[Elecraft] K3 sensitivity + preamp: looking for wisdom

[DOUGLAS ZWIEBEL]

Hi...

The following request for information is not specific to the K3 (or
even to Elecraft).  Please craft your response to be generic.

Despite being an active ham/contester for over 40 continuous years
(but not being an ee), I just do not understand how a "preamp" works.
It is not a "smart" device, so how does it "know" to amplify the
signal and not the noise?  I first used a preamp way back when
(1960's) when I was a teenager...it was an Ameco job.  It was more
"fun" than useful, because I could give out "big" S meter reports.

Later, when listening in the noise became more important to my style
of operation, I never could notice a difference between putting the
preamp "on" and just turning up the volume (many rigs with built-in
preamps too).  Sometimes, turning up the AF gain worked better.

So...HOW does a preamp help to "bring a signal out of the noise"
without the use of some fancy computer algorithms backed by lots of
computing power?  Further, does a preamp really "improve" sensitivity?
 I have yet to find a signal that I could not copy (or hear) with any
preamp "on" vs increasing the AF gain.  PS...I am talking about HF.  I
know that lots of guys use a preamp on 160m with their beverages, so
the effect must be real...right?  HOW does a "dumb" device do it?

de Doug KR2Q