Preamp Injector / Inserter Location - AVS Forum
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post #1 of 23 Old 08-22-2014, 11:58 PM - Thread Starter
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Preamp Injector / Inserter Location

Is there a performance advantage to be had if one were to position the power inserter or injector more near the antenna?

Typically, I believe most introduce power within the living area, at or near the TV, which is at end of the line.

I currently have a CM-7777 on the mast. The output from which runs to behind the TV, to where the power injection device is located. When I try to visualize moving the power further up, more near the antenna, common sense seems to tell me that would be beneficial, more so on a longer run. However, common sense is not always correct...

I'm looking for every little edge I can get in a fringe location, so even if the benefit is marginal at best, with power in the attic, I would make this easy change. I'm curious if this theory is correct, and if so, why...

Thanks much.

Edit / Add:

Attenuation of 100' of RG6, for the frequencies of most interest to me (approx. 250 to 350 MHz), is a little over 3 dB. That would equate to about 1 dB loss for every 30 feet. If one were to put the power injector 30 feet closer to the antenna, should that theoretically make for a 1 dB benefit? I realize I might be missing something...

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post #2 of 23 Old 08-23-2014, 04:30 AM
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No, it doesn't work that way.

For the power supply's run to the amp, the DC resistance, not RF attenuation, is the factor of interest. You have to know the current demand of the amp, the DC resistance per unit length of the coax, the loaded output voltage of the power supply, and the minimum DC input voltage of the amp at maximum current drawand then run some basic calculations. Most power supplies put out several volts volts more than the amp requires so that a reasonable voltage drop across the coax isn't an issue for most installations.

In other words, if you're using a good quality RG6 with solid copper core, you can go a significant distance with the power run without running into problems from that issue.
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post #3 of 23 Old 08-23-2014, 08:59 AM - Thread Starter
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ProjectSHO89, thank you for setting that line of thinking straight. I had communicated with a CM representative several weeks back, one who apparently was "technical", and upon asking the same question, he said that he inserts power in the attic for that reason. After much researching on the web, I found nothing to support such though. Curiosity got to me though, thus the thread start... It seems that having the amp near the antenna as possible is good thing, but power insertion can happen anywhere in the run without consequence.

Might there be a negative aspect of inserting power further up? Since there is no performance reason to do so, at least in the way I asked, it would be nice I suppose to have less clutter behind the equipment. Also, I wonder from an interference standpoint, might it be better to have power inserted earlier, like in the attic, where there is no other adjacent AV equipment, power lines, connecting cables, speaker wires inches away. It does get damn hot in the attic, and certainly colder up there as compared to the living space, but behind all of that equipment, in cramped quarters, it will get toasty too...

Thoughts? I appreciate the knowledge share...
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post #4 of 23 Old 08-23-2014, 11:13 AM
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Quote:
Originally Posted by ;
Might there be a negative aspect of inserting power further up?
No, if it feels good to you, knock yourself out.

Quote:
Originally Posted by ;
I wonder from an interference standpoint, might it be better to have power inserted earlier, like in the attic,
The coax is shielded, it doesn't matter.

Quote:
Originally Posted by ;
It seems that having the amp near the antenna as possible is good thing,
Yes, almost only. There are rare exceptions such as for future serviceability or when a little extra attenuation is desired to cool off a signal that's too hot for the amp.

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power insertion can happen anywhere in the run without consequence.
Yes, as long as you don't trip over the factors in my previous post.
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post #5 of 23 Old 08-23-2014, 11:50 AM - Thread Starter
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ProjectSHO89, much appreciated.

I'll simply insert power in the attic to avoid additional clutter behind the DVR, where there's already a large confluence of A/V stuff...

Thanks again.
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post #6 of 23 Old 08-24-2014, 06:40 AM
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Got the same amp as OP. The power supply and amp are both in the attic. I put the amp in the attic to keep it out of the weather. Also my splitter is up there going to other rooms. Yes it gets very hot up there, but it's worked for 10 years with no issues.

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post #7 of 23 Old 08-24-2014, 09:38 AM
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There are two versions of the 7777: The "Classic" which was replaced by the "New" one in 2012. Other than their names and the metal shell of the amplifier, they have very, very little in common.
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post #8 of 23 Old 08-24-2014, 03:00 PM
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Quote:
Originally Posted by ProjectSHO89 View Post
There are two versions of the 7777: The "Classic" which was replaced by the "New" one in 2012. Other than their names and the metal shell of the amplifier, they have very, very little in common.
So your saying I lucked out by having the older unit?

A little off topic, but I was a two way radio tech for 35 years. The old radios had discrete components and I had to trouble shoot to the component level. The new radios were microprocessor controlled,no crystals,surface mount technology.Component level trouble shooting was no longer needed, all I had to do was swap a board.

The down side was the radios no longer worked as well as the old ones. All I got was complaints about range and interference. My response was "I didn't design this radio".The guys knew the difference in how their old radios worked.

