Hello Doug,
Here some basics about preamps. I will try to make it short, but I could
expand to pages....
A preamp only makes sense if the noise-figure (NF) of the preamp is better
than the NF of the normal input stage (usually the normal frontend input).
Each stage in a receiver (frontend, mixer, pads, amplifiers and buffers)
adds his internal noise to the combination of signal and noise injected into
that stage. If the noise-figure (NF) of the first stage is better then the
NF of the following stage, the amplification factor of the first stage is
determining if, and how much, the S/N ratio becomes better (or not). So, to
stay with your words: The preamp is not some kind of intelligent amplifier
selecting signal out of the noise but it should present a better signal to
noise ratio to the next stage because it adds less internal (thermal) noise
to the signal than the following stage.
The best result in signal to noise ratio is coming out of your antenna
connector (at the antenna) everything done after, is worsening the
situation. The first decrease in S/N ration is due to attenuation of the
coaxial cable. So the best place to put the preamp is at the antenna so the
signal + antenna noise is amplified to a "maximum" level. The amount of
noise added by the preamp and coaxial cable is only relative small compared
to the amplified antenna signal.
A preamp has got 1 big disadvantage: it decreases the dynamic range of the
original receiver setup (with about the same number of dB's as the gain of
the pre-amplifier).
The best solution whatever is a frontend that has to little gain of it's own
but a relative low NF (let's say 3 dB) and capable of handling BIG signals.
A separate preamp that has a NF of 0,5 dB and a gain of 13 dB would be good
enough to have a good overall performance (low NF and not too much gain
before the first mixer stage).
Most preamps in HF equipment are TOTALLY useless and only worsening the
situation (but looks good on your S-meter). The discussion about the need
for a low NF preamp on HF is easy for me: The less noise the preamp adds
from itself (and so worsening the S/N ratio) the better it is. The effect of
a low noise preamp is lessening when the atmospheric noise is higher, but
the effect will never be zero!
On low band the atmospheric noise is high and local signals very strong and
so stressing the dynamic range of the receiver. On these bands it's
sometimes better to switch the preamp off.
73's Evert PA2KW
-----Original Message-----
From: elecraft-boun...@mailman.qth.net
[mailto:elecraft-boun...@mailman.qth.net] On Behalf Of DOUGLAS ZWIEBEL
Sent: Monday, December 15, 2008 17:15
To: Elecraft Reflector
Subject: [Elecraft] K3 sensitivity + preamp: looking for wisdom
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
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