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I have the following hypothetical question. Suppose I have a subwoofer rated for 250watts RMS power handling. I have a choice of powering it with a 250watt RMS amplifier or a 500watt RMS amplifier. Assume that both amplifiers have 80% efficiency at their maximum RMS output. I have heard that amplifiers are less efficient below their maximum RMS wattage capability. Does this mean that the 250watt amplifier will run more efficiently compared to the 500watt amplifier? (assume they were calibrated for the same SPL output)
 

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I don't think you would (with a class AB) see better efficiency until right near max output, so if you played the 250w all the time near clipping it may be better than the 500w. It would likely take a little less power, and since the 500 has more power it could be more dynamic where the 250 starts to clip and that would take more power. In the end I doubt you would see that much difference if you compared them on a meter making exactly the same output, the 500 would certainly take more at the higher outputs it can provide.

If you are worried then get a hybrid or class D, they are far more efficient at partial outputs. Significantly better than class AB.

Don't forget a hybrid is basically a class AB output amp with a variable power supply. When a class AB output has less difference between the rail power and output (meaning it is at max output) then it is more efficient, so the power supply limits power at lower output to make this happen much more of the time. Class D is pulse modulated, but the full range are getting quite good now.
 

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Remember in a car people don't care that much about partial power efficiency because the amp is taking less power for less output anyway; the output level has a much larger affect on power use. Since dB is a log function compared to power used, that means the amp will not use a lot of power until 90% of max output for example. 75% output uses far less power. Max power will only be needed right at clipping (the max the amp could provide), and power required drops off quickly as less output is needed. So even a less efficient amp will not use much power for say 0-50% output.

Efficiency only measures the loss power in to power out and does not account for the dB to power use ratio, only for changes in efficiency at different power levels.

This is important to people running limited power such as boat systems, getto boxes, tailgate setups, etc. A class D will run longer at partial output with its higher efficiency compared to a standard class AB, before it depletes the power source. Also becomes important when your sub amp kills your headlights or alternator, of course. Even though efficiency is better right at max output for a class AB, it is not as good as class D.
 
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