The following is a visual guide to amplifier distortion vs amplitude with a schiit Vali 2+ and Ultron 7DJ8 tube as the example. Above is a typical tube or hybrid amp 1 KHz distortion spectrum. This graph will look unusual due to the dual axis delineations. On the y-axis blue is linear in units dBu while yellow is logarithmic in % distortion. Only the log scale is showing. The x-axis also has dual delineations with blue logarithmic in units Hz while yellow is linear in units dBu. This graph has been initialized for a measurement sweep of amplifier % distortion vs level in dBu. Amplifier gain has been adjusted for 0 dB (unity) and load is 300 Ω. In this measurement the signal generator is sending +20 dBu to the amplifier. There are five meters displayed in the graph above. Upper left is a continuous reading of amplifier output level in dBu. The right column consists of four FFT derived readings: 1) THD+N: a measure of the entire spectrum with the fundamental (1 KHz) removed. 2) D2: 2nd harmonic distortion only 3) D3: 3rd harmonic distortion only 4) 4+HD+N: the distortion that remains after fundamental, 2nd and 3rd harmonics has been removed 4+HD+N is what I call the Crap Factor™ (CF). It includes AC mains noise, higher order harmonic distortion, intermodulation distortion, inharmonic distortion, SMPS noise, system noise, etc. This spectrum is from a Vali 2+ with 7DJ8 / PCC88 tube at +20 dBu. At this level a typical dynamic headphone will produce between 120 and 125 dB SPL. Not a very likely use case if the listener wishes to avoid a rapid decline to deafness. THD+N and D2 are almost the same value indicating THD+N is dominated by D2. D3 is considerably lower by a factor of approximately 5x. 4+HD+N is observed to be much lower than THD+N, a considerable disparity. In this measurement CF is dominated by the higher order harmonics. Reducing the signal generator to 0 dBu (Vali 2+ remains set for unity gain) observe a substantial reduction in THD+N, D2, D3. Higher harmonics have nearly disappeared. Also observe how the 60 Hz mains noise remains constant. At this level D3 and CF distortion readings are nearly identical. At this level typical dynamic headphones will produce between 100 and 105 dB SPL. Harmonic distortions in a descending order and a CF of 0.0054% are consistent with a amplifiers that typically sound pleasing if accomplished with a low amount of negative feedback. Performing the sweep from +20 dBu to -20 dBu observe the following 1. THD+N and D2 are nearly identical above -10 dBu 2. Higher harmonics begin to disappear below 0 dBu 3. CF will hit a minimum between +5 and 0 dBu then begin to rise as AC mains noise become dominant in the measurement 4. CF will rise and cross over D2 at -15 dBu Watch 60Hz, and to a lesser degree, 300 and 420 Hz remain relatively constant throughout Pay particular attention to the spectrum from which the sweeps are derived based on the specific measurement parameters. While the sweep graphs are useful, watching the spectrum over the level changes is considerably more informative. The final result of a sweep from +20 dBu to -20 dBu at 5 dBu per step Above -15 dBu THD+N is primarily D2. At -15 dBu, D2 and CF are approximately the same value. Below -15 dBu level 60 Hz AC mains noise dominates both CF and THD+N. A refined measurement sweep from +20 dBu to -20 dBu at 2 dBu per step THD+N sweep alone over-simplifies the underlying complex behavior exhibited by this amplifier and is true for nearly every amplifier. The behavior with a 1 KHz sweep over various amplitudes has been observed. Sweeps at several other select frequencies would be useful to determine the amplifier behavioral consistency. In the above and subsequent measurements bandwidth for 4+HD+N will be altered from 22 Hz ~ 22 KHz (normal) to 400 Hz ~ 22 KHZ (AC mains noise reduction) Vali 2+ noise below 400 Hz is primarily a -90 dBu 60 Hz AC mains noise. By removing it from 4+HD+N result the amount of information reduction that occurs observing THD+N alone will be highlighted. In the graph above THD+N primarily reflects the contribution of D2 above -15 dBu. Below THD+N is comprised of mostly 60 Hz AC mains noise. The modified 4+HD+N now reflects system noise below 0 dBu. Above +2 dBu, 4+HD+N is dominated by higher harmonics. 100 Hz distortion sweep for Vali 2+ with 7DJ8 from +20 to -20 dBu 1000 Hz distortion sweep for Vali 2+ with 7DJ8 from +20 to -20 dBu 5000 Hz distortion sweep for Vali 2+ with 7DJ8 from +20 to -20 dBu This amplifier demonstrates relatively consistent behavior at the various frequencies used to test amplitude vs level. Compare and contrast the foregoing extensive amount of data obtained from measurement sweeps against a single observation such as: 0.46% 1KHz THD+N at 2V. Not only is the level ridiculous as being far above a normal use case, such a simple, single point observation represents a profound reduction of information to evaluate amplifier performance for this attribute.