Total Harmonic Distortion — THD — is one of the most quoted specifications in amplifier marketing, and one of the least useful in isolation. A low THD figure is routinely presented as evidence of superior sonic performance. The reality is more complicated, and in some cases, an amplifier with measurably higher THD can sound subjectively cleaner and more natural than one with a lower figure.
THD measures the level of harmonic distortion products — multiples of the input frequency — produced by an amplifier relative to the fundamental signal. A THD of 0.001% means that harmonic distortion is 100,000 times below the signal level. This sounds vanishingly small, and in absolute terms it is. But THD is a composite figure that conceals the distribution of those distortion products across harmonic orders.
The auditory system is not equally sensitive to all harmonic distortion. Second and third harmonic distortion — the lowest-order products — are the least audible and, in moderate quantities, the least unpleasant. Higher-order harmonics — fifth, seventh, ninth — are far more audible and more frequently described as harsh, grainy, or fatiguing. An amplifier with a THD of 0.5% composed almost entirely of second harmonic content may sound more natural than one with a THD of 0.01% composed of high-order odd harmonics.
This is why some tube amplifiers, which measure relatively poorly by THD standards but produce predominantly low-order even harmonic distortion, are found to sound smooth and musical by experienced listeners. Solid-state amplifiers using heavy negative feedback can achieve extremely low THD figures but may shift the distortion spectrum toward higher-order components in ways the headline figure does not reveal.
Always ask not just how much distortion an amplifier produces, but what kind. The spectrum matters more than the total.