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post #1 of 166
Thread Starter 
HUM FAQ, by Bill Lummus (Beta version)

Does your system hum? Here is some basic information and a troubleshooting algorithm

How do I figure out what is causing the problem?

1. Disconnect everything from the amp except the power cord and the speakers. Do you still have the hum? If so, see number 2

2. Disconnect everything from the preamp but connect the preamp to the amp and plug them both into the same outlet. Do you have the hum now? If so, see number 2

3. One by one connect each source to the preamp. When you hear the hum you’ve localized the problem. What is most common is that when you connect a source that has any connection to the video system (like a DVD player connected to the TV or a cable box) then the hum will appear. Go to 1.

NOTE: optical interconnects are immune to ground loops and EMI/RFI.

1. Ground loops

Most common in the home theater environment is a ground loop. What is a ground loop? Simply put, it is when the ground currents have more than one path from the component to earth ground. This is most severely and most commonly cause by having two separate grounds. One main electrical house ground and one for the CATV, antenna, satellite, etc… This can result in a safety hazard at worst and some nasty current flow and noise at best. Usually this is manifested as a low pitched 60 hz hum.

Tuner box----------------------CATV/SAT ground
DVD Player
Amp------------------------------House Ground

You can see that if the two grounding points are at different potentials you will get current flow through the system and noise.

Systems with a properly unified ground can also have ground loop hum simply because the impedance of the different roads to ground are different.

Different equipment can have dramatically different sensitivities to ground loop current. Very powerful amps, amps with high input sensitivities, amps with high input impedances, and systems with very sensitive speakers will all be very sensitive to ground loop hum. Other types of systems may not be so sensitive. This is why a system which has never hummed before can hum when a component is added or changed. It may be that a ground loop was created, or it may be that an existing ground loop is now noticeably audible.

The solution?

A. The best solution is to unify the grounds in your system. First and foremost be sure your CATV/SAT/ANT is properly grounded to your main house ground. You can find your main house ground next to your utility meter. It’s a copper rod driven into the ground and it has a large gauge copper wire connected to it. You can run a wire from the splitter for your video source to this grounding rod. All the ingredients are available at your local hardware store, Home Depot, or Lowes. Frequently this will solve the hum. If you don’t feel like doing this yourself, call you cable or satellite installer. It’s code- they are supposed to do it correctly.

It is important for safety reasons to have all the grounds bonded together. Say, for example, a bolt of lightning struck the earth near your service entrance. This is going to cause a large voltage surge at the ground. As long as everything is connected and bonded to that ground- no problem. There will be no difference in voltage and no current over the systems in your house. But if you have a CATV or satellite dish ground somewhere else you have a problem. That ground is going to be at a dramatically different potential than your main house ground. Guess what those grounds are connected through… your equipment! Now you have a surge of up to 6000 volts traveling from one ground site to another through your house and equipment. Equipment damage is likely and you or your family could be injured or killed. “Cheater plugs” are OK for troubleshooting and testing, but should not be used for a long term solution.

B. Now that everything is grounded properly we can proceed to other possibilities. Remember that ground loops can propagate even if everything is grounded properly. All that is needed is more than one path from your equipment to the grounding point. One solution is to plug everything into the same outlet. If you demand on having more than one outlet for your system, you may need one of the ground loop isolators. These are isolation transformers which break the ground loop without affecting the sonics. There are several available. Probably the most famous are from Jensen transformer and they have excellent white papers about proper grounding and system hum available on their website.

C. It can happen sometimes that a particular component (audio or video) will dump a lot of noise on the safety ground. If you connect everything back one by one and, for example, when the TV is brought into the loop the hum appears. But you disconnect the CATV/SAT from the wall and the hum persists then this is probably the case. See B above for solutions.

2. You could be having a problem with EMI/RFI, you could have a power problem, or you could have a defective component.

EMI/RFI usually has more of a hiss quality than a deep hum.

A. Use shorting plugs on every unused input. EMI can enter your system over unused RCA or XLR inputs. You can buy shorting plugs at various location, or you can make your own by soldering the center pin to the grounding pin on cheap RCA connectors. If you need to short unused XLR plugs, check with the manufacturer. DO NOT SHORT THE OUTPUTS- you will damage the equipment.

