Ground loop problem

[howiez]

I could use a new perspective on ground loop isolation. Prior to yesterday, I had a Klipsch powered sub in a corner 20 feet away from my audio system. I had in wall RCA cable run to that corner via the basement. I had occasional ground loop hum but very subtle, and used an in line ground loop isolator you'd use for a car stereo, and it eliminated the problem 100%.I know normal convention is to keep your source, signal processing and amplification at once spot, and use a common outlet (or balanced cables-no pro sound gear ).

I built a BFM THT to replace the Klipsh, and it worked great in that corner, however the eventual goal was to move the THT to a side wall turning it into an end table, (and build a sibling some day for the opposite side wall for room modes).

I purchased and ran a new RCA to the new location, and now have TONS of hum, more than ever before at new location. I swapped cables, with and without Isolator, and got nowhere. I finally plugged the sub amp into the same Panamax power center all my AV equipment is plugged into, and still get the hum. Shouldn't that common ground eliminate that problem? I'm pretty sure both outlets where subs are plugged in are on the same breaker so what gives?

[Bill Fitzmaurice]

Shouldn't that common ground eliminate that problem?

No. The amount of hum is determined by the length of wire connecting the grounds of the various devices in the system. What's worse is that hum increases by the wire length squared, so twice the wire equals four times the hum, and four times the wire equals sixteen times the hum. That's why the best place to put gear is all in the same area, so that the ground connections are as short as possible. Even then you may need to use isolation, especially if there's a TV cable in the system, as it usually will connect to ground at the AC service entrance, and that can amount to a lot of wire.

[dswpro]

Hum is not always "ground loop" though it is a likely culprit when running wires all over the place. Other sources include :

Induction -- your shielded cable is running right next to or coiled up very close to a magnetic field or power cord.

Bad Electronics -- Something in your amp or elsewhere is malfunctioning and producing 60 Hz hum as a result. which you are attributing to ground loop hum.

Bad Cable -- you sure the shield on the cable is not severed somewhere? could it be frayed? If you "ran" it in a wall could it have chafed on something exposing or even breaking the shield?

Long Run -- Even Shielded cables are susceptible to induction hum, especially unbalanced ones. Runs over ten feet are frowned upon. You could consider a pair of direct boxes and a balanced shielded XLR cable.

I would run everything in one place using whatever wire you need to rule out faulty gear or wiring, Then move things into position using any "built-in" wiring.

[byacey]

Is the powered sub a 2 wire power cord, or 3? If only two, it's likely not a ground loop that causing the hum, as there isn't a secondary ground reference.

[howiez]

Thanks everyone for the quick replies.

Everyone hit on one aspect of the puzzle. It is a long cable for sure, I had hoped being a bit better shielded it would be clearer than the old. Apparently the 12 year old cable was made better! It is a 2 prong plug so no dedicated ground on the sub amp, either of them. Using a shorter cable didn't seem to have any effect, 25 ft vs. 50. I didn't try anything under 12ft, as It wouldn't work in my space anyway. Strangely enough, I stumbled upon a near solution. It turns out the old cable did indeed get chaffed in the basement rafters. It had been pulled too hard one way and draped across a duct and the metal strap holding it. That 'cut' had actually been grounding the wire 'most' of the time. That explains why I had only 'some' issues with ground loop and my isolator cleaned up the rest. The funny part is, I removed the isolator and plugged my 50ft directly to my Behringer DSP1124P and the awful hum went away. I still can hear a faint hum if I put my ear in the mouth of the THT, but under normal use we'll never notice it. I'll search for a better shielded cable, and maybe one 35ft will fit.

[byacey]

If you want to make your own cabling, RG/59 miniature with a copper braided shield is excellent for this purpose.

[howiez]

If you want to make your own cabling, RG/59 miniature with a copper braided shield is excellent for this purpose.

What about RG6, the only reason I ask is I have a 500 ft spool? OK, maybe it was 250 ft new, but for sure I have enough left! I had wired the house 12yrs ago when I moved in, as the previous owner bought some RG( zero?) and screw on F-connectors. You couldn't get a single channel without noise on any TV in any room in the house.

*edit* now that I think about it, mine might be aluminum braid, I'll check. Either way, sounds like a project, I can get the length I need, no more and better shielding. THX!

[byacey]

Aluminum shield is no good, it can't be soldered and it's higher resistance compared to copper. Also, RG-6 is pretty large to be cramming into RCA or 1/4" connectors.

[billkatz]

Aluminum shield is no good, it can't be soldered and it's higher resistance compared to copper. Also, RG-6 is pretty large to be cramming into RCA or 1/4" connectors.

The difference between aluminum and copper resistance is negligible. If you want to use the RG6, crimp F connectors on it and get F->RCA adapters.