Sadly, there's a cohort who feel its their duty to religiously prosecute the narrative that 20-20kHz is all that matters. Because a deeply vested interest wrote a white paper or they read something a guru wrote and took it to heart as gospel.
But, if you or I had created Class D amplifiers like Hypex or Purifi, we'd attempt to paint them in the best light possible- I've said that before. But I wouldn't stoop to blatant misrepresentation and neither would you. Pure marketing wins and it seems, some people are falling for it. Not me.
I have power amplifiers where the HF distortion is precisely trimmed to a minimum level during setup. A 20kHz signal is injected and the harmonics (up to 200kHz) are monitored and the HF THD trim is tweaked to a minimum. That is so they
produce their rated (0.003%) THD across the entire specified bandwidth and meet their advertised specifications in accordance with regulations. Try doing that with a 20kHz measurement bandwidth.
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Another, a 1982 Perreaux PMF-2150B 200wpc@8R/400wpc@4R power amplifier has a rated THD at 20kHz and 200 watts RMS continuous of 0.009% or lower. The HF THD adjustment must be made at full power and at 20kHz, monitoring harmonics. Try that with a Class D. The last one I adjusted, was 0.004% at 20kHz at 200w. A difficult trim, but they took their responsibilities to adhere to specifications seriously.
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Hypex/Purifi et al, just throw all that out the window and draw a line at 20kHz and say "nothing to see here- move along". Their THD, power and efficiency numbers have been, continue to be and likely will remain, cherry picked. That is until some authority pulls them up and requires honest disclosure.
But we have more in common than not. Everyone here on ASR is interested in fidelity, music and HiFi gear. Some people come at it a different way than others and place more emphasis on standards and disclosure- others just want to press a button and listen to music. Everyone is different.