B. Be sure to use shielded interconnects. Unshielded interconnects are very susceptible to ambient noise.

C. If you do all this and it still doesn’t help, next is to troubleshoot power problems. Unfortunately, there is no easy way to do this. A lot of household appliances dump noise on the power lines. Most notorious for this are halogen lights and dimmer switches- both commonly used in HT construction. You can take the equipment to another part of your house and hook it up to see if the problem persists or you can just carry it down to you local HIFI shop and have them plug it in, If the hum goes away with the equipment in another location- you have a problem with your power source. Power conditioners may or may not help. You may need an isolation transformer, balanced power, or a power regenerator.

D. If the hum is there no matter where you plug it in, you could have a defective component. Take it back to where you bought it or send it back to the manufacturer to be tested. You could also have a very sensitive system and just be hearing the noise floor of your equipment. This is a more common problem with 100+ dB sensitivity speakers.

If you still can’t figure it out, post a message.

This is a beta version, please send me a private message with suggestion for improvement, do not post them here!
post #2 of 166

The most common ground in an A/V system is the third wire in the power circuit, which ties back into the main distribution panel. On mine it and the neutral (white) on 120V circuits both meet at the neutral bar.

Should the neutral bar tie directly to the house ground stake?

How does one prove that to be a solid connection?
post #3 of 166
I don't get it.
First you disassemble the entire system.
Then, if the amp still hums I add the preamp?

Why not start by:
Disconnect the video (cable, sat, etc).
If the hum goes away, the cable needs to be grounded.
If not,
Disconnect the sub.
If the hum goes away . . . is the sub on a different circuit, if so we found the problem.
If we still have hum and it is not from the two most common sources,
Turn off all the sources (DVD player, cd player, etc.).
If the hum is gone, turn them on one at a time until it comes back.

If there is still hum with all the sources turned off, then disconnect them.
If the hum goes away, add them one at a time until it comes back.

Otherwise, if there is hum with only the pre and power amp connected:
disconnect the pre. If the hum goes away, it is the preamp.
If you still have hum, the amp is bad
post #4 of 166
Thanks for this Bill. Much appreciated:


Very timely for me .....
post #5 of 166
Hey - I have a halogen lamp that makes my amp humm when it's in the dimmer mode (at full lighting, everything is fine - no hum). Would one of those Tripplite Isobars w/ their isolated power banks be able to correct this?
post #6 of 166
Just a quick note on ground loops caused by cable. If you do not want to wait, or you cannot get your cable company to come and ground your cable properly, it is very simple to do.

First you should have one of these where you cable comes in to your house (usually where all utilities, electric, telco, etc.) come in. If you do not, then get one. Next you will ground this to your house ground (cold water pipe or rod) you will see your telco and electric breaker box attached to a place on the cold water pipe or rod, just tie into one of these or get another clamp. That is it, you are done.

If your cable is grounded but you do not see where it is grounded to, just change it to the same place as the rest of your house, no more ground loop.

I know this was stated above, but I though I would help out just a bit. I have included a diagram for those that would like.

As for halogen lamps, move them to a different room on a different circuit or get a different lamp. If you have AC buzz, either run a dedicated circuit, or if that is not possible, try an isolation transformer.
post #7 of 166
So the regular Tripplites won't cut it, huh? Guess it might be time to find a better halogen.
post #8 of 166
I have my doubts as to if they will take care of it fully, they may help. I moved my halogens to a different room. I have not found a halogen that did not add AC noise. Chokes are not as good with extremely noisy devices as these lamps are, isolation of the electronics from the noisy source is they way to cure it. The isobars use chokes and do not truly isolate. You may cut it back some, but I think you will still hear it.

Time to get a non-halogen.
post #9 of 166

Originally posted by DMF

The most common ground in an A/V system is the third wire in the power circuit, which ties back into the main distribution panel. On mine it and the neutral (white) on 120V circuits both meet at the neutral bar.

Should the neutral bar tie directly to the house ground stake?

