Watts and power

In a way, I like @millercarbon's answer, despite leaving me with the feeling that there really are no good answers.  So far, that squares with my experience; any time I think I see a correlation between any particular amp parameter and the quality of sound I hear, along comes another amp that breaks the rule.  Having said that, I would still like to find a book that tries to address how power, output current, speaker/crossover characteristics and other factors affect what we hear from a given amp/speaker pairing.  Does anyone know if such an animal exists?

Miller as usual puts a lot of words down that mean exactly nothing. Actually your last post said absolutely nothing. Lots of snark and veiled insults but no actual info.


Tube amps may hit above their weight because they can often be driven into clipping without sounding awful and YES people with low power tube amps do this.


Other than that, looks at watts at the low of your speakers impedance. One amp that does 100 watts and 4 ohms and 200 at 2 ohms is not the same rating as 100 at 4 and 150 at 2 if your speaker dips to 2 ohms. 


Other than that, mainly comes down to headroom provided by the power suppl both the voltage and capacitance and a beefy enough output stage to deliver short term peaks. There is no magic, no ethereal effects ... Just basic low level engineering. Flowery words are meaningless.

There are few variables and they are easy to understand if you take the time to do it. Some people want it to be complex because THEY do not understand it and wish that others do not, so that keeps them on level ground.

The main usefulness of amplifier power ratings, IMO, is that they can make it possible to determine a reasonable approximation of how much volume (i.e., sound pressure level, or SPL) can be produced by a given amp/speaker combination at a given distance. In doing so they can make it possible to **rule out** candidates for purchase that would not be suitable in that respect, relative to the particular listener’s preferences.

Here is a calculator which can be used for that purpose:

https://myhometheater.homestead.com/splcalculator.html

In using that calculator an attempt should be made to find actual measurements of the sensitivity of the particular speakers, rather than published specifications which are often optimistic by a few db. If Stereophile has reviewed the particular speaker the measurements section of the review will provide a good indication of that.

In using that calculator it should also be kept in mind that speaker sensitivities are most often specified based on an input of 2.83 volts, rather than 1 watt. 2.83 volts into 8 ohms corresponds to 1 watt, but 2.83 volts into 4 ohms corresponds to 2 watts. So in the case of a 4 ohm speaker the response to an input of 1 watt will usually be at least 3 db less than the published specification.

Regarding output current specs for amplifiers, such as 60 amps, 100 amps, or other such outlandishly high figures, those are among the most useless of specs. As explained in the following thread what they usually represent is how much current the amp can supply into a dead short (i.e., zero ohms) for an unspecified tiny fraction of a second. And as indicated by Atmasphere in that thread they might even correspond in many cases to the output of the amplifier’s power supply under such conditions, rather than to the output of the amplifier itself:

https://forum.audiogon.com/discussions/current-limit-onset-definition

Atmasphere has also explained in other threads that one reason some very low powered amps may sound more dynamic and powerful than their power ratings would seem to suggest is related to how the distortion they produce varies as a function of output power. In particular, single-ended triode (SET) tube amps, which often have power ratings in the vicinity of 8 watts or even less, can sound much more powerful than that because our hearing mechanisms use certain harmonic distortion components as loudness cues, and the distortion produced by SET amps varies from being vanishingly small at low power levels to being very considerable as their maximum power capability is approached. The result being a subjective perception of greater dynamics and more power than the ratings would suggest.

Generally speaking, though, once candidates that can be determined to be unsuitable matches have been ruled out (such as by determining that the pairing would not be capable of producing acceptably high SPLs) the best way to determine a preference is usually by some combination of listening, trying to determine what kind of amplification the speaker manufacturer prefers, and researching what owners have found to be suitable amplification for the particular speakers.

Regards,
-- Al

P.S. to my previous post: The SPL calculator I linked to assumes box-type dynamic speakers, as opposed to planars (i.e., Maggies or electrostatics) or "line source" speakers. In those cases the calculator will typically underestimate the SPLs that can be produced at a given distance for a given input power, by several db at typical listening distances. SPLs produced by planar or line source speakers fall off less rapidly as listening distance increases than in the case of box-type dynamic speakers.

Regards,
-- Al