Signal Amplifiers (Amps, Preamps, Distro Amps) - See Chart in Post #1

[roger1818]

Originally Posted by nezdepain

On my paper instructions for my AP8700 it says "A maximum preamp-to-power injector coax length of 150 feet RG-6 copper clad center conductor, or 300 feet of RG-6/U with solid copper center conductor, or 150 feet RG-59 is acceptable"

That makes sense and is probably a good rule of thumb for most pre-amps (unless otherwise specified). When it comes to RF, there is not much difference between copper clad and solid copper because of skin effect, but for DC power, solid copper will be more conductive and thus have a lower voltage drop.

As others mentioned the power injector will provide unregulated DC at a higher voltage than required and the pre-amp will have a voltage regulator to bring it down to the voltage required, so as long as the voltage drop is within tolerance, everything will work just fine.

[roger1818]

Originally Posted by majortom

The ap8700's power injector has a voltage regulator inside it.

That would be very surprising. Most pre-amps have the voltage regulator inside the pre-amp iteself. The power injector will provide unregulated DC since the voltage drop in the coax is unknown, so there is no point regulating the voltage before that drop.

[majortom]

Roger, I know that, but see posts 1896 thru 1903...Perhaps there is another at the preamp end.

http://digitalhome.ca/forum/showthread.php?p=1096505

Also keep in mind they didn't always ship their preamps with this same inserter.
Just for the last few years.

[roger1818]

^^^That is strange. Are they using a switching power supply now? If so, that might explain it, as a voltage regulator would help clean up some of the switching noise. You wouldn't want to combine noisy DC with RF.

[fireflimoon]

Sorry if this is not the proper area for this question...
When you have strong and weak signals and have too strong an amp for the strong channels, will your tuner still lock on the weaker channels? Only those stronger channels will not be accessible right?
I'm thinking about an a/b switch but need to know the answer to the question above.

Thanks

[hoopitup2000]

The weakest channels are lost first. The stronger channels will likely remain.

[fireflimoon]

Ok Thanks
So its like the noise level (overamping) is increased across the uhf/vhf band for all frequencies when you have an overloading amp. Only the loudest signals able to overcome the noise get heard by the tuner?

[hoopitup2000]

Yes, the dynamic range of the amp is crushed when it is overloaded.

[fireflimoon]

Last question if you are ok with that...
I think I get what you are saying...its like a bunch of people talking in a room the farther you move away from them, the harder it is to hear everybody - even if you boost everyone;s voice by the same factor, the loud ones drown out the quiet ones at a certain point so that you can't make them out. I guess that is signal to noise ratio. I never thought about it but is the signal to noise ratio different for each frequency that is available from one antenna gevn the same cables, balun, tuner etc?

[majortom]

fireflimoon,
The reason you don't want to overamplify has to do with intermod distortion.
try reading about jack & jill, here.

http://digitalhome.ca/forum/showthread.php?p=1232026

[fireflimoon]

Thanks Majortom

[holl_ands]

When two strong signals pass through an amplifier, they can generate Intermod
products (e.g. 2f1-f2, 2f2-f1) that can fall on top of desired, weak channels.
And note that each Third Order Intermod product is THREE channels wide.
With three or more strong signals, the number of 3rd Order Intermod products
that can fall within the desired frequency band is much higher, affecting many
more channels.....

SNR on affected channels is Sd/Ni, where Sd = Desired Signal Strength and
Ni = Intermod Product signal strength, which depends on the strengths of f1 & f2.

SNR on non-affected channels is Sd/No, where No = Thermal Noise Floor,
which is the same on all frequencies (No = -106.5 dBm).

A Preamp should be chosen to maximize the SFDR (Spurious Free Dynamic Range),
wherein the resultant Intermod Products are no stronger than Thermal Noise. In
SOME situations, where there is TOO much amplification, a small amount of LOSS
(typ. 3-4 dB) can be placed prior to the Preamp to reduce these Intermod Products,
since there will be 3 dB reduction in Intermod Products for every 1 dB reduction
in signal strength (hence 9 dB reduction when adding 3 dB attenuator before Preamp).
For more info, search for my posts re "SFDR".....and Spreadsheets found here:
http://imageevent.com/holl_ands/files/ota

[fireflimoon]

Thanks Holl_Ands...
Are there any cons to getting an amplifier that has a variable output setting for amplifying the antenna's signal? If you follow the Jack and Jill example, as long as you stay in the linear region of amplification, you shouldn't get any "child" frequencies created and therefore won't fudge reception of other frequencies. Why would I buy a non variable preamp in the first place - if a variable one can be used to back off on the amplification of the signals coming from the antenna when they get close to the non linear region as described in Majortom's link to the Jack and Jill article? Wouldn't I alwsys want an amp that was "variable"?.

[laurienicol]

I tried a CM7001 tuner, that was replaced by CM with a PAL that I love. I also tried out a CM7777 pre-amp that didn't improve my reception. Both are doomed to collect dust till they get thrown away. Is there someone in the Greater Vancouver area that has a use for either or would like to try them out? I will either lend or give them away.
Laurie in Ladner BC

[fireflimoon]

Gross oversimplication probably...

Let's say you are in a site where the strongest signals are about 35db higher than your weakest signals that you want to get. Plus you have two preamps, but one at a time is attached to the overall configuration you are using which is one antenna, 100 feet of RG6,
and one tv.

Let's also say that your weakest channels are at -1db (nm) and pwr(dbm) is at -92 (from tvfool).

All else being the same regarding the amplifiers, If you have amplifier A (1 DB NF)with a lower NF than Amplifier B (3 DB NF) by 2db, does this better amp lower the noise floor to give you a better signal to noise ratio so that the tv tuner has a better chance of locking on to the signal or is there something else going on?