On distortion by Bob C
Posted 25th March 2011 at 08:30 PM by klewis
I don't find it hard to believe that very low levels of certain kinds of distortion are detectable when at the same time virtually un-measurable differences between passive elements like cables and capacitors are widely believed to be audible.
THD-20 does not tell the whole story on amplifier behavior, but it has also been given a bad rap because single-number usage of THD-20 can obscure the real story. If you have THD-20 with a full-spectral analysis out to the 7th or 9th harmonic, it tells a lot more, but most THD analyzers start cutting off at 80 kHz. One should always take note of the waveshape of the THD-20 distortion residual as well. If it is rather sinusoidal, that is often not too bad. If it is spikey, that is potentially very bad.
CCIF 19+20KHz with full spectral analysis is much better because the IM products lie in-band.
At the same time, moderate levels of benign THD, like mostly 2nd and 3rd, may not bother anyone very much. This is often the case with tube amplifiers.
Finally, amplifiers often behave differently in the real world than on a test bench driving a sinewave into a resistive load. For example, does the amplifier clip cleanly and/or softly? Is the amplifier able to supply high currents into difficult loads? Does the amplifier's frequency response change when driving a load whose impedance varies with frequency? Does the amplifier burst into brief parasitic oscillations on signal peaks when driving certain kinds of speaker cables? All of these and more real-world behavior differences can make amplifiers sound different completely independent of THD measurements. This doesn't mean that we throw out THD measurements, but it does mean that they must be evaluated carefully in context and understood that their difference may not be responsible for the difference being heard.
Very low THD-20 is neither necessary nor sufficient for extremely good sound, but that does not mean that THD-20 and similar distortion measurements are not valuable. It also does not mean that the pursuit of low distortion, when done properly, is without value. The act of properly achieving very low distortion via very linearly-designed circuits, and the attendant necessary attention to detail, will often cause the designer to do things that raise the overall quality of the whole circuit in ways that may not just show up on a THD test.
If you have an amplifier producing 0.1% THD-20, there is a lot of room for there to be nasty stuff in there like crossover distortion. On the other hand, it could all be benign second harmonic. But if you have an amplifier that produces only 0.001% THD-20 under all conditions, with 5th harmonic on up being under 0.0001%, then there is very little room for there to be audible levels of nasty crossover distortion and the like in there.
Cheers,
Bob
THD-20 does not tell the whole story on amplifier behavior, but it has also been given a bad rap because single-number usage of THD-20 can obscure the real story. If you have THD-20 with a full-spectral analysis out to the 7th or 9th harmonic, it tells a lot more, but most THD analyzers start cutting off at 80 kHz. One should always take note of the waveshape of the THD-20 distortion residual as well. If it is rather sinusoidal, that is often not too bad. If it is spikey, that is potentially very bad.
CCIF 19+20KHz with full spectral analysis is much better because the IM products lie in-band.
At the same time, moderate levels of benign THD, like mostly 2nd and 3rd, may not bother anyone very much. This is often the case with tube amplifiers.
Finally, amplifiers often behave differently in the real world than on a test bench driving a sinewave into a resistive load. For example, does the amplifier clip cleanly and/or softly? Is the amplifier able to supply high currents into difficult loads? Does the amplifier's frequency response change when driving a load whose impedance varies with frequency? Does the amplifier burst into brief parasitic oscillations on signal peaks when driving certain kinds of speaker cables? All of these and more real-world behavior differences can make amplifiers sound different completely independent of THD measurements. This doesn't mean that we throw out THD measurements, but it does mean that they must be evaluated carefully in context and understood that their difference may not be responsible for the difference being heard.
Very low THD-20 is neither necessary nor sufficient for extremely good sound, but that does not mean that THD-20 and similar distortion measurements are not valuable. It also does not mean that the pursuit of low distortion, when done properly, is without value. The act of properly achieving very low distortion via very linearly-designed circuits, and the attendant necessary attention to detail, will often cause the designer to do things that raise the overall quality of the whole circuit in ways that may not just show up on a THD test.
If you have an amplifier producing 0.1% THD-20, there is a lot of room for there to be nasty stuff in there like crossover distortion. On the other hand, it could all be benign second harmonic. But if you have an amplifier that produces only 0.001% THD-20 under all conditions, with 5th harmonic on up being under 0.0001%, then there is very little room for there to be audible levels of nasty crossover distortion and the like in there.
Cheers,
Bob
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