Noise is a sensitive subject among audiophiles, and many are obsessed with eliminating it completely from their systems. Depending on the noise level it can certainly make ones listening experience less than desirable. When it comes to noise in an audio system, we can look at it from several different perspectives, but for this discussion we will just focus on a few. There is 60 Hz hum, which is power, or cable related, or at times caused by ground loops. Also, if the hum has some harmonics, it will sound like 120 Hz buzz, another culprit of noise in an audio system. Then there is hiss and the occasional crackle that some audiophiles like to call tube rush if it occurs with a tube component.
However, let us take a moment to dispel a myth. While tubes can certainly produce noise in a system, solid state devices, whether transistors, integrated circuits (ICs), or others are not immune from producing noise either. Another myth we can dispel is the belief that speakers make noise. To be clear, speakers do not make noise. There cannot be some bad part in the speaker that makes the types of noise previously referenced. If your speaker makes noise when you are playing music that is distortion. What we want to understand is the background noise of your audio system when for example you initially turn on your system or push pause on your volume control during music playback.
Now here is where it gets a little tricky and some audiophiles will have a tough time getting a handle on this. If you are assessing noise in your system, it is not correct to advance the volume control above your normal listening level, let alone to the full clockwise position which I know many audiophiles do. It would be quite difficult to listen to a system at full volume without significantly loading the room or blowing out speaker drivers. So why would anyone assess noise at that level? The proper way to assess noise in your system is simply to adjust the volume to your typical listening level or perhaps a bit higher, say 10 dB, in case there are some situations where you might want to listen louder.
Now with the volume level properly set, if you have noise at your listening position, or a reasonable distance from the speaker with no music playing, that can generally be fixed. Yet many audiophiles feel the need to place their ear against the speaker to assess noise. Seriously, how many people do you think listen to music that way? If your ear is against the speaker and there is a little bit of noise that generally should not be a problem. While it may not be an ideal outcome in your mind, that should not decrease the enjoyment of music in any way. I know it will be hard for some obsessive-compulsive audiophiles to accept but seriously, you are doing just fine if you cannot hear noise at your listening position so sit back and enjoy the music.
For those of you who do not have a set or multiple sets of shorting plugs these are a good investment for conducting noise tests. Just remember to only use them on the inputs, never the outputs unless you want to damage your component in which case have at it. So now that we have addressed a few facts and debunked a few myths about noise let us hear from Roger on the topic of noise specifications and how noise is best measured.
Noise Specification Standards
Roger A. Modjeski
Most published noise specifications are useless as they do not give a proper reference. When they are given, it is often in reference to full output, which still requires some recalculation if we use the amplifier at less than full output. For instance, if an amplifier’s noise spec is -80 dB referenced to full output at 100 watts it is only -60 dB at 1 watt and -40 dB at 0.1 watt where folks with quiet rooms, sensitive speakers, and probably sensitive ears may likely be listening. Those are not quiet amplifiers.
I have lots of suggestions for the industry. One is that I believe we should stop using full output as a reference because it allows noisy high-power amplifiers to have good noise specs. Instead, we should reference it to 1 watt. Since speaker sensitivity is rated at 1 watt/1 meter we can then easily calculate the noise at 1 meter. If your speaker sensitivity is 100 dB at 1 watt/1 meter and the amplifier is -60 dB below 1 watt. Then your signal to noise will be 60 dB at 1 meter and at your listening position wherever that is. I would love to rate my amplifiers 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 must take the line section of the preamplifier into account.
Line sections typically have their gain stage after the volume control thereby putting the full noise of the line into the power amplifier. This is why in most instances the noise does not go down with the volume control setting. If the noise does go down with the 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 have gone to a lot of effort to make very quiet power amplifiers. When I get a note from an owner who thinks it is the power amplifier making noise in their system, I always advise trying shorting plugs in the input to test for noise. The result is that 99% of the time the owner reports the noise has disappeared. In addition, I smile when the preamplifier or source component is responsible for the noise. Hum is often cable and ground related and floating the preamp and signal sources fixes this. Do not forget the cable isolator if you have cable TV coming in anywhere near the system, of just disconnect the cable as a test.
Here is what happens with a noisy preamplifier. Very few preamplifiers are below 20 uV of output noise and the quietest ones are 5 uV. It is hard to do much better than that. You cannot even get near those numbers with a 6SN7, a truly inappropriate tube to use for a preamplifier. If you have 20 uV of noise and a typical power amplifier 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 amplifiers resident noise. So, you see, just because the amplifier is connected to the speaker, it is not the amplifier that is at fault, it is just doing its job which is to amplify what it is given.
Another way is to rate noise in absolute values by simply stating it in millivolts or microvolts if your amplifier 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 0.4 mV and the RM-10 typically 0.15 mV and these specs are achieved with AC heaters. The RM-10 noise spec 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 1 meter. Since we have 2 speakers add 6 dB to that and more if your chair happens to be in a standing wave peak at 60 Hz or 120 Hz. Every time we cut the output noise in half, we get 6 dB less noise (or increase in signal to noise if you like). Reducing the noise to 0.28 mV as I have reduced the noise by 20 dB!