My point was, that spike on the THD vs F graph is not really there because the X-Axis is the test frequency, not the actual frequency you would be playing when listening to music.\n\nFor clarity, when you apply a 6.5 kHz signal, you get that spike, but the magnitude of the spike, looks to be about just over 0.002% is the "Total harmonic distortions" so it included all the harmonics in the ultrasonic band that is likely well pass the audio band of 20,000 Hz. ASR uses 45 kHz or higher, but in Hypex's, that did not specific the test bandwidth, and that's part of the reason I can only take an educated guess.\n\nAnother point to note is, that spike was there when tested at 200 W output. There are no such THD+N spikes at 2 and 20 W output level.\n\nIt would have been nice if Hypex published the FFT too, like ASR typically do, then you can see the frequency of the actual distortions. As it is now, you are only seeing the total, you cannot tell what that 0.002% is made up of, and as I said, if I guessed right, it would be made up of harmonics at frequencies much higher than 20,000 Hz so you wouldn't have to worry about the 2nd, and 3rd (12, 18 kHz).\n\n\nPENG, this is terrific information. The distortion figures, from memory, are calculated using the root-sum-squared equations of the numerous harmonics. So you are right, you don't know how many of the inaudible harmonics are used and at which amplitude. Personally, I would have used only the highest 2nd or 3rd harmonic - whichever the higher, at this frequency rage.