One way to look at it is 0 dB SPL is inaudible by definition. A reasonably quiet room for example has 40 dB of ambient noise but this sound is not generally in the frequencies that humans are most sensitive. It is lower frequency. I have read that humans can detect sound 30 dB below the noise floor. So a very conservative estimate 10 dB of distortion. An amplifier that produces .03% distortion has distortion attenuation of -70 dB. Driving an 87 dB efficient speaker at .03% THD has 17 dB of distortion. Theoretically, this could be audible depending on character and frequency.
Most amplifier specifications are at near maximum power where they have the highest S/N. The performance at 1 watt and below may be better but is often worse.
Amplifier THD specifications do not include transient amplitude errors and do not include phase. I am stating that these are issue, they may not be. However, we are clearly not measuring everything that matters.
That said, in my experience well built Class A/B amplifiers operating within their range sound similarly good. Some however, some have more transformer hum and hiss. This is not always constant as well. Those with very efficient horn speakers find such amps unacceptable. I found transformer hum unacceptable.
I understand the thinking of many on this and other forums, essentially stop me before I kill again (meaning spend useless money). I try some gear and endeavor only to spend useful money within my budget. I will relate my impressions and reasoning. Since, I remain a student, I hope to continue to learn.
- Rich
