The distortion produced by your audio amplifier may be louder than the noise produced by a major appliance!
John Siau took Stereophile's THD+N measurements for 7 of their top-rated power amplifiers and replotted the data in terms of sound pressure level at the listening position. Once percent is converted to dB relative to our threshold of hearing, the test results become meaningful, and somewhat shocking.
John shows exactly how much noise each amplifier produces, and he shows exactly how loud the distortion is at every playback level. All of this information is hidden within traditional THD+N vs. power plots. The meaning is obscured by the traditional percent and power units. These traditional units have no direct relationship to the real world. John converts these units and the data comes alive!
John shows that the noise and distortion is well above the threshold of normal hearing, even when selecting some of the very best audio amplifiers. One of the seven amplifiers is radically different. It is the only amplifier out of this elite group that produces absolutely no audible noise or distortion.
6 comments
T.N.:
A 3 kHz tone can be detected by the human ear when the tone is up to 30 dB lower than a white noise masking signal, provided the tone is above 0 dBA. The measured level of the noise in a room does not directly determine the quietest sound that can be heard. Our ears have the ability to hear tones that are well below the level of the ambient noise. I frequently demonstrate this to visitors at Benchmark. On the other hand, a white noise signal is quite effective at masking a lower-level white noise signal. RMS summation of the noise signals will show that if one noise signal is 6 dB lower than the other, the overall noise will increase by 1 dB and this is about the lower limit of what can be detected. When there is a greater difference between the levels of the two noise signals, the lower level noise signal is fully masked.
When you call the flat line distortion that we typically see below one watt (or so…) you call that noise… but noise is production of energy without any input stimuli while distortion is an unwanted energy that requires an input signal to be produced/measured. Here is a questionable aspect of your discussion – I would bet no one hears any “noise” from their listening position which you imply is a problem as it is “8 dB SPL”- also, given that even e very quite room is still 30dB SPL this distortion (noise?) is drowned in background noise. The questionable in your video is to claim that noise is at 8 dB SPL at listening position and that that is a real experienced figure and a problem. The corresponding % distortion and the dB SPL calculation is correct but – distortion is not noise… I think you should have gone all the way and also not only converted watts to voltage b ut to dB – that would have been even more descriptive – perhaps together with how dB SPL add..
Thank you John for this clear & concise video explanation & comparison. Stereophile & similar would do their readers a great service by adopting your method.
Very nice video, John. Honestly, none of my amps produce audible noise at “input off” conditions (92dB sensitivity speakers).
Isn’t is possible the lowest levels of thd+n (the dishwasher levels) are mostly distortion, such that the spectral content is highly related to the musical signal?I know your logic about audibility still applies, but I am wondering what you have found in measurements, since you consistently called those lower level signals “noise” in the video.
Sharon, I completely agree and I tried to point that out several times in the video. What I said was: “of all the electronic components in a typical audio system, the one
component that’s responsible for more distortion in most cases is the power amplifier.”
The speakers will make more distortion than the electronics, in most systems, but we have the ability to eliminate the electronic side of the equation.
Speaker performance should never be used as an excuse for making poor-performing electronic components. Too often, a manufacturer will say “we are better than the loudspeakers so we are good enough”. Historically this has been especially true when loudspeaker manufacturers added amplifiers to their speakers. We are starting to see better amplifiers being added to speakers, but loudspeaker manufacturers are still more focused on the drivers (as they should be).
- JS