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Thread: Preamp Comparison - RCA vs. CM-7777

  1. #1
    Join Date
    Jan 2015
    Location
    Akron Ohio
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    639

    Default Preamp Comparison - RCA vs. CM-7777

    I got my hands on a new-in-box vintage Channel Master 7777 preamp from Chad Bryan...
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    Up until I received it, I was using the commonly available RCA TVPRAMP1R with separate VHF and UHF inputs. I'm only 1 to 2 miles from several FM and DTV broadcast stations that overload the preamp unless I filter them out before the preamp. I use a couple Tin Lee filters to filter out the strong DTV blowtorches...
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    I was doing a side-by-side comparison of these two preamps, and I definitely noticed improvement using the CM-7777 over the RCA. The CM-7777 will not overload nearly as easily. With the RCA, I had to use a full-band FM trap before the preamp or else VHF TV would be severely overloaded. With the CM-7777, I can use a selective FM notch filter before the preamp, and it won't overload like the RCA did. Here is the custom FM filter I use...
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    This helps because I can now DX both FM and VHF TV at the same time using this preamp.

    Another observation is that the noise is much less using the CM-7777. By comparing signal strengths on the HD Homerun tuner, the vacant channels look much cleaner. The CM-7777 provides way more amplification than what I needed, so I applied some attenuation after the preamp to help reduce possible distortion and lower noise. I determined that I needed to add about 11.5 dB of attenuation (in addition to the long coaxial cable) before I began to see decreases in the SNRs.

    I'm much happier using the CM-7777. It's too bad it was discontinued.
    Andrew

    My TV and FM DX Photos from Akron, Ohio...
    https://www.flickr.com/photos/133179000@N04/albums

  2. #2
    Join Date
    Dec 2010
    Location
    Poughkeepsie, NY
    Posts
    734

    Default

    Channel Master has a new CM-7777 pre-amp, called the "Amplify", model CM-7777HD. Using "7777" again is going to cause confusion. I'm thinking of trying one out here next Spring. It is UHF/VHF, and it has choice of 30db or 17 db gain, and has FM trap and LGE filter.
    Chris - Poughkeepsie, NY
    DTV DXer since April 2009
    DTV DX screenshots at: http://www.flickr.com/photos/dtvdxer/sets
    DTV DX Videos at: http://www.youtube.com/user/dtvdxer

  3. #3
    Join Date
    Oct 2012
    Location
    Fort Myers, FL
    Posts
    446

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    Anybody try the 7777HD yet? I'd like to replace my aging Radio Shack amp and built-in cell tower filtering sounds pretty appealing.
    :: Ryan Grabow
    :: egrabow.com/dx
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  4. #4
    Join Date
    Jul 2005
    Location
    Indianapolis, IN
    Posts
    356

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    Ryan, I have not tried the Channel Master 7777HD yet, but I just ordered one through www.channelmaster.com I tried to order one from this site a few weeks ago but was told it was out of stock and not available. Today I found it available. It's $89.00 but they do provide free shipping on orders over $49. I have found it elsewhere but at higher prices. I did find a few open box units at Ebay for slightly less dollars.

    I will try to perform some testing with it once it arrives but because it is Winter in the Midwest, I'm not sure how productive the tests will be right away. I have a great DTV DX test location in mind where a nearby cell tower seems to pose a few audio issues on a couple UHF channels, so it will be interesting to see how well the built-in LTE filter works.

    BTW, thanks Andrew for your fine review above between the RCA TVPRAMP1R and the vintage Channel Master 7777. I own both of those units and hope to compare them with the 7777HD unit sometime next year.

  5. #5
    Join Date
    Jul 2005
    Location
    Indianapolis, IN
    Posts
    356

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    With 65 deg. weather yesterday, I lowered the tower and replaced the Winegard HDP-269 preamp (12 dB gain) with the new Channel Master 7777HD preamp (17dB gain or 30dB gain) and the Channel Master 4251 UHF parabolic. Since I live 5 miles from the Indy antenna farm, I'm using the low (17dB) setting. A button on the unit allows you to select between low and high gain. So far, absolutely no good tropo DX to check on distant reception. As for local reception, with the 4251's high gain, even the low setting is overloading when the antenna is aimed at the local towers, as a couple stations actually drop below the decode threshold. This didn't happen with the HDP-269. Once some decent tropo shows up I'll know more about how it works with weak, distant reception. It may not stay on the tower very long, as I would like to perform some comparison testing along with some other models in an area away from the city with less high RF issues. http://www.channelmaster.com/Amplify.../cm-7777hd.htm

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