I guess what I'm trying to say is the old 7777 amp probably used superior components. By cheaping out on design all they are going to do is lose business.

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post #9 of 23 Old 08-24-2014, 03:28 PM
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The new design isn't "cheapened out". It simply is not the same amplifier anymore. It's actually a very good amplifier, it's just quite different.
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post #10 of 23 Old 08-24-2014, 06:17 PM - Thread Starter
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I can't speak to the particulars as to what makes the newer CM-7777 different, but what I liked about the older 7777, the one that disappeared a couple years ago, was that it was dual input (2 antennas). Dual input amps are a rare breed these days...
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post #11 of 23 Old 08-24-2014, 07:13 PM
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The RCA preamp is one of the few remaining preamps with dual inputs. There may still be an Antennacraft version, but they have a bit higher noise figure. As for the newer CM-7777, it is higher gain at 30db power level, and that is too much for many applications. Whereas the newer CM-7778 at 16db gain may be a better choice.
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post #12 of 23 Old 08-24-2014, 07:20 PM - Thread Starter
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I have both the RCA amp (dual input) and the new CM-7777. Given our location, the 7777 is desirable. That said, the RCA did a great job, especially allowing me to tie-in a VFH antenna. With the DB4e now getting channel 8 for us, not so sure I need a dual input amp now, allowing me to use the 7777. Also, the Kitz amp was quite impressive...
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post #13 of 23 Old 08-25-2014, 08:47 AM - Thread Starter
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I have a related question, seemingly appropriate here in this discussion...

I am now contemplating a splitter, a basic 1->2 one up in the attic. I realize of course there will be an associated insertion loss of approximately 3.5 dB, possibly up to 4 if the splitter is not one the of the better ones... This a simple reality as power is being halved.

I wanted to avoid this scenario given our fringe location, but I wish to try it before throwing in the towel, hoping the CM-7777 can compensate for this loss. I plan to raise the antenna (DB4e) a bit too, to help overcome this split...

In doing some reading, I found this relevant diagram from Antennas Direct:

https://www.antennasdirect.com/cmss_...itter_Opt2.pdf

Questions:

1. I need to look more carefully at the splitters I have in my old cable box, but is one port always going to be a "DC Pass Out"?

2. If one does as described in the diagram, does the benefit of amplification apply to all outputs on the splitter, and if so, equally?

3. Much more related to the original topic here, especially if #2 is "no".... Would there a performance benefit in this set-up to have power insertion prior the splitter? I'm guessing for the same reasons provided earlier, the answer would be "no"....

4. Given that the splitters i have are 20+ years old, almost all are oxidized or corroded from moisture having gotten into the bag some how, I plan to buy a new one. Any really good ones that can be recommended, with particularly "low loss"?

Thanks much.
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post #14 of 23 Old 08-26-2014, 10:59 AM
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1. No. Such splitters aren't particularly common. Usually, they'll be non-DC PASS or, if a "satellite"-rated splitter, will have all outputs DC PASS via steering diodes.

2. Yes, all splitter outputs are amplified relative to the input (Amplifier gain minus splitter insertion loss).

3. No. Splitter location and type may be selected based on the combination of the location of a needed AC receptacle vs where the coaxial cables all terminate. See the notes on the diagrams you referenced above.

4. Non-DC PASS, the Ideal-branded splitters from Home Depot have been tested and shown to be about the best available from a B&M retail store. For "satellite"-rated splitters, the Ideal from Home Depot would also be suggested. Realistically, any of the splitters from almost any retail store will be "good enough" except in very critical situations. I measured up to about a db's difference between best to worst in a recent test shootout I performed.
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post #15 of 23 Old 08-26-2014, 11:26 AM - Thread Starter
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ADTech, thank you. I have found your posts helpful, even on other forums (assuming "ADTech" is you elsewhere).

I will pick-up an Ideal at THD later today and give it a go.
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post #16 of 23 Old 08-26-2014, 12:06 PM
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If you use a splitter between the power injector and the preamp, then you ideally need a splitter with one port passing power. Or if you use a satellite power passing splitter, you will have to use voltage blocks on the ports not connected to the power supply. I use a Holland splitter with one port power pass, as I do not have a power outlet in the attic, so the power injector is downstairs at the TV.
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post #17 of 23 Old 08-26-2014, 12:29 PM
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Quote:
Or if you use a satellite power passing splitter, you will have to use voltage blocks on the ports not connected to the power supply.
Generally, no, you don't. The sat splitters are diode steered which will prevent power from going anywhere except the common port when fed from any of the outputs. The advantage is that you don't need to keep track of which splitter output port is use. The disadvantage is that you have the diode's forward voltage drop of around 0.7 V to contend with which, if one has very long cable runs, might be an issue. In all the sat splitters I've cracked open, there's a 1N4007 in parallel with a small value capacitor feeding each of the output ports.

