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Modern amplifier differences

I read an article recently that mentioned an equation you helped develop regarding the correlation of subjective tests with THD. There was also an older attempt at making a better correlation by a guy named Shorter. As his equation was much simpler to use than yours (one did not need to know the transfer function of the amp) I have applied this to 1 Khz distortion spectra that are published on the Soundstage.com archives. What I found was rather interesting. There was a vast difference between most solid state amps and tube amps when the THD was weighted using Shorter's equation (basically the order of the harmonic squared/4 then times the amplitude of that nth harmonic).

For sure the SS amps still had a lower overall total distortion...even with the weighting factor. But what was most interesting was when I looked at what percentage of that total was to be found in the harmonics greater than the 5th. In this regard, the SS amps had much more of their weighted THD in the 6th to 20th harmonic (the limit of the measurements used). There was a large disparity between the SS amps and most PP tube amps and then a gap again between them and SET amps (or hybrids and PP tube amps with no negative feedback). I can tell you which amps I find to sound the best (I have heard about half of the amps that I did this calculation on) and they are all ones that scored a lower higher harmonic:total ratio.

Will one be able to tell the difference blind between various well made SS amps? Perhaps not. Will they be able to tell the difference between an amp that has 80% of its weighted harmonics in the higher order range (> 5th) and one that has only 5% of its total weighted harmonics greater than the 5th harmonic? I should hope so or else there is no need to correlate subjective hearing and measurements at all because they all sound the same. My guess is the difference will not be subtle and could be reliably detected under blind or DBT conditions.

If you would like I can email you a copy of the spreadsheet I have compiled. If it is possible to apply your GedLee equation to an FFT spectrum of a 1Khz sine wave let me know how and I will use the same data with your equation.

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