How does one prove that to be a solid connection?


With some panels, your neutral and your ground can be on the same bus bar. Thus, your ground/neutral bus bar will connect to the ground (rod or cold water pipe).

You can use a simple plug-in circuit tester that will test for ground. You can purchase these at home improvement centers, electronic stores, etc. If the outlet you check passes the ground test and your cable feed is tied to the same ground. You are most likely grounded properly.
post #10 of 166
Thanks Dr Spike! Any suggestions for a replacement lamp? Should I go for say, a flourescent lamp?
post #11 of 166
Fluorescent can cause AC noise as well, although they tend not to be nearly as bad as halogen. I have had fluorescent on the same circuit with no problem. You could try one. Incandescent is always a good choice, always nice and quiet.
post #12 of 166

Originally posted by DrSpike69
You can use a simple plug-in circuit tester that will test for ground. ... If the outlet you check passes the ground test and your cable feed is tied to the same ground. You are most likely grounded properly.

"A "Must" for power tool users and musicians"

Well, I do use power tools...

Yes, I've got one of these, and use it for polarity etc... But how does it check for a relationship between ground/neutral and a water pipe?
post #13 of 166
It will tell you if you have an open ground in what you are testing, if all your outlets come out with an open ground you have an open ground. Best way is to physically look and see if your power main, cable, and telco are connected to the common ground. To see if you are properly grounded you can check the ground impedance, it should be 25 ohms or less.
post #14 of 166
Okay, I can see that. No current flow, no path to ground.

One of these days I'll pull the panel and check neutral bar to water pipe. (I don't have any ground loops or other wierdness, just trying to understand how to diagnose someone else's future problem. Thanks.)
post #15 of 166
No ground loops, sounds like you are in great shape. The panel is easy to come off, just be careful not to touch certain areas, even if you turn the main breaker off, the box is hot. You can also get a good zap if you touch the neutral bar and become the ground for a circuit. For the most part, I have always added circuits with the mains left on. I was taught that way, probably not the safe way. I guess the safe way is to have the power company shut off your power. Unless you have a breaker by the meter, not all places do.

Glad to shed some light on the subject.
post #16 of 166
place a .15uf cap and a 1.5k resistor in parallel across your volt meter leads then check for leakage by grounding the black lead and touching the red lead to metal on the chassis.
post #17 of 166
I've been racking my brain for about a week now trying to get rid of the hum in my subwoofer... Hopefully someone has some ideas.

I have a SunFire subwoofer hooked up to an Integra DTC 9.4.

So far, this is what I've tried...

I've disconnected everything and found that the "hum" doesn't appear until I hook up the cable from my PreAmp to my subwoofer. Then it doesn't go away until I unplug the RCA cable. If I plug the Subwoofer in without the cable, I get no hum. But once I plug in the cable, there's the hum...

Any ideas how to fix this?
post #18 of 166
Thread Starter 
Please don't post on this thread. Start another one for individual issues. This would get overly cumbersome if everyone jumped in.
post #19 of 166
While running a link checker on my PC, I found that certain links no longer existed. Well that's fine as I tend to overbookmark anyways. However a good one regarding Ground Loops was gone. Turns out there's a site that actually archives old sites...well a lot of them anyways. Typing in the old URL into the WayBack Machine resurrected what I think is a pretty good and informative article. If you've got some spare paper on the printer, this is one of those keepers that provides a good background, plans for the GL elimination, and still has some nice, informative graphics to illustrate points. For anyone whose interested, just click here.

Update on other sites that also have information regarding hum.

Ground loop problems and how to get rid of them

Ground Loops or Let Me Hum a Few Bars

FAQ About Ground Loops with Frequently Given Answers

Ground Loops What They Really Are (rescued via the internet archive site)

Balanced Line Technology (rescued via the internet archive site)
post #20 of 166
I noticed a ground hum/loop I dont know what to call it but I notice that when I grab the end of the coax wire that comes in to my house with one hand and the coax wire that goes to my HT with the other I get a little buzz in my hands(these two wires are normally connected via a splitter). If I took the wire that runs to my HT a ran that through the coax inputs on my Monster HTS 5100 would that solve my buzzing problems?

