Distortion spectrum. Please again forget the THD+N number, +N comes from the measuring system and not from the amp. Unfortunately THD is a measuring system limit as well. Please note very low level of mains related components, though it is supplied from a conventional bridge+capacitors linear supply.
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I'm not a technical person at all, but having spend several decades looking at distortion measurements in the popular hi-fi press and -- very importantly -- listening to the comments and preference of audiophiles, (most of them subjectivists), I have established my own conjecture and strong believe.
My believe is that most of all it is
the harmonic distortion spectrum determines enthusiasts SQ tastes. Specifically I suspect the following with regards to the spectrum:
- Higher order distortions sound bad, (harsh, grainy, shrill)
- 2nd, and to a lessor extent, 3rd order products sound pleasant, (i.e. more than merely benign), to many listeners. Hence to preference many have for vacuum tube amplification
- 2nd and 3rd order harmonic tend to mask the nasty higher order harmonics in one's equipment, not only in the same component but also up & downstream components. Hence the strategy of putting a tube preamp ahead of a S/S power amp, etc.
- 2nd/3rd order harmonics also tend to mask distortions and excessive brightness in recordings.
(Don't get me wrong: I'm not advocating for components that have high, 2nd/3rd order harmonics. Personally I prefer lowest overall distortion -- at least in case of above average recordings.)
I also really like the idea of measuring the HD spectra at several frequencies along the audible range
versus the standard practice of measuring at this or that single frequency.