Natural sounds (not recorded) contain many harmonics, and primarily even ordered (second, fourth, sixth, etc.). It is the combination of harmonics that allows us to distinguish a trombone from a french horn when they are both playing the same note. Amplifiers produce an artifact called “harmonic distortion”, which means that they create harmonics where they don’t exist, or exaggerate harmonics that are already there. Even ordered harmonics (second, fourth, sixth, etc.) are pleasing (consonant) to the ear, while odd ordered harmonics (third, fifth, seventh, etc.) are irritating (dissonant). Think of it in terms of the sound of a barber shop quartet. If the three harmony parts are all singing in tune with each other and the soloist, all is well as far as the listener is concerned, no matter how loud the harmony parts are singing. This is consonance. However, if just one of the harmony parts sings out of tune, even quietly, the listener finds this irritating. This is dissonance.
Tube amplifiers, particularly the single ended type, produce even ordered harmonic distortion, primarily second order. Push pull tube amplifiers, particularly solid state amplifiers, tend to produce odd ordered harmonic distortion. If you are purchasing a single ended triode tube amplifier, 1% total harmonic distortion (THD), which means 1% of the sound is harmonically distorted, is actually pleasing to the ear, because it is even ordered. However, with a solid state amplifier, THD as low as 0.5% can be irritating, because it is odd ordered distortion. Therefore, when purchasing a solid state amplifier, make sure that the THD is no more than 0.5%. All high quality solid state amplifiers meet this specification, but you should note the fine print as to the THD in the rear channel of surround sound integrated amplifiers for the distortion factor. Often, it is larger than 0.5%. On the other hand, the bottom line in all cases, is whether the sound pleases you or not. “Specsmanship” as it is called (emphasizing the specifications on the technical data sheet supplied with a component) should not be the deciding factor. Decide with your ears, but ask to see the technical data sheet before you write the check.
Negative feedback is the process of taking a portion of the output, electrically inverting it, and then feeding it back to the input. The purpose of negative feedback is to flatten the frequency response, reduce harmonic distortion, lower the output impedance, and also to reduce the effects of “parasitic oscillation” that can occur when parts of the circuitry cause an induced current to flow in other circuits where it is not wanted. However, too much negative feedback can sometimes be used to produce an artificially low harmonic distortion specification. Second order harmonics will be reduced, but fifth order harmonics will increase, and this odd ordered distortion is much more noticeable than the original second order distortion. A tell tale sign of too much negative feedback is an excess of sibilance in the human voice (the “s” is exaggerated).