Most published noise specs are useless as they don't give a proper reference. When they do it's often to full output, which still requires some recalculation if we use the amp less than full output. For instance, if an amp's noise is -80 dB re. full output of 100 watts it is only -60 dB at one watt and -40 dB at 0.1 watt where folks with quietest rooms, sensitive speakers (and sensitive ears) may likely be listening. Those are not quiet amps.
I have lots of suggestions for the industry as you well know. We should stop using full output as a reference because it allows noisy high power amps to have good noise specs. Instead let's reference it to 1 watt all the time. Since speakers sensitivities are rated at one watt/one meter also we can then easily calculate the noise at one meter. If your speaker sensitivity is 100 dB at one watt, one meter and the amp is -60 dB below 1 watt. Then your signal to noise will be 60 dB at one meter and at your listening position wherever it is. I would love to rate my amps this way and I might include this handy spec, but, unfortunately it does not tell the whole story of what will happen in your system because we still have to take the line section of the preamp into account.
Line sections typically have their gain stage after the volume control and put the full noise of the line tube into the power amp. This is why the noise doesn't go down with the volume control setting in most instances. If the noise does go down with volume setting then the noise is ahead of your volume control which puts it in your signal source. This is important information for those of you trying to locate your noise source.
I've gone to a lot of effort to make very quiet power amps. When I get a note from an owner who thinks its his power amp making the noise I always tell him to try shorting plugs in the input. 99% of the time the noise disappears, I smile and he finds it's his preamp making the noise. Hum is often cable and ground related and floating the preamp and signal sources fixes this. Don't forget the cable isolator if you have cable TV coming in anywhere of the system, of just disconnect the cable for a test.
Here's what happens with a noisy preamp. Very few preamps are below 20 uV of output noise the really good ones are 5 uV and its hard to do much better than that. You can't even get near those numbers with a 6SN7, a truly inappropriate tube to use for a line amp. If you have 20 uV of noise and a typical poer amp gain of 30 dB (which also happens to be 30x) the noise at the speaker will be 600 uV or 0.6 mV which is a lot higher than my amp's resident noise. So you see, just because the amp is connected to the speaker, its not the amp that is at fault, it's just doing it's job which is to amplify what it is given.
Do all of us who make power amps a favor; try shorting plugs first. You can make them by taking any old RCA cable, cut it off at 2 inches from the plug, strip and twist the hot and ground together. If you are using and XLR, twist all three wires together and plug it into the input.
Another way is to rate noise in absolute values as I do in my amps. Simply state it in millivolts or microvolts if your amp is that good. Then knowing the gain of the power amp and the noise of the preamp you can figure what the noise at the speaker will be and which component will be the limiting factor.
For comparison the early amplifiers that David Manley sold often had hum of 2.0 mV at the speaker terminals. My RM-9 was typically 0.5 mV and the RM-10 typically 0.15 mV and these are achieved with AC heaters. The RM-10 noise at that level is very hard to beat. In my amps hiss is even lower.
If you have a speaker with 100 dB sensitivity, 2.83 mV will produce noise at 40 dB SPL at one meter. Since we have 2 speakers ad 6 dB to that and more if your chair happens to be in a standing wave peak at 60 or 120 Hz.
Every time we cut the output noise in half we get 6 dB less noise (or increase in S/N is you like). Reducing the noise to 0.28 mV, as i have reduces the noise by 20 dB!
For those who want further education on the different reference levels that are proper I refer you to Wikipedia where I rarely find anything incorrect (unlike the internet in general where almost every technical article I read has some flaw or is outright wrong).
I don't have time to carefully proofread this right now. Feel free to point out errors in language or failure to make my point clear. It's a complex issue and something I will put in my book. I would love for my book to be a collection of writings like this and have an editor to put them together, point out need for clarification etc. Who wants the job? The successful applicant gets to have his name on the title page as Editor.
