If it's at all frequencies and levels up to clipping, with the load you present, then of no concern.
S
Correct, and consistent with my experience.
My hearing ain't what it once was. But when I listen to ABX comparisons of distortion levels with real music, I run out of the ability to detect a difference around -36 dB of harmonic distortion. That means an amp with 1% THD (-40 dB) is as effective for me as an amp with a THD of -120 dB. I'm sure others have hearing that is better than mine, but it's not a matter of experience--I have as much experience listening critically as most audiophiles. Maybe more.
But it certainly is a matter of training. There are those who have received special training in techniques they can use to isolate the effects of certain kinds of distortion at very low levels, but to hear them they have to use special listening techniques, such as cranking up the amplification dramatically during the reverberation tail and in the quiescent signal level that follows leading into a silent bit. I have not received that training and am just working from memory of what other have described, but I can hear distortions in the gaps at extreme amplification. (Don't try this at home. When the gap is over and the music returns, damage will result at the levels I'm talking about.)
And harmonic distortion is less noticeable than enharmonic distortion of various types. Some noise sounds like performance space ambience in a recording, and the only noise that makes any audible sense to me is quiescent hiss. Some amps produce more than others, and with efficient speakers that his can be audible in some listening environments.
Distortion early in the amplification chain is a bigger problem than later. The typical voltage gain of a power amplifier is in the upper 20's of dB, and for a preamplifier is in the range of maybe 10 dB. Together, a line level source might be subject to as much as 40 dB of voltage gain. Noise early in the chain gets amplified, but then so does the music, so it's still best to compare the noise to the maximum signal. If a source produces noise at -100 dB while the maximum signal is at 0 dB, then it will be difficult to amplify it enough for the noise to be audible when the peaks are tolerable. But a line level source at 1% distortion (-40 dB) might become audible when the 0-dB peaks are amplified to barely tolerable levels of, say, 110 dB SPL. In that case, the distortion products will be at 70 dB SPL, which is quite audible. But will we notice it when our ears are being assaulted by 110-dB music signal?
Summarizing: The ability for music lovers to hear distortion in real-world listening situations is great exaggerated, and those exaggerations are supported by much arm-waving and usually no actual measurement or testing at all. Thus, the person who is worried about that 90's amp with its 95 dB S/N would put their own mind at ease if they actually compared them with enough controls to avoid accidentally deluding themselves. Such comparisons have been made and the findings are clear.
But the distortion of an amp that is clipping can be quite audible, depending on how the amp handles clipping. I recall an article from quite a lot time ago that analyzed a number of magazine-held listening comparisons, and determined that in those tests, the amps were driven to clipping about 1% of the time. Meaning: The smaller amps of the past, when used with inefficient speakers that were and continue to be popular, clip more often than we might realize if we enjoy listening to music at levels that "seems like" a live listening experience. (An easier target for acoustic music than amplified music, of course.) When I started out in this hobby, amps that could do that were rare and expensive, because power at that level was not that easy to do. That is no longer the case. Even so, I do see the clipping indicators flashing on my amp that is capable of a solid 300 wpc into 8 ohms at the point where distortion first begins to rise noticeably. But that happens only playing very dynamically recorded percussion equipment
very loud.
But that is clearly not a problem with those Classe monoblocks, which are about twice as powerful as my amp. Those are rated at -60 dB THD (0.1%) at rated power, with noise not specified beyond the signal/noise ration of 100 dB.
Rick "on the assumption that we are not being trolled" Denney
