mics noiser than preamps

[panang]

ok, forgive me if this is a stupid question.. i'm just trying to grok the numbers.
signal to noise ratios for microphones seem to be spec'd at 75dB and under. but S/N for mic preamps is much higher, more like 100dB.
why bother having the preamp noise floor 25 dB under the mic noise floor? that's 25 dB of clarity devoted to reproducing microphone noise, right?
ok so maybe i'm missing something-- is it that mics S/N is referenced to a lower operating level, so they have a higher dynamic range, so the overhead is much greater?
or something?

 

[The Axis]

Regardless of the ratio of signal to noise for each component, noise is always additive for each component in the chain, so less noise is always better. The SN ratios for mikes are for comparing to other mikes, and the SN ratios for preamps are for comparing to other preamps. Lower is always better (all other factors assumed equal).

Peace,
Rick

 

[panang]

true, my perspective is maybe a little different because i'm recording for research use, and on a shoe-string, rather than cutting the next la vida loca. i guess it comes down to how much you pay for how much noise.

at 100dB of S/N, it seems like the noise contributed by the preamp is 25dB below that of the finest microphone...

correct me if i'm wrong, but in terms of power that's like adding a noise signal that is 0.0032 times the power of the microphone noise... or in other words, the increase in noise, or (mic noise + preamp noise)/(mic noise), is
10 log_10 (1.0032) dB = 0.039dB of added noise. is that even audible?
by the same calculation, for a mic with 75dB SNR, if your preamp SNR is 90dB your total noise goes up by 0.13 dB.
to add 1 dB of noise or more the preamp SNR would have to be no more than 6dB greater than the mic's SNR...
But even the cheapest mic preamps i could find have a SNR of over 90dB.
Considering that microphone S/N ranges from 40dB for cheap mic elements, to 75dB for expensive ones, it seems like the preamp noise is basically not a significant factor. most of the money and effort should go into getting quiet mics and exploiting their full dynamic range...
(opologies if this is obvious... i'm learnin)

 

[panang]

actually i see what the deal is:
the headroom *is* higher with most mics because the SNR is measured at 94dB SPL, whereas the maximum SPL can be 50dB higher than that. In contrast with a preamp, the headroom is only 25dB.

that means that if the SNR of a mic is 80dB, the dynamic range could be 120dB. whereas if the SNR of a preamp is 80dB, the dynamic range will only be 106dB.

so if i'm recording something as loud as 124dB SPL on my mic spec'd at 80dB SNR, then the effective SNR at that level is 100dB, and a clean preamp is necessary to preserve that SNR. For recording a conversation at 64dB SPL, on the other hand, the effective SNR for the same mic will be only 50dB. to bring this signal up to line level requires more gain, which amplifies the mic noise more.
in this case it probably doesn't matter whether my preamp has 100dB or 80dB SNR, because the preamp noise is negiligible compared to the amplified mic noise.