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But I'm pretty much out of steam on this argument, so maybe we can call it a draw?
Hehe... Cooper, this is only starting to get interesting.
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The impedance of any speaker varies dramatically with frequency, regardless of the power output. It becomes a stronger factor as power levels increase.
I don't know what exactly you meant by "a stronger factor." But as you stated yourself, the speaker impedance curve essentially stays the same regardless of the power output. It does NOT suddenly become more important from amp's perspective above certain power output levels, as I explained in detail above.
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That is a good point. However, I don't believe it's as clear cut as you're indicating. I have to fall back on my argument about transients.
It is indeed as clear cut as I indicated -- there is nothing hidden or mystical here. The hard fact is, if the amp can maintain an essentially flat frequency/phase response at continuous 100W output with a real-world load, then it will maintain the same frequency/phase response through the highly dynamic music playback, AS LONG AS the transient power peaks do not exceed 100W. In other words, the problem of power-supply regulations, dynamic current draw etc. are already covered here when measuring at steady-state 100W output, as a worst-case scenario so to speak.
Now, if the dynamic power demand overtly and frequently exceeds 100W (or whatever maximum available), that is an entirely different story. But I strongly doubt that people (including the "expert" magazine reviewers) routinely put the SS amp into that kind of situation during a typical subjective listening test using typical music materials. In the vast majority of home audio settings, I would predict that your ears start to hurt way before the amp starts to exhibit stresses. As an example, my Adcom power amp (rated 325Wpc into 4 ohms) has nifty instantaneous distortion indicators; I have NEVER managed to turn them on even when listening to highy dynamic music/soundtrack materials at insane (basically intolerable) volume levels in my 5000 cu. ft. room.
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It's oh-so-easy to measure continuous current draw situations, but with dynamics... well, remember the wonderful world of derivatives? If the current goes from 1 amp to 100 amps in .001 seconds, what does the current graph look like? A straight line?
The dynamic behavior of an amp is (unlike that of loudspeakers) in fact quite simple, again as long as you do not swing the amp beyond the maximum continuously available power. All you need to fully describe the amp's transient/dynamic behavior is the (complex) transfer function across the entire audio frequencies, measured at near-maximum power outputs. EVERY transient/dynamic behavior will be covered by this transfer function, including your example if the amp is actually capable of doing it. [incidentally, not many amps can ever do it in the first place, because 100A into 4 ohms and 8 ohms correspond to 40kW and 80kW, respectively.] An important fact here is, the transfer function of an amp can already be fully determined by obtaining simple power/phase response curves over the entire audio frequency range. There are many ways to do this, including the classical frequency sweeps, pink noises, square waves, and more fashionable impulse and step responses, but these are in fact 100% equivalent to each other when measuring amps (again, they can derive totally different results when it comes to loudspeakers, because they produce so much time-domain aberrancies).
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One very well-established, but counterintuitive, observation in high-end equipment design is the effect power supplies have on the sound of a component. I'm not just talking about amps; even preamps and digital sources are affected. The more "overhead" the power supply has, the less constrained the sound. This is one of the reason high end components weigh so much.
I do NOT think that observation is "well-established" outside the audiophile marketing machinery for expensive audio gears.
I fully agree that the more expensive the equipment is, the bigger, more over-spec'ed power supply it tends to boast -- this is important because they have to justify the higher cost in some ways. I, for one, love to look at the beautifully built innards of some of those expensive gears, which alone would hugely boost the pride of ownership for me. But I am NOT aware of any reliable (subjective or objective) reports convincingly demonstrating that these over-spec'ed parts actually produce a better sound quality. Let us not forget that ALL audiophile magazines operate within the realm of the huge marketing machinery (in fact, they ARE perhaps the most important part of the whole marketing mechanisms).
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On a $9000 100wpc Linn Klimax Twin, Stereophile measured a 3dB rolloff at 58kHz. I will quote from the article: "This curtailed ultrasonic response also rounds off the leading edges of a 10kHz squarewave, but the waveform is refreshingly free from overshoot or ringing." The graph indicates this very obviously, and it also has an interesting curve when dropping down to the low part of the signal.
This is an entirely expected, "text-book" example of beautiful squarewave response. Of course, if the amp's high-frequency pole is at around 60kHz, it will NOT perfectly reproduce a 10kHz squarewave; its 7th harmonics is already down significantly. These sort of things only reinforce my assertion above: when it comes to electronics, all these seemingly different measurements are in fact functionally equivalent. Indeed, I am NOT aware of any modern SS amps/receivers that behave badly or unexpectedly on squarewaves (if squarewave overshoots or rings, there must be a peak(s) in the frequency response, somewhere in the treble-ultrasonic region, which I've never seen these days).
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I'll stop here, as I can no longer make a strong assertion about exactly what causes the frequency-specific variation. But most measurements vary greatly with frequency, such as channel separation (by 10s of decibels), noise floor (often by 10s of decibels), and other things.
Yes, these measurements (especially noise floor) can vary wildly across audio frequencies. But, unless the amp is defective, it is a variation from, say, -120dB to -90dB -- it remains essentially inaudible in most modern SS amps.
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Given the fact that plenty of people on Audioreview will comment on the "brightness" of a $400 Yamaha amp or the "warmth" of a Harman Kardon, I am inclined to believe there is some cause.
Now, we are finally back to the "faith" domain. I fully respect your faith on these anecdotal reports. However, my own version of faith here, faced with this same assortment of reports, is that the listening impressions of many of these people might well be already influenced and pre-conditioned by the very widespread notion of the "Yamaha brightness." I simply cannot convince myself with these reports, especially given that I myself do not hear the alleged brightness on the Yamahas, and because I do not see any (measurable) physical foundation for the Yamaha's brightness.
