dfira wrote:Sorry, I meant increase the signal to noise ratio, and as a result, be able to lower the gain on my preamp.(which means a lowered noise floor?) (Is that technically correct?) I hope that was clear.
In general noise floor is due to thermal conditions and the current in the circuitry, and for a mic simply the fact that there's air molecules around the capsule hitting it at random.
So it stays where it is.
However a booster - basically using two clean gain stages instead of one - allow you to keep the signal in the best performing band of the final preamp (and nowadays, the A/D conversion analogue front end).
1) my preamp being the cause of much of the noise, - i suspect it is because with no mic plugged in, but the gain at where i would have it anyway, the noise is floating around -66dBU* when i plug the mic in it increases only very little, and that includes ambience.
Without anything plugged, you're just listening to the impedance noise, which means nothing: if the preamp isn't broken, when you plug in a microphone that noise will drop.
That's the reason why you mute any preamps you don't use.
2) the focusrite pres to be noisier than the cheap mic booster. If the booster is even equally as noisy as my preamp, then i get the benefit of being able to record quieter sources, but not a lowering of the actual noise, which is what i would like to achieve here.
* Don't catch me our on incorrect decibel annotation... I'm writing it from memory!
Preamplifiers are exactly designed to amplify very small voltages (mic level goes from about 0.001 to 0.01 volts) without adding much noise and distortion. By definition any good ones are _not_ noisy. That's one of the reason most stuff works at line level (higher voltage), because it's far easier (and cheaper) to make as it does not need be so good as a preamp.
A fixed-gain clean booster will not need the variable gain circuitry, so everything else being equal it will generally be a little less noisy than a regular preamp with an equivalent gain setting.. but just a little, down from an already very small value.
The benefit of using it is, as above, that by staging the gain that way you can keep the variable-gain preamp in the sweet spot where everything is running at best (and heat is kept down. A lot of electrical noise is due to heat in resistors - put some ice under the pre and it will be a trifle quieter).
It's very unlikely your Focusrite is particularly noisy, but if you have to crank the gain you will amplify ambient noise. Any noise you hear when you get the gain you need is ambience (or actual noise you're producing, clothes are a killer for that).
With microphones, the only way to reduce the signal-to-ambient
noise ratio is to reduce the ambient noise, or work nearer to the mic.
And about the decibel notation - it is not something you can ignore. "decibel" is just a ratio, so if you don't say what you are taking a ratio of, the number gives no meaning. Being unsure of "what" decibel you're talking about is like going to a supermarket and say you want to buy "something".