Re: Pre-Amp or Distribution-Amp... What's the difference?

On 2 Mar 2007 07:47:45 -0800, "NadCixelsyd" <nadcixelsyd@xxxxxxx>
wrote:

What's the difference between a pre-amp and a distribution amplifier?
Do they do anything different?

Sorry about emphasizing all the weasel words, but there are a number
of exceptions that should be taken into account when purchasing one of
these devices.

There may or may not be any difference depending on the particular
amps, but *generally* a preamp is designed to boost weak signals and
should be mounted as close to the antenna as possible. The reason
being there may be a useful signal at the antenna but it's too weak to
be useful by the time it goes through the coax to the TV set.
*normally* a preamp only has one port in and one out.
Mounting a preamp at the TV set and then moving it to the antenna can
produce a phenomenal difference in the signal.

Distribution amps on the other hand are *usually* designed to take a
signal that is already at a useful level, amplify that signal and
"distribute" it across a number of ports. The signal at the output
ports may be a bit stronger than the input but the total output is
usually not a lot stronger than the input.

There are also splitters and combiners which are passive devices.
Generally a splitter can be bidirectional. IOW *generally* a splitter
can be used in reverse as a combiner as when two antennas are
connected through a combiner to one cable.

*generally* splitters have an even number of ports, but not always.
If it's a true splitter all output ports will have the same insertion
loss. I have several 3 and 5 port splitter/combiners here.

I live about 8 miles from 8 UHF HDTV stations. However, there are
other stations about 30 miles away that I do not receive at the
current time.

When that close to a station or stations you have to be careful about
overloading a preamp. Typically a distribution amp will not have
enough gain to cause a problem in this situation, but there is no
guarantee. If the signal is strong enough splitters are usually
adequate.


It's about a 40 foot run from the antenna to the first splitter in the
attic. It's a 4-way splitter going to four different rooms. Two of
the rooms have an additional 2-way splitter (DVR & TV). I could put a
pre-amp at the antenna. I could also replace the 4-way splitter with
a distribution amplifier.

*usually* it's not a good idea to cascade splitters. When a signal is
divided by two each signal is 3 db (half the original) weaker plus
the insertion loss of the splitter. In the case of the 4 port each is
1/4, or 6 db less than the original plus the insertion loss which will
probably be more than the two port divide. An additional two way
splitter will divide drop those signals by an additional 3 db so you
are getting very near a 10 db loss and certainly more than that when
insertions losses are taken into account.

If the 4 port splitter were replaced by a 4 port distribution amp you
most likely would end up with about the same signal strengths at the
TV sets (down wind of the 2 way splitters) as you have at the input of
the distribution amp. You might possibly even end up with a tad more.

There isn't a great deal of loss in a 40 foot run of good coax with
good connectors properly installed and you really shouldn't need a
preamp for stations only 30 miles distant where as stations only 8
miles distant could easily overload a preamp.

Assuming (assuming is never safe) you have good coax and connectors
with the connections properly made then my choice would be a
distribution amp to replace the 4-port splitter and leave everything
else as is. That does not come with a guarantee, but it's where I'd
start.


Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
.

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