Ground Loop Question

Discussion in 'Archived Threads 2001-2004' started by Jim Noble, Sep 6, 2002.

  1. Jim Noble

    Jim Noble Extra

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    After installing my Rotel RMB1095/RSP1066 combo, I immediately noticed a loud hum, which many of you here correctly identified as cable-induced (thanks to everyone for the help).
    I have an analog cable feed going to a ReplayTV 4160, and a DirectTV feed going to a Sony SAT-B55 receiver, which is connected to the ReplayTV via RCA (analog) cables and an S-video cable.
    When I disconnected the analog cable feed, the hum went away-even with DirectTV connected as described. However, over the past two days, the hum has come back [​IMG](with cable still disconnected)-only now, it goes away when I disconnect the satellite feed.
    Does this make any sense? I'm not too sharp on the electrons, but if the hum is a result of different grounds for the amp (house current) and the offending device (cable or satellite), shouldn't it always be there? Can it come and go as described? Or is it a gremlin?
     
  2. Claude M

    Claude M Stunt Coordinator

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    Perhaps the reason why the problem comes and goes has something to do with moisture level of the ground or a loose connection somewhere on your sat rig.
     
  3. Chu Gai

    Chu Gai Lead Actor

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    I think Claude is correct in that whatever ground was in effect when your Direct TV was installed has apparently come 'undone' for some reason. Consider a service call as they can address the problem without you having to spend additional funds.
     
  4. Jim Noble

    Jim Noble Extra

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    More interestingly, when the hum from the satellite feed is "on", I can get it to go away by disconnecting and reonnecting the satellite feed. It is as if current "builds up" in the satellite cable, is discharged when the cable is disconnected and reconnected, then "builds up" again.
     
  5. Claude M

    Claude M Stunt Coordinator

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    Static build up on the dish itself?
     
  6. Jeffrey Chin

    Jeffrey Chin Second Unit

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    i witnessed the exact same problem, except i have comcast digital cable. i have it connected to my replaytv as well. save yourself time and headache by buying one of these babies. problem solved.
    3 words:
    MONDIAL MAGIC BOX
    http://www.audioadvisor.com/store/pr...V%20I solator
     
  7. Clay Autery

    Clay Autery Stunt Coordinator

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    Claude has it in one or the other suggestions... 1) The SAT system ground is either unstable or the earth ground is borderline and seriously affected by slight changes in soil moisture... or 2) you have bigtime static buildup on the SAt antenna (due to NO/INEFFECTIVE ground of the dish itself).

    Here's the deal as "I" see it....

    The ground potential for cable, SAT, OTA, and electronics (AC mains) MUST be as close to exactly equal as humanly possible to reduce the potential for ground loop hum et al. Eliminating grounds is NOT an option in my opinion.

    THIS is how I did MY system to make sure I didn't (and I don't) have ANY ground loop induced hum...

    My AC mains originally terminated to a rebar that passed through the foundation footing below the AC panel and into the soil... who knows how deep... the terrestrial cable and AC grounded to this bar... the rebar was rusty and the clamps were crappy which resulted in a higher than desired resistance to ground... There was no convenient way to get my SAT/OTA grounds terminated to the AC mains earth ground, and THAT ground was crappy anyway (5 ground connections: OTA antenna, 2 SAT dishes, OTA antenna lead, and the ground from the Switch 64 (SAT leads)).

    So... I installed a NEW earth ground to use as a common for everything... I bought an 8+ foot copper grounding rod (thick copper clad steel actually I think), a large copper bus bar, and a ton of heavy gauge solid, bare copper wire (4 gauge from bus bar to rod, and 8 gauge from each device to the bus bar).

    I drove the 8+ foot rod into the earth to a depth of 8+ inches below grade at a distance of just over 8 feet from the foundation of the house. I then connected the 4GA copper conductor to the rod with a large copper clamp (1/2" clamp bolt with high strength conductive thread lock on the bolt). Then I used a heat gun to heat up the contact area to a very high temp to drive off any moisture. After heating, I sealed the entire contact area with 2 part epoxy 1/2" thick in all dimensions (built a small form around the connection). Then I covered the cured epoxy joint with a poly sealant in three thick coats.

    Then I routed the 4GA conductor back to the bus bar (sub-surface) and terminated it to the middle contact on the bar.

    Finally, I routed 8GA solid bare copper to grounding clamps/points for the SAT/OTA antennae and leads and routed the terrestrial cable ground wire and AC mains ground wire to the bus bar (I used a chemical copper cleaner to dress each old wire before terminating them).

    I have NEVER had a single problem with hum again...

    NOW, some geographic locations make it very difficult to achieve a good earth ground... If the water table is more than 8' down or the soil moisture content is very low, you may have a problem getting the earth ground resistance down to less than 1 Ohm (mine is currently 0.3 ohms on my longest run). There are ways to address this... If you have a very deep water table or some other soil condition that makes earth grounds difficult, post back and I'll help you design a fix...

    Hope this helps...
     
  8. Jim Noble

    Jim Noble Extra

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    Thanks for the magic box suggestion, Jeff. I have already put a Jensen transformer on my cable feed, which eliminated hum from that source. Unfortunately, all of the cable in-line ground transformers block the DC return to the dish which is needed for DirectTV to function.

    Clay, you are a man who knows grounds! My dish is not grounded, I am looking into doing it (the downlink cable runs to a powered and grounded splitter/amplifier). We have a very high water table and grounding the house current has not been a problem.
     

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