The single-port power passing splitters use a low-value coil (around 100 uH) connected between the input and the selected output to provide the DC power path.

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post #18 of 23 Old 08-26-2014, 12:30 PM - Thread Starter
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Thank you tylerSC.

A DC pass splitter it is... The Home Depot has the aforementioned Ideal 1->2 splitter and it passes DC. Since there will be no open ports, no need for blocks or terminations...

While there is no performance edge to be had by placing the power inserter further up, I think I will do it to avoid more clutter in a somewhat tight space where the A/V equipment is housed...

As an FYI, I'm doing this split to feed a Tablo DVR which will reside in the attic. I still await Tablo's "approval" to do so... The total coax run from the antenna to the Tablo should be about 12 to 15 feet. I contemplated a separate mast & antenna just for the Tablo to avoid the insertion loss. It's that rough out here...

The other side of the splitter will drop down into the living room TiVo box.
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post #19 of 23 Old 08-26-2014, 04:13 PM
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Denny's Antenna site has a good diagram on how to install a splitter with a preamp, and the use of voltage blocks. But if you place the power injector in the attic before the splitter, then that will resolve those concerns.
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post #20 of 23 Old 08-27-2014, 10:53 AM
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You won't find voltage blocks or splitters that need them in retail stores. You'd have to work hard to find them online as they're just not needed very often. Personally, I've yet to actually hold one such splitter in my hands.

Tony,

See attachment for a sketch of a basic two antenna system with separate amps.
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post #21 of 23 Old 08-27-2014, 12:01 PM - Thread Starter
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ADTech, thank you for making the time to draw that up. Nothing like a visual.

Please allow me talk out loud so-to-speak, asking a question or two along the way...

Two antennas, one UHF (like the AD DB4e or C4) and the other VHF. I don't have a VHF only antenna at this point, but a small combo one, the RCA ANT751. I'm really only after one VHF channel (channel 8), and it's somewhat the opposite direction of the UHF channels in San Francisco. Would a combo UHF / VHF antenna be problematic in such a set-up? Heck, as mentioned previously, maybe just the AD VHF kit is all that I need. The DB4e gets channel 8 pretty well surprisingly, so the dipole addition might negate the need for the separate VHF antenna?

Again, here's the location. Apologies if I provided it earlier in this thread:

http://www.tvfool.com/?option=com_wr...ec2f44cdac3547

The DB4e is aiming at 315 to 320 degrees (true) to get the meat of all that we seek. Channel 8 is at 168...

Back to your drawing, which I assume is two antennas on the same mast. Amp is in reference to preamp, mast-mounted. Any commentary with respect to the desired gain on these preamps in relation to that of the disti amp further down? I asked because I learned in this somewhat related thread that it might be beneficial to have a preamp with less juice than the disti amp... That other discussion, post 6 in particular by Calaveras:

http://www.avsforum.com/forum/showth...7#post26921417

The combining of the VHF and UHF feeds is done by a UVSJ? Pretty certain, but seeking clarification....
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post #22 of 23 Old 08-27-2014, 01:12 PM
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The existing mall combo is fine for channel 8 if it's doing the job.. The UVSJ that combines the signals will simply reject (throw away) any UHF signals from the combo.

The two antennas in my schematic may be on the same or on separate masts, it doesn't matter as far as the functional schematic is concerned. You'll have to determine what's both practical and functional on your rooftop. Two general rules to follow: 1) Do not place one antenna behind the other and 2) Keep them physically separated by several feet.

My general rule of thumb for selecting amplifier gain is to add up all the downstream insertion losses (splitters, coax, tuner input, etc), then add 6-8 dB for a minimum estimate of gain required. Installing an amp with more gain than what is needed is not useful unless you have future expansion plans for the system. Adding an excessive amount of gain, more than 10 db than what is required, is usually an invitation to new problems. An excellent case history is the "new" CM7777 with its 30 dB of gain which has caused a tremendous amount of problems due to confusion with the older lower gain version. In any event, if you properly select the gain of the pre-amp(s), you won't need a distribution amp.
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post #23 of 23 Old 08-28-2014, 02:25 AM - Thread Starter
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Thank you ADTech.

The new CM7777 is one of the amps I have here... A weird story, but I actually got it for free. Overstock.com had it listed as the older dual input, which I wanted, but when it arrived, it was newer single input. I requested to send it back for a refund, they apologized and said to keep it! So, here it is, along with the RCA dual input amp...

Given my fringe location, with nothing in green or yellow (TV Fool), 62+ miles away from the desired towers, a 2 edge path, I am of the belief that 30 dB gain of the 7777 will not be problematic for me. But, that's just my common sense speaking...

I like the concept of vertical stacking and might give that a try with another DB4e. I'm currently playing around with tilting the antenna, and I'm actually seeing some positive effects doing so...

Thank you for addressing my questions and sharing your words of wisdom. Greatly appreciated.
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