Axiom Home Page Forums General Discussion Home Theater Grounding and hum

I read an interesting article in MIX magazine this morning and found it on their website: Mix Interview: Arthur Kelm (A WELL-GROUNDED APPROACH TO AC POWER PROBLEMS).

Although his background is more in recording studios, the information would be helpful to those with hum and grounding issues in their HT.

So, since you can't solve your square room problem, you're now solving problems you don't even have.

Rock on, Mark.

I was a little miffed when I got to the $4500 Fluke meter as the beginning of a solution to bad wiring; I'll take the band-aid approach for that kind of coin, thank you very much.

Then I read on and really liked the 220V step-down transformer to 6 outlet idea. Boy, could I have used that that when rewiring. I could have had the entire AV room on a single, isolated circuit...

Oh well, so far I'm passed the hum and buzz portion of the process, so I'm not going to go back and retro-fit.

Scott

Since we are on the topic...

What level of hum is considered a real ground loop problem? Since I've introduced a separate amp (Newcastle AM-9080) I've noticed a hum at the speakers that wasn't there before when just using my AVR (Onkyo 806). I tore my system apart to isolate the issue last night and traced it back to a few sources.

1. Coax digital input from HTPC to AVR (now acting as a pre/pro).
2. HDMI input from HTPC to AVR (yes, there is a common well-known theme here baout PCs and EMI/RFI)
3. HDMI input from TivoHD DVR.
4. HDMI output to TV/Projector

Now, the hum is really low. You can only hear it if you are within about 6" of the speakers (M60s). When I remove the HTPC connections and the coax from the DVR the noise level drops to almost imperceptible but barely there. I've already ordered a Cable ground loop isolator, and will replace the coax digital from the HTPC with an optical connection, but there really isn't that much I can do about the remaining HDMI connections.

So, what is considered a ground loop problem? A loud (audible from a distance >1 ft.) hum or any hum at all?

It definitely isn't the amp. I tested the noise floor by hooking up an iPod directly to the inputs as well as the AVR with a single optical input connection then standard interconnects to the amp and both test show zero noise...

Thoughts?

Forgot to mention... All my gear in the HT run back to the same ground on the same circuit...

If it’s not bothering you from your closest listening position I wouldn’t worry about it. AFAIK ground loops exist to some degree in all our systems it’s just a matter if it’s loud enough to be a problem.

Based on what you’ve said it sounds like your TV cable my be the worst culprit and it sounds like you’ve already moved to solve it.

I had ground loop problem between my subwoofer and DirectTV antenna that I resolved by routing the antenna coax through my Trip-Lite surge suppressor. Here is a link that might offer some help if not I’m sure someone else will have better advice.

http://www.epanorama.net/documents/groundloop/index.html

Your situation seems to mirror what the article mentioned: A little here, a little there adds up through the chain.

If you have to be 6" away to hear it though, I wouldn't worry about it.

On my pretty busy small-city street, if I have less than 70dB of ambient noise, I'm happy. In my case, I could have a good hum (insert joke here) and never hear it anyway.

Thanks for all the input guys.

Here's an update: I wandered out to the cable drop to check out the grounding on the cable coax. I found the grounding block... and notice that while they hooked up the coax to the grounding block, and ran the grounding wire through it, they didn't bother to screw down the binding post screw!!!! The grounding wire was barely touching the body of the grounding block. So, I tightened the screw and that killed almost all the hum coming from the coax.

I'll probably try the isolator since I've already ordered it to see if it helps any at all on top of that.

I still have to go deal with the HTPC though, but I think the optical connection will help tame that quite a bit. Those PC switching power supplies and HDD motors are pretty noisy in terms of EMI/RFI. If I have to I may migrate the HTPC to a spare laptop... Running directly off of a DC power source will probably clean up the HTPC a lot.

Mark, I wish you hadn't sent that article! This morning I was calling around to a couple of electricians to get quotes on chemrod grounding posts!

They send me kickbacks....

Hi Foobarred,

I'm glad you've mostly solved the hum problem. You shouldn't have any audible hum from your speakers or subwoofer, even listening up close. And ground-loop troubles are almost always traceable to TV-cable inputs.

Those chemrod grounding posts are exotic! New to me.

Regards,
Alan

Alan, I always thought it wasn't at all unusual to have some residual noise when listening up close to a speaker and having the volume up to "listening levels"?

