I have experienced the same thing with the 'crawl' on the TV screen, and yes, this is definitely ground-loop-borne. Furthermore, if I turn the gain way up on the 990, I can hear the line noise (60, 120, 180 etc Hz) whereas if the cable is disconnected from my Motorola DVR (but all connections left intact from the DVR to the 990) the hum immediately disappears (completely) and the screen crawl stops. After $inking what I have into amps, speakers, pre/pro, HDTV etc, the thought of just 'living with' the hum and crawl just doesn't cut it.

One simple fix that I have used is to take two 300 Ohn to 75 Ohm 'baluns' (Radio Shack or equivalent) and wire them back to back. That is, solder the two pair of 300 Ohm terminls together, resulting in two 75 Ohm screw connectors. This effectively isolates the grounds but lets the signals pass through. With this in place there is neither screen crawl nor hum from my system...and the fix costs all of about $9.00 to implement.

However...what I have noticed is that there is definitely a loss of signal to noise ratio on the 'lower' tier of cable channels (and regrettably, I seem to have made a rather effective notch filter for channel 7 in the process), though those above 100 or so don't seem to be adversely affected one iota. Fortunately for me, I don't watch those channels much (and more importantly, neither does my wife...), so that really doesn't bother me. Another plus here is that the HD channels don't seem to be affected at all either....so I'm willing toi live with the lower S/N on the channels I seldom watch. Compromises...

Cool thread though...I'll have to check out what some of the others have proposed. Maybe I can eliminate the ground loop hum and crawl, and not lose as much signal (on the lower channels) in the process.

One last bit...can anyone here say how good the proposed isolators are in terms of maintaining signal to noise ratios on aALL channels?

Mark

In my case I had a ground loop problem once with a satellite feed.

There was no opportunity to use the dual 75 to 300 ohm balun trick because the LNB requires power from the satellite STB. Even if one uses a powered multiswitch, for DirecTV systems, the STB uses different DC voltages to select the LNB polarization. Dish systems use DiSeQ switching control which uses a 20 Khz carrier, which would not pass too well through a balun pair. I used a different trick.

At first, I ran a heavier #6 ground lead from the dish ground rod to the main house grounding point, but this only helped a little.

In my case the loop was between the satellite feed and my subwoofer which had a third prong on the power supply. Rather than break this safety ground to break the loop (potentially very dangerous), I build a little extention cord, terminated in a single gang utility box with a GFCI mounted in it. Thus, I could safely lift the ground, and still have some fault protection via the GFCI. This worled well.

I would not encourage others to do this though, unless they thorougly understand what they're doing: there is a danger of the GFCI not tripping even if there is a short to signal ground, which can travel back on an interconnect shield, possibly causing a fire. It's unlikely, but not a risk that exists with a real ground in place. In my case I consider it about as likely as a "real ground" working itself loose and thus gone, so I accepted the risk.

Another possiblity is to break the loop at the output of your cable STB: use optical digital audio interconnects, and video baluns on the video lines (one, two, or three, for composite, svideo, or component outputs). This won't help digital video signals, though, on DVI or HDMI connectors, unless you use a paid of electrical/optical DVI converters, typically used for long runs. That can get expensive. (Even video baluns can run several hundred dollars each, for good ones).

I love it when stuff just works out...

So, I perused this thread and some other sources (including some of the Outlaw manuals) and found a 'reasonably' priced isolator readily available - the Jensen VRD-1FF ( www.jensen-transformers.com ), bought it (around $50 or $60), and took delivery a few days later.

So, I installed it between the service point and the cable TV amplifier / distribution box. I then removed the cheesy home-made variant (that I reference in one of my prior posts in this thread) and connected the cable directly to the Motorola DVR.

Result?

Absolutely ZERO hum and ZERO video crawl. Works like a champ. Also, the lower tier channels that suffered poor S/N at the hands of my cobbled isolator are now much, much lower (video) noise (though I doubt I'll watch many of them anyway).

I should say that theoretically, putting one isolator between the cable service point and the cable amplifier / splitter block (for those who have them) may not completely solve hum and crawl for all systems. This is because (and I think others have pointed this out) all of the cable shileds are tied to one point (the splitter or distribution system), so theoretically, a ground loop could still result by virtue of the cable referencing ground in another part of the house.

I think it's fair to say that no two installations are exactly alike, and this goes for potential found at the ground at a given location. However, I think it's also fair to say that the difference in potential between the cable at the service point and the house ground are considerably greater than what's likely to result from the grounds all being tied together at the splitter or cable TV amplifier / distribution point.

All I can say is that I installed the isolator where I did, and there are three other TVs in the house in various rooms connected to the cable at their repsective locations. None of them have hum or crawl.

So, I'd say try 'going cheap' first by trying it the way that I did. If you still have problems then if you were to install them on each cable send (or at each TV / /DVR / Cable box / audio system) I think the chances are very good that you'd kill any and all hum / crawl at every location (because seperately-isolated feeds would ensure isolation on each cable, and therefore, from each other).

Oh, and the folks at Jensen give you a nice short F to F cable with the isolator, so no trips to the store or having to crimp your own. In less than 1 minute my problem was solved.

Life is good.

Mark
PS: Indeed, this will not work for Satellite systems, but it worked wonders on my cable.

Outlaw Audio has some suggestion about the "ground loop" problem in the manual of their new amplifier 7125 on page 8 and 9.

RW