a power amplifier is only 50% efficient suggests that a 100 watt amp might only be able to output 50% of its rated power. After all, it’s only 50% efficient. And that’s where we get in trouble.

Hopefully, it helps to understand an amplifier’s efficiency refers to the losses of input power potential incurred while producing its rated power. An amplifier with a rated power of 100 watts always produces its stated output power level. Efficiency merely explains to us the cost of producing that power.

Thus, when a class AB amplifier outputs 100 watts into the speaker, it is drawing from the wall 200 watts. 100 watts are going into the production of heat, 100 watts are delivered to the load.

Tomorrow I’ll confuse the issue more by explaining the mysterious efficiency of a class A amplifier.

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25 comments on “Amplifier efficiency”

  1. An ‘A’ class amplifier’s efficiency is rated by how fast it can cook, to medium-rare, a 2 pound T-bone steak during Beethoven’s 9th Symphony.

    1. Now that provoked a Laugh Out Loud! 🙂

      Sorry for the delayed response, I’ve been catching up on sleep (as far as such a thing is even possible) and otherwise “takin’ care of business.”

      “Freude, schoner Gotterfunken, Tochter aus Elysium, . . .”

  2. The debate of amp efficiency is great and all, and if I had multiple racks full of amps running in an environment where power savings was part of the focus, then my concern could tend to look at more efficient designs, however, in my home environment and for my listening pleasure Class A doesn’t bother me at all. It behooves me to think we spend inordinate amounts of our savings on equipment and have some great concern over a few percentage points of efficiency. If I have to sacrifice my listening pleasure because of more efficient designs, I will choose earlier designs that meet my listening preferences, just as I use an amp from the 80’s for just this very reason. And while it is certainly not real efficient, it’s sound product exceeds much of today’s market choices, and much less impact on the budget, which savings far exceed the question of savings from efficiency.

  3. Just for fun Paul, can you say what the power consumption is in Kilowatt hours of your fully operating IRS V system Vs charging your Tesla vehicle.
    Efficiency of either would be hard to predict I guess.

    Do you have any figures? You lucky guys over there get your power way cheaper than us in the UK.

  4. The problem with heat is that it needs space and you have to get rid of it.

    Solve the heat problem with amplifiers that are 90% or more efficient (or less efficient but good design like Hegel) and you can pack the entire audio system in close proximity in one box, with multiple benefits – no warm-up time, short signal paths, few if any cables, much better EMF/RFI insulation, reduced grounding issues, integrated functionality and modular upgradeability. They are significantly cheaper and take up far less space.

    There really is no limitation to power – the Hegel 590 is one box with 300w/8ohms Class A/B, the Devialet Expert Pro 1000 is 1,000w/6ohms Class A/D. The operating temperature of the latter never gets above 43 degrees C.

    I’m convinced amplifier efficiency is the fundamental issue in modern consumer audio because it facilitates high quality and better value integrated components that people want to buy – and seem to win most of the awards and seem to be taking over the market. As John Darko commented recently, inefficient component systems are as much a lifestyle choice.

    1. I think Darko‘s comment is just an attempt of a story line for those who want to believe that what is convenient and mass adequate, is similar sounding as what’s state of the art and unfortunately the opposite in convenience and efficiency. I think there’s really no one doubting the quality difference except consumers who want to talk their choice into a status of „good enough generally“ while it’s rather just „efficient, convenient and on a certain level which is good enough for them (which is fine)“. And I’m aware that comments like his are what everyone wants to hear about what’s affordable. I just never experienced such arguments to be true. Not regarding cameras, bikes, cars or Hi-Fi.

  5. Wait, if an amp is 50% efficient, wouldn’t it draw 200W and generate 100W of heat and 100W of speaker driving power? If it draws 150W to make 100W of speaker power, isn’t that more like 66% efficient?

    1. Your explanation was crystal clear until the second to the last paragraph. I was scratching my head and wondering whether to comment until I saw Deki’s reply.

    2. I was going to say the same thing but had a minor thing to do at the hospital today so didn’t get to this till 10 hours later. (nothing ‘major’ at hospital)

  6. Is here a reason to even care about it, other than saving the environment (which I can appreciate)? I have owned huge class A amps in the past, and it’s not like the electric bill had a noticeable change.

    Is there some other negative aspect to it being less efficient, like less expected product life?

    1. Besides the numerous negative aspects of inefficient heat-producing amplifiers noted above, I owned the superb Musical Fidelity A1 (solid state Class A, 20w) that had an additional negative aspect, in that there was a fairly good chance it would set your house on fire.

      1. I owned a Threshold T200 (100/ch) for quite some time. It’s the only piece of audio gear that I regret selling. It also got hot enough that you wouldn’t want to keep your hands on it very long.

        1. You guys just do not appreciate the added benefits of inefficient amps. I used to have a pair of 140 W tube mono amps. The darn things drew 400 W at idle just from the heaters on the eight output tubes. They would help keep the room warm. We would listen to a lot more music in the winter than in the summer. 🙂 Today I have this behemoth 750 W per channel ( into 4 Ohms ) SS stereo power amp. Even if I drive the speakers hard enough to make your ears bleed it barely gets warm.

  7. Paul, If I understand you correctly you have just defined amplifier efficiency as ( power drawn from the wall – power to speakers ) / power to speakers. If we say power to speakers is 100 W and power drawn from wall is 150 W then we get ( 150 W – 100 W ) / 100 W = .5 x 100 = 50%. Now if an amp draws 200 W from the wall to send 100 W to the speakers we have ( 200 W – 100 W ) / 100 W = 1 x 100 = 100%. This says that an amp the requires 200 W to deliver 100 W of power to the speakers is twice as efficient as an amp that only requires a 150 W to deliver 100 W to the speakers. This makes no sense. What am I missing here?

    1. So Paul, If instead of what you originally called efficiency ( please see above ) we call the above percent power lost, it all makes sense.

      1. Right and as I have been corrected because of my terrible math skills, whatever we wish to call it, for the example I gave delivering 100 watts to the load requires 200 watts from the wall.

        Efficiency is probably a poor choice of words but that’s the industry standard lexicon.

        1. Hey Paul, even Albert Einstein needed mathematical assistance in solving the Theory of Relativity and afterwards at Princeton. Einstein was a great visualizer of what could be or what might be.

  8. “If an amplifier were 100% efficient it would generate no heat and whatever wattage it pulled from the wall would be delivered to the speakers.

    The typical class A/B power amplifier is about 50% efficient. This means that for a given number of output watts, let’s say 100, the amplifier will produce 100 watts of heat.”

    I’m no mathematician, but believe your 50% efficiency statement above (a .5 ratio) is Correct Paul! 100% efficiency would be a 1 to 1 ratio (power in=power out…doesn’t exist). Maybe I’m in on the conversation too late…were corrections made earlier to your post?!?

    Ahhh, checked my e-mail…corrections Were Made to your on-line post! 😉


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