A while back I was taken to task by someone via email for attempting to discuss the headroom versus noise trade off associated with the location of a volume control attenuator in the design of an analogue signal processing chain.
Apparently, I was some kind of imbecile for not being cognizant of the allegedly fundamental fact that, for all practical purposes, a systems signal-to-noise ratio is always entirely defined by the first gain stage.
I often see this BS being touted as some kind of truth as fundamental as the 2nd law of thermodynamics, or as verifiable as the fact that night always follows day. If you have an objection, then you are clearly some kind of technically illiterate dope who doesn't even know the basics!
In reality, anyone who regurgitates the quote in the title of this thread, ironically, reveals themselves to have next to zero experience in the design and analysis of analog signal processing chains (audio or otherwise).
Consider the following example (which is just one amongst countless hypothetical constructions):
We have an audio system and the majority of the system gain comes from the first stage - that being a MM phono pre-amplifier. This phono pre-amplifier has a voltage gain of 40dB (that is times 100) and an input-referred voltage noise of 3.5nV rt/Hz (for a [unweighted 20kHz BW] S/N ratio of about 80dB ref. 5mV). So, fairly typical figures for a good design.
That 3.5nV rt/Hz of input-referred noise is amplified to a whopping 350nV rt/Hz signal at the output of the phono pre-amplifier by its 40dB of gain.
350nV rt/Hz is obviously a huge amount of noise that will easily swamp out noise contribution of the following stages....
Right?
Well, yeah......
BUT
Believe it or not some of us strange folk don't find it particularly practical or at all desirable to listen to our systems with the volume wound up to 11 all of the time.
So we might typically incorporate this conceptually mind-blowing thing called a volume control attenuator somewhere in our signal chain. As a matter of fact, a location that it is often found is right at the output of the phono pre-amplifier.
For typical, casual listening in the evening, it would not be even remotely out of the ordinary to have the volume control set to give in the vicinity of 40dB or so of attenuation. This is especially so if you have a high-powered system with efficient loudspeakers.
Well, that 40dB of attenuation will effectively nullify the gain contribution of the phono pre-amplifier.
That volume control attenuator doesn't magically attenuate just the music signal whilst ignoring the self-generated noise present at the output of the phono pre-amplifier - it attenuates both signal sources equally.
So at the wiper of the volume control pot (or stepped attenuator or whatever) we are now back down to a noise signal voltage of just 3.5nV rt/Hz. I am keeping things simple here by ignoring the thermal noise contribution of the potentiometer itself, but that is perfectly OK as at 40dB attenuation even a potentiometer as large of 10kohms will present as a source resistance less than 100 ohms, so its noise contribution is negligible. People mostly stopped doing dumb things like specifying 5Mohm volume control potentiometers 30 years before the end of the era in which thermionic tubes were dominant.
3.5nV rt/Hz is quite unlikely to now be the dominant source of noise in the system. A power amplifier is generally doing good if it has an input-referred voltage noise significantly less than 10nV rt/Hz.
Now if that volume control potentiometer is followed by a moderate amount of active signal processing circuity such as cascaded active crossover and/or tone-control/equalisation filter stages, then these are almost certainly going to be the dominant source of self-generated system noise almost all of the time by maybe an order of magnitude or even two.
And before someone pipes up, yes, the contribution of record surface noise changes the outlook and relaxes hardware requirements significantly, but that is completely irrelevant to a technical analysis of the contributors to a systems total self-generated electrical noise.
Well that is my rant for this evening.
Apparently, I was some kind of imbecile for not being cognizant of the allegedly fundamental fact that, for all practical purposes, a systems signal-to-noise ratio is always entirely defined by the first gain stage.
I often see this BS being touted as some kind of truth as fundamental as the 2nd law of thermodynamics, or as verifiable as the fact that night always follows day. If you have an objection, then you are clearly some kind of technically illiterate dope who doesn't even know the basics!
In reality, anyone who regurgitates the quote in the title of this thread, ironically, reveals themselves to have next to zero experience in the design and analysis of analog signal processing chains (audio or otherwise).
Consider the following example (which is just one amongst countless hypothetical constructions):
We have an audio system and the majority of the system gain comes from the first stage - that being a MM phono pre-amplifier. This phono pre-amplifier has a voltage gain of 40dB (that is times 100) and an input-referred voltage noise of 3.5nV rt/Hz (for a [unweighted 20kHz BW] S/N ratio of about 80dB ref. 5mV). So, fairly typical figures for a good design.
That 3.5nV rt/Hz of input-referred noise is amplified to a whopping 350nV rt/Hz signal at the output of the phono pre-amplifier by its 40dB of gain.
350nV rt/Hz is obviously a huge amount of noise that will easily swamp out noise contribution of the following stages....
Right?
Well, yeah......
BUT
Believe it or not some of us strange folk don't find it particularly practical or at all desirable to listen to our systems with the volume wound up to 11 all of the time.
So we might typically incorporate this conceptually mind-blowing thing called a volume control attenuator somewhere in our signal chain. As a matter of fact, a location that it is often found is right at the output of the phono pre-amplifier.
For typical, casual listening in the evening, it would not be even remotely out of the ordinary to have the volume control set to give in the vicinity of 40dB or so of attenuation. This is especially so if you have a high-powered system with efficient loudspeakers.
Well, that 40dB of attenuation will effectively nullify the gain contribution of the phono pre-amplifier.
That volume control attenuator doesn't magically attenuate just the music signal whilst ignoring the self-generated noise present at the output of the phono pre-amplifier - it attenuates both signal sources equally.
So at the wiper of the volume control pot (or stepped attenuator or whatever) we are now back down to a noise signal voltage of just 3.5nV rt/Hz. I am keeping things simple here by ignoring the thermal noise contribution of the potentiometer itself, but that is perfectly OK as at 40dB attenuation even a potentiometer as large of 10kohms will present as a source resistance less than 100 ohms, so its noise contribution is negligible. People mostly stopped doing dumb things like specifying 5Mohm volume control potentiometers 30 years before the end of the era in which thermionic tubes were dominant.
3.5nV rt/Hz is quite unlikely to now be the dominant source of noise in the system. A power amplifier is generally doing good if it has an input-referred voltage noise significantly less than 10nV rt/Hz.
Now if that volume control potentiometer is followed by a moderate amount of active signal processing circuity such as cascaded active crossover and/or tone-control/equalisation filter stages, then these are almost certainly going to be the dominant source of self-generated system noise almost all of the time by maybe an order of magnitude or even two.
And before someone pipes up, yes, the contribution of record surface noise changes the outlook and relaxes hardware requirements significantly, but that is completely irrelevant to a technical analysis of the contributors to a systems total self-generated electrical noise.
Well that is my rant for this evening.
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