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RE: You missed the point...

"While it's probably reasonably true today that the correlation between good meas performance and good sound is mildly positive, IME"

Earl Geddes actually found out that it is a slightly negative correlation but of course the R^2 is rather poor...so probably no correlation is more correct.

The only reason there is no correlation is that THD is nearly meaningless and so is IMD. The specific harmonic pattern IS significantly more important (See Gedlee metric or Cheever metric). This is the point that Ralph is trying to make. The metrics ALL indicate that low order harmonics even in relatively high % are benign and the higher order harmonics need to fall off exponentially to essentially zero. This monotonic PATTERN is more important than having very low, but evenly distributed harmonics that nearly all push/pull SS amps with negative feedback produce. This pattern is wrong to the ear/brain...if it wasn't then you would not have the poor correlation with THD and IMD that clearly exists.

Now, you claimed that there are some very good measuring amps that also sound very good. Care to name them? I have yet to hear one and I have heard nearly all of the top praised über SS amps, most of which measure tops. Ironically, the best sounding SS amps are the ones that measure somewhat poorly by the usual industry benchmarks (Pass, darTZeel, Vitus etc.). Please don't tell me that Soulution sounds good...I am not sure which psychadelic coolaid the reviewers are taking when they praise this brand...it just isn't on I tell you. Sure the latest and greatest SS beasts don't sound hard or etched like a poor SS amp from the past but they also don't sound really natural and still electronic or "hifi".

Based on psychoacoustics, NO push/pull amp, whether Tube or SS, actually qualifies as correct from an inherent design flaw...the cancellation of even order harmonics. That is usually further exaccerbated by the addition of negative feedback and Class A/B that further reduces low order even and odd harmonics and gives rise to an infinite series of high order harmonics. The maths are clear as are the measurements. With all due respect to Ralph, his OTLs are wrong as well but perhaps less wrong than most.

Nelson Pass also realizes this and his white paper on distortion is a very interesting read.

Given we have no truly linear amplification devices at our disposal (if we did then these discussions wouldn't exist) the best we can do is to "hide" the distortion behind our perceptive blind spots. SET done right gets the cloeset to mimicing our ear/brain pattern and puts those distortions in our blind spot...mostly. The only real downside to SET compared to other designs is power and cost.

For sure a short cut in SET execution will result in the bloated soft sound that many accuse the design for but it is the execution. A good output transformer that won't saturate within the power bandwidth is neither easy nor cheap to make. Making sure the driver doesn't distort before the output stage takes a sharp designer as well. Also, load line setting and parts matter because there is no negative feedback to squash it all out.


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  • RE: You missed the point... - morricab 09:09:32 11/13/15 (0)

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