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Re: I'm just not sure about one thing

Let's clear a couple of misstatements up right now. First of all, it is not the alleged roller coaster impedance curve that causes the audible frequency response variations, but it is in great part the of the interaction of the output impedance of the amp with the impedance curve of the speaker.

Second, I have never maintained the FR variations in speakers are insignificant, and I have no idea why you would think so. It is certainly something you have made up.

Third, as to the variations caused by the interaction of the impedances of the amp/cable and speakers, I make two basic points about that. First of all, that the FR variations of amps with a high output impedance into many speakers loads is likely to be audible, sometimes quite audible, whereas the FR of low output impedance amps varies very little into most speaker loads as compared to driving a resistor. Of course, as you pointed out, an amp with a high output impedance driving a speaker with a pretty flat impedance vs. frequency curve would change its FR much--but then, it would sound pretty much like an amp with a low output impedance!! That's hardly much of an argument for favoring tubes . . . but if you prefer tube amps, that's fine with me. I'm not trying to convert you, but appear to find my preference for solid state threatening.

A second point is that I generally audition speakers with good solid state amps (which mostly have a low output impedance). This removes a variable which is likely to be significant from the audition.

If I thought some FR shaping would improve my speakers, I would get an EQ (1/3 octave or perhaps parametric) rather than trying to try out different tube amps in the hope that something would improve. But, then I make every effort to get speakers I really like and I have succeeded in doing so over the past decades.

I'll have to suggest the same thing to you as I do to RGA: if speaker measurements don't mean anything to you, then don't bother with them.

Now, I can only tell you what I know, and there are people who know a lot more than I do about this. If you really want to find some answers to your questions about what is significant from technical people, why don't you look at Dr. Toole's White Papers? I think this is the most relevant one:

http://www.harman.com/wp/pdf/Loudspeakers&RoomsPt2.pdf

JA, who took the measurements of the Paradigm Signature S2 for Stereophile, commented that "the balance overall is impressively flat," and it is. In JA's measurements, there is no such thing as a "+8 / -13 db response" in his curves above the bass. The only thing about +8 I can see is tweeter ringing above the audible range at maybe 27 kHz (many speakers have this, even some 'stats, I think), and on axis there is a very narrow interference dip (I should guess)around 12 kHz, but not off axis. JA pointed this out, BTW, but perhaps you missed it.

Now, though I find JA's dispersion measurements to be very revealing, I think the NRC measurements are more accurate, and even JA likes to compare his results to them when a manufacturer such as PSB provides them. And the NRC measurements as shown on the Soundstage site, linked below, are in many respects extraordinarily good for a speaker, such as the Listening Window curve in Chart 2. While you might prefer the response of the S2 with a tube amp, then again you might not. In any case, if I want a more downward sloping response with some recordings, there is the Tilt control on my Quad preamp. Much handier than changing amps!
____________________________________________________________
"Nature loves to hide."
---Heraclitus of Ephesus (trans. Wheelwright)

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