Heat/Efficency of Speakers

Can you explain how that's the case? As I understand it now amplifier efficency only covers wall -> speaker terminal efficency. The wattage output to the speaker will mostly all be turned to heat. Where I stand Class A appears the answer only because there are 2-25W amplifiers all over the market where the alternatives all have higher output.

The heat gain in your room will be affected by the amplifier since nearly 100% of power consumed by the speaker is converted to heat.

If you have two amplifiers, one being a 100 wpc Class A and the other a 100 wpc Class AB, both will pass through the same power to the same speakers. However, the Class A amplifier will consume about 1,000 watts of power at idle and at rated power and that will be converted into heat as long as the power is on. The Class AB amplifier will consume about 175 watts at idle and 400 watts at rated power. The heat transferred to your room will vary from 175 watts to 400 watts. Therefore the room heat gain will be a lot lower with the AB amplifier since the heat gain from the speakers will be the same with both amps (listening SPL and duration being equal).

Air condition your room, or go to class D power amp!

Buy a horn speaker.

...does the power usage scale with the volume and demands of the speaker?

Yes, absolutely, in the case of class AB, and even more so in the case of class D. For class A, though, the AC power drawn from the wall outlet will be determined by the amplifier and will be essentially the same all the time, with the power that may be supplied to the speakers at any instant subtracting from the power that is dissipated (converted to heat) in the amp. (But as stated above almost all of the power supplied to the speakers will then be converted to heat in the speakers).

Also, keep in mind that most of the time under most circumstances just a small fraction of the amp’s power capability will be utilized. Brief dynamic peaks in the music will typically result in much more power being delivered to the speakers than the time-averaged amount of power that is delivered. Although different recordings can differ greatly in that regard. Recordings that are dynamically compressed to a severe degree (which is particularly common among pop and rock recordings) may have less than a 10:1 peak to average ratio of the amount of power delivered to the speakers, while there are some classical symphonic recordings having minimal or no compression for which that ratio may be more than 1,000:1. In the latter case very little power will be supplied to the speakers, on average, and hence a class AB or class D amp will not generate much more heat than when idling.

Regards,
-- Al

To make this a bit simpler, Class A amps will typically generate the most heat because they draw full power all of the time. Class AB amps will generate less heat but the amount will vary depending on whether they are heavily biased into Class A (more heat) or less.  Class D amps generate the least amount of heat.  The situation where speaker efficiency will make a significant difference is if you go with very high efficiency speakers, because you can then use a very low power tube amp. 
















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