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1326965[/URL]]The rated output is irrelevant. That’s the maximum output at its rated distortion (which, interestingly enough, is never provided for guitar amps but is a routine spec for audio amps). The input signal level determines the instantaneous output power. Class A and A/B amps with seriously overbuilt power supplies have even more headroom (but at higher distortion levels) above rated output. Most class D amps don’t have this luxury.
Most jazz players never push an amp beyond a few Watts, even on gigs. Speaker sensitivity (which is thankfully a routine spec in our world as well as the audiophile’s) tells the story. Many 12s generate 100 dB from a one Watt input, measuring SPL at one meter in front of them. SPL drops off about 6 dB for every doubling of the distance to a point source. Since speakers are somewhat directional, it’s a bit less depending on the radiation pattern of the speaker in question.
So 100 dB at 3 feet is about 95 at 6’, 90 at 12’, 85 at 24’ etc. The actual level may even be 2 or 3 dB higher, depending on the environment (reflecting, diffracting, and absorbing surfaces etc) and the speaker’s directivity. 85 dB is loud.
Normal conversation in our living rooms is about 60 dB and a home vacuum cleaner puts out about 70 dB at 10 feet. “Bedroom volume” is somewhere between them. So at home playing levels, even a “100 Watt” amp is putting well under a Watt into any speaker, including the least efficient ones (which make 85 dB or even a little less at a meter from a 1 Watt input).
The power level needed to sustain an unmic’ed 90 dB in the far reaches of a venue is another matter entirely. Sound energy is absorbed, reflected back at the source, and otherwise dissipated by everything and everyone in a room. Power level needed to achieve a given SPL also goes up exponentially with distance from the source. And a doubling of output in Watts means no more than a 3 dB increase in SPL at the same listening location. So getting 85 dB at the back of a crowded bar 50’ in greatest dimension could require 200 Watts from a guitar amp without sound reinforcement.
So you can safely and effectively use any amp for home playing. The output level will be the same regardless of its maximum potential.
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