One single number says nothing, regardless it is expressed in % or dB (any technically competent person is able to transfer % to dB and vice versa).
I cannot imagine a competent class AB amplifier with high crossover distortion products at 10mW power. So I would respectfully request for
@John_Siau to post a proof in a form of a distortion spectrum graph of the ill behaving amplifier. It is the only way to lead a scientific discussion based on facts and not on the stories based on one’s authority, especially if there is a business interest.
Here are the plots for the offending amplifier (shown in green on all graphs). The second amplifier used in the ABX test was the AHB2 (shown in red on all graphs). These tests plots were made in 2016. The offending amplifier was an RA-500 "studio reference" amplifier. We had two samples that were both purchased new at the time of the test. Both behaved identically. This amplifier was passively cooled and was designed to be rack mounted. I suspect the amplifiers may have been under biased because of thermal issues related to the passive cooling and the need to run in a studio equipment rack. It is also possible that there was some problem at the factory and these units got shipped with incorrect bias settings. I searched our facility and we no longer have these two samples. In any even, the RA-500 is no longer available.
The first plot is an FFT of the 1 kHz, 1Watt output spectrum of both amplifiers immediately after a cold startup.
The vertical axis (dBrA) is dB relative to 1 W.
From this FFT, you can see that the offending amplifier was working properly when stone cold. It did display a fair amount of power supply noise, but probably low enough to be inaudible with the 87 dB speakers used in the test. The 120 Hz and 180 Hz power supply noise may have been slightly above the threshold of hearing, but I specifically do not recall hearing AC line noise.
Also please note that H3 and H5 are too low to reach 0 dB SPL with 87 dB speakers.
Additional notes:
The 1kHz fundamental was removed with the AP2722 notch filter to improve the resolution of the FFT. This was a technique that we had to use with the older Audio Precision test stations. As many of you may know, the APx555 uses this technique internally, but then splices the 1 kHz fundamental back in.
I am trying to locate the FFT that shows the crossover distortion after the amplifier was warm but not overheated. These measurements were made in 2016 and we have since upgraded to APx555b test stations, so these plots may be lost. I will post them if we locate them.
I do recall that the perfect scores on the ABX tests were achieved by listening for harmonics of the 1 kHz tone.
The THD+N vs. Power curves always displayed the effects of crossover distortion. The minimum THD+N of the offending amplifier (red curve) is -74 dBW (at 0.01W), and this is what was measured and documented when the ABX test was run.
Please note that this "malfunctioning" amplifier still met the manufacturer's specifications for THD+N.
Here is the THD vs. Output Power sweep. The THD+N level is -72 dBW which is just 2 dB lower than the THD+N test above. This confirms that the THD+N reading was mostly distortion. Note that the -72 dBW THD would have produced about 15 dB SPL at the listening position. This agrees with our written notes regarding the ABX test. The harmonics caused by the biasing problem were high enough to be audible.
So what we have here is a very specific case of one brand and model of class AB amplifier that easily drifted into an under biased condition. Once it warmed up and entered this under biased condition, THD could be heard on a 1 kHz tone at very low power levels (0.01 Watt) and a listening level of 67 dB SPL (amplitude of 1 kHz tone). The sum total of the harmonics would have reached about 15 dB SPL at the listening location.
Conclusions:
Harmonic distortion at 15 dB SPL was heard when playing a 67 dB SPL 1 kHz tone. This was confirmed with perfect ABX scores from multiple listeners. The high scores indicate that the distortion was easy to hear when using a pure 1 kHz test tone.
The distortion was caused by crossover distortion in the offending amplifier (see green curves). We had two RA-500 amplifier samples and both had the same problem on they each had it on both channels. They were purchased at the same time, so it is hard to say if it was a production problem or a design problem.
The audibility of the distortion was only confirmed on a 1 kHz test tone. This may imply that it could be heard on some music samples, but the answer to this question is well outside of the scope of this simple test. A much more involved test would be required to determine audibility when playing music.
This crossover distortion defect was present in both channels of two band new RA-500 power amplifiers. This problem could have been solved with thermally stable bias circuits, or by proper factory adjustments, or a combination of both. Clearly these two test samples were not thermally stable. For those of us who design power amplifiers, it seems incredible that these amplifiers shipped with this biasing problem. It is hard to know if this is unusual, especially in lower cost class AB power amplifiers. The RA-500 was a low-cost "studio reference" class AB power amplifier.
This is why we need people like Amir and John Atkinson, who have provided us with a large database of tests.
Thanks to everyone who has made comments regarding this ABX test. I appreciate the need to keep the conclusions specific to the test that was conducted.