Distortion Surface: old measurement, new approach The following metric is typically included for amplifier manufacturers’ specifications: THD+N < 0.05% The above specification is incomplete, with operating conditions left out. 2Vrms 300 Ω load 1 KHz THD+N < 0.05% This is a specification that allows replication of the measurement conditions exactly. It is my long held opinion THD+N at a single operating point is nearly useless for assessing audio sound quality. Please see the following posts for reference: https://www.superbestaudiofriends.o...-of-6-amps-thd-n-and-thd-specifications.6194/ https://www.superbestaudiofriends.o...thd-and-thd-n-specifications-and-graphs.5074/ https://www.superbestaudiofriends.o...distortion-vs-amplitude-a-visual-guide.12279/ However, sweeps of distortion over operating level and over frequency might provide better insight into differentiating amplifier distortion behavior. This is the very first measurement visualization of this type Using an iFi Audio iDSD Diablo in ECO mode with a 300 Ω load. In the above measurement the red dot represents THD+N at 1 KHz, 2Vrms which measures 0.0016% or -95.9 dB. Not only is it above a reasonable operating level for most listeners and headphones, it is a ridiculous over simplification about the amplifier distortion performance. An enormous increase in information is presented by the surface which sweeps THD+N over Level dBu and Freq Hz. Observe increase in distortion as level drops. Typical operating levels are -20 to -10 dBu which for many headphones will produce listening levels of 80 to 90 dB SPL. Consider a very well regarded amplifier, ECP Audio DSHA-3F (original) With the same 300 Ω load this measurement reveals a drastically different distortion surface. Let that sink in. Be mindful of scale changes for the graphs, especially in the Distortion dB axis, for best visualizations of nuances. Rotating the view for nuanced view of the distortion surface This is only the beginning. Remember THD+N is also an information reduction. Decomposing THD+N into the following: D2 (second harmonic) D3 (third harmonic) 4+HD+N (everything else) will provide additional insight into the amplifier distortion performance. D2 and D3 distortion surfaces vary considerably from THD+N and from each other. You would have never known without such measurement detail. Future research might decompose 4+HD+N into 4+HD and N as separate surfaces which would increase the level of information. Many amplifiers are harmonic distortion dominated at higher operating levels while noise dominated at lower levels. Knowing the level of crossover and how much of each are likely to provide further insights.