Daniel Smith
post #21 of 166
Thread Starter 
Once again- please don't post specific issues to this thread. Start your own individual thread.

If you have something like Chu added- this is the place.
post #22 of 166
If you want to work in your circuit breaker box but would like none of it hot, you can remove the meter outside of your house.

1) Turn off all sensitive equipment (computers, etc).
2) Go to breaker box, turn off your main breaker.
3) Go to your meter box outside, open it up and remove the meter.
4) Do your work in the breaker box.
5) Once done, double check to make sure that your main is still off.
6) Replace the meter outside.
7) Flip your main back on.

If you have any large loads that are still on while you are working, you should turn those circuit breakers off one-by-one before step 2. And then turn them back on one-by-one after step 7.
post #23 of 166
Is a shorting plug the same as an AC ground floater? Another website (Audiovideorevoluton.com) recommends using AC ground floaters on each of your components until you find the one with the hum, then leaving the floater on that one component. That worries me though, because I thought that AC ground floaters just remove the ground and leave you with an ungrounded connection. Any thoughts on this?

I hope this general query conforms to this thread. Feel free to delete it if it does not.
post #24 of 166
Thread Starter 
A shorting plug and a ground floater are not the same. A shorting plug is most like a jumper between signal and ground. A ground floater is a "cheater plug" which lifts the AC safety ground.

I don't mind so much the use of cheater plugs for troubleshooting purposes, but don't leave them there. It is called the "safety" ground for a reason.
post #25 of 166
What are your thoughts on isolated grounds? Isolated grounds run to seperate grounds? If the seperate grounds are all good deep grounds (8' rods or burried copper water mains) in different places around the house perimiter does this present a potential difference and a pos. hazard as in the lightning strike scenario? Should these seperate grounds be bonded together if they are used? Does the point of connection to the ground system matter? ie. Connecting cable ground block wire to an outlet ground with your av equipment vs. running it back to the ground rod.

Thanks for this thread.
post #26 of 166
Thread Starter 
All grounds must be bonded together. It's code. It's the law. Once they are bonded together it doesn't really matter where they are, as long as there is one at the service entrance where N-G bonding occurs.

Yes- there is a risk to you and your equipment if this is not done properly.

Re: CATV grounding. Always be sure there is a grounding block at the service entrance that is properly grounded. As long as this is present feel free to connect additional grounding blocks as you choose. I would connect them to the chassis ground of your equipment rather than an outlet. If the wire inadvertently connected to the hot wire from the outlet you'd have a problem!
post #27 of 166

Originally posted by Bill Lummus
All grounds must be bonded together. It's code. It's the law. Once they are bonded together it doesn't really matter where they are, as long as there is one at the service entrance where N-G bonding occurs.

It goes a loooong ways towards eliminating ground loop(hum) problems also.

post #28 of 166
If the cable company came to your house and put a ground rod in the ground and that is it, that is not considered a ground. If they came to your house and split bolted to your utility ground or a pipe clamp to your water main that is a true ground. But if a sat installer or a cable installer ever come to your house and put a copper rod in the ground it is not a ground. Tell them to get back out there and fix it. The only exception to this is if they run a #8 bare wire underground and no more then 20 ft to the main ground or else they have to ad another ground rod and go another 20 ft or less until they reach the utility ground.

post #29 of 166

If the cable company came to your house and put a ground rod in the ground and that is it, that is not considered a ground.

Not when it is a 4' rod no matter WHO installs it!

A eight foot rod, buried away from the house eight feet below ground as far as possible (at least a foot) with the heaviest gauge cable you can muster is a REAL ground!
post #30 of 166
I have had a passive subwoofer for sometime but haven't used it as I have not had a spare amp to power it. However, I have recently got my hands on a power amp which is powerful enough to do the job. The problem is that I get a hum is the rest of my speakers when I connect this amp to the sub pre-out on my av amp. This occurs only when a power supply is connected to the power amp even when it is pluged into the same outlet as the av amp. Does anyone have an ideas as to a solution or should I get another power amp.
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