My system is dead silent at listening levels with nothing palying and the system powered up.

My EP500 Sub arrived yesterday, which was good timing because I only just finished installing the QS8s that had arrived with the rest of my Epic 80 system on Monday.

Anyway, I switched on the system last night for the first time with everything connected (which was previously working just fine with only the fronts and center speakers) only to find very loud mains hum coming from all the speakers!!!!

Luckily, I had actually read the sub woofer manual before setting it up, so rather than spending hours isolating the problem, I just removed the Chassis Grounding screw, as instructed, from the sub and that solved the problem!!!

It was funny, I was holding onto the screw when we powered the system up again, and both my wife and I had absolutely no confidence that this would solve the problem.

I'm out of state at the moment, but have to check out my system when I'm home again...

Hi Mark,

With older vintage solid-state gear ('60s and '70s), you'll definitely hear some thermal hiss (not hum) when the volume control is well advanced and your ear is close to the speakers.

You'll also normally hear some thermal hiss on AV receivers on the Phono input (moving-magnet--MM), which adds another gain stage, especially so on a unit that has a moving-coil (MC) cartridge input. The latter adds yet another stage of gain, so unless it's a really expensive low-noise design, you'll almost always have a bit of residual hiss on the MC phono input.

Modern inexpensive stereo or AV receivers may have a bit of thermal hiss when the volume control is near maximum, but you shouldn't have any hum if all the grounding is correct.

Well-designed separates (separate preamp and power amp) should be virtually dead quiet, even at maximum rotation of the volume control and when you're really close to the speakers.

Regards,
Alan

I assume this is without music going through? Otherwise it would become deaf quiet...

Exactly! ha, ha.

I would bet the majority of people with ht systems deal with ground loops, interference or other related problems at one point or another. Ground loops seem very common to me (that is atleast my impression). I had to deal with it after adding a second sub. The hum was coming through the subwoofer and not the speakers.

Also at one point I noticed a moving horizontal line on the LCD only when using the xbox 360. It turned out the computer in the next room caused this when it was on while playing the 360.I moved the computer upstairs and the problem was solved.

As of this moment my system has no "noise" and I hope it stays that way but you never know whats going to happen when adding new equipment in the future. These types of problems can be a real PITA.

Now that you mention it, it's the slight hiss I remember hearing, not the hum. My memory is going quicker than my ears.

Rattle and Hum and all that.

And that memory thing only gets worse.

They say it's the first thing to go. Now what was the second?...

Scott

Alan,

Do you have any advice on dealing with HTPCs? Fixing the poor ground on the cable tv line has almost eliminated that as a source, but the HTPC is definitely introducing some noise (EMI/RFI I'm sure) into the system. I was going to switch to an optical connection for the audio, but there is also a small of noise being introduced by the HDMI connection from the HTPC. Not really sure what the options are here aside from trying a whole different HTPC such as a laptop based system that uses an isolated ungrounded power supply and runs via DC only internally...

Anyone have any ideas?

At this point the hum is around the <15db level (I'm guesstimating). It's really really low, but the idea that it should be completely silent is driving me nuts.

Also, I doubt it's actually an issue, but in term of interconnects from pre to amp... Should I be using RG-59 or RG-6 coax interconnects here? Do I really need quad shielded RG-6 to go the 2 ft or so? Right now the interconnects don't seem to be introducing any noise. I can verify that by taking out all the sources form the pre and connecting my iPod to the Aux in... everything is dead silent even with my current RG-59 based interconnects.

Foo, on your point about the pre-out connections, RG-59 would nearly always be adequately shielded for a typical home environment, even for considerably longer distances than that. RG-6(which may or may not be more heavily shielded)isn't needed.

Hi Foo,

I'd agree with JohnK on the RG-59 as being entirely adequate and your not needing RG-6.

Your idea of using a laptop with the isolated power supply may be a solution, but frankly I'm not experienced with HDMI/grounding issues using a home theater PC in a system.

Of course we all want dead-silent noise-free systems, but from a practical point of view, without a lot of tedious trouble-shooting, it's almost impossible to entirely eliminate all noise from a system.

I think the goal of achieveing no audible hum/hiss from your listening area when the volume is advanced to a level that would normallly generate very loud sound with CD or movie playback is a worthy one and achievable without becoming obsessively preoccupied with eliminating all audible hiss/hum with your ear next to the speakers and volume control fully advanced.

Regards,

Alan