Most Home Theater users have experienced a "Ground Loop" at some point, even if they have no idea that's what it's called. A Ground Loop is garbage voltage which travels between the devices in your system -- over the cable shields of the cables connecting them -- looking for a path back to Ground so that current can flow. In the process, it can cause havoc with various different parts of your system. The most common symptom is an annoying "Hum" from your Subwoofer.
Getting rid of a Ground Loop can be a trying experience -- made more so if you are operating in the dark about what's going on. So in this post, we'll talk about Ground Loops: What they are, where they come from, and how to squelch them.
As mentioned in my post on XLR Cables, better cables are SHIELDED cables. That is, the cable includes an electrically conductive shield, running the length of the cable inside its protective wrapper, and completely surrounding the signal line(s). The purpose of the shield is to catch external Interference sources before they can reach the wires inside the cable and alter the Audio or Video you are trying to transmit. (For higher frequency cables, such as Video cables, the shield also plays a role in limiting emissions FROM the cable which might affect your other gear.)
The shield is connected to the chassis ground of the devices at either end of the cable via the electrically conductive outer part of the cable plug at each end. Thus any Interference signals captured by the shield can be harmlessly dumped to ground without affecting the desired transmission signals.
Note this means the chassis of all the devices in your system are electrically connected to each other via those cable shields built into the cables between them. Devices don't even need to be turned On to be electrically connected this way.
EXCEPTION: Optical (S/PDIF) Digital Audio cables -- see my post on The Great Connector Conspiracy -- include NO electrical connections. There are no electrically conductive wires for the signal itself, and no cable shield conductor either. The optical fibers inside the cable are not sensitive to external Interference signals, and thus need no shield. Sometimes you can USE this fact when trying to ferret out the source of a Ground Loop. Because if the only connection between two devices is an optical cable, there can be no Ground Loop current flowing across that cable!
Most of the Interference captured by these cable shields is quite small. You still want to capture it, of course, because the signals you are transmitting are not that large either. But dumping these small Interference signals to ground causes no problems.
However, SOME Interference signals can be quite a bit larger! The most common culprit is Power Line Interference -- at 60 Hz in the US, or 50 Hz in Europe. Power Line Interference is pretty much everywhere, as it is emitted by the power lines inside the walls in your house, and also by sources outside your house such as utility poles and their transformers.
If enough Power Line Interference current flows, it can play various sorts of havoc with your gear. The most common symptom is "Hum" from your Subwoofer -- since both 60 Hz and 50 Hz are audible frequencies which your Subwoofer is particularly designed to reproduce.
Power Line Interference can also show up as horizontal noise lines on your TV! Typically these will slowly scroll upwards as you watch them because the refresh rate of the imagery on the TV is actually slightly slower than power line frequency. (See my post on Digital Video for details.)
It can also cause poor reception of Cable and Satellite TV channels.
In extreme cases, Power Line Interference can even cause problems with networking, and even remote control between the devices in your setup.
TECHNICAL NOTE: Another common complaint in Home Theater setups is "Hiss" from the speakers. This is caused by Radio Frequency Interference, and is not usually related to Ground Loops. I'll likely talk about Hiss Interference in some future post.
I've mentioned several times about Ground Loop voltage looking for a path back to ground so that electrical current can flow.
Indeed it is that flowing garbage current which causes the problems. If there IS NO path to ground, or only a poor one, the Ground Loop voltage present on your cable shields (and device chassis) usually produces no symptoms.
In most cases, this plays a key role in the discovery that you've got a Ground Loop in the first place! Why? Because you add a new device to your system and suddenly you've got problems! For example, you might get the Subwoofer Hum I mentioned above.
It is almost invariably the case that this new device is *NOT* the source of your problems. It is, rather, that the addition of this new device into your set of interconnected gear has finally provided the missing link -- a good path back to ground -- so that the Ground Loop current can finally flow!
This most commonly happens if the new device uses a 3-prong power plug. That 3rd prong directly connects chassis ground on the device to your house ground -- assuming your power outlets are properly grounded. It is an ideal path for current to flow if that garbage voltage is already present on the device's chassis.
There are adapter plugs out there which allow you to plug a 3-prong plug into a 2-prong power outlet. (These also include a wire which is connected to the 3rd prong, and which you are supposed to attach to house ground -- but most people ignore that or even snip it off.)
If you add a new device with a 3-prong power plug, and suddenly discover you have classic, Ground Loop symptoms -- such as the Subwoofer Hum mentioned above -- one simple test you can do is to use one of these 3-prong to 2-prong plug adapters (with the extra wire left unconnected). If the problem goes away entirely (or even just mostly) you have solid evidence there is a Ground Loop in your system!
IMPORTANT NOTE: Such 3-prong to 2-prong adapters are a useful tool for confirming the presence of a Ground Loop, but they should *NOT* be used as a cure! Proper AND SAFE operation of your devices with 3-prong plugs requires that the 3rd, grounding prong be properly attached. You might use an adapter like this temporarily to deal with the annoyance of a Ground Loop, but you should set aside time as soon as possible to fix the REAL, underlying problem. Seriously: Ground Loops represent problems which should be FIXED, not merely concealed.
TECHNICAL NOTE: 2-prong power plugs can also be "polarized". You may have noted that some 2-prong plugs have a one prong which is wider to make sure the plug will only fit into the socket one way around. Sometimes you can "conceal" a Ground Loop problem by using an adapter which lets you reverse that 2-prong plug in the power outlet. As with the 3-prong to 2-prong adapter, all you've done is reduce or eliminate the path back to ground via that polarized 2-prong plug. This is NOT a fix, but, again, it is good evidence you actually HAVE a Ground Loop problem which needs to be addressed.
So you've discovered you have a Ground Loop! Perhaps you spotted problems like Subwoofer Hum. Perhaps you tried the tests with power plug adapters just mentioned. Perhaps your problems only appeared after adding a new device, and you've confirmed the problem goes away if you disconnect that device.
So NOW what?
Well NOW you need to ferret out the source of the garbage current!
First, as stated above, if your problems appeared when you added a new device it is almost always the case that your new device is NOT the culprit. So what ARE the likely culprits?
These days, the single most common source of Ground Loops is garbage current coming into your Home Theater along the cable shield of a Cable or Satellite TV feed line.
Cable TV lines in particular are subject to tremendous amounts of Interference, as they typically run along the same poles carrying power lines. Satellite TV lines may only run from your roof, but they are still subject to the generally present Interference from sources outside your house.
THIS source of Ground Loops is PARTICULARLY easy to confirm. Simply disconnect the Cable or Satellite feed line where it comes out of the wall into your Home Theater room! If the problems go away then you KNOW that feed line is the culprit.
The FIX for such problems coming into your house on a Cable or Satellite TV feed is to correct the grounding of that feed wire at the point where it ENTERS your house. I.e., give all that external garbage an easy path back to ground BEFORE it enters your house.
TECHNICAL NOTE: This is also a safety factor for your Home Theater gear. Although even proper, feed line grounding won't fully protect you from a direct lightning strike, it WILL protect your gear from the surge that might happen if a distant lightning strike puts a voltage spike on the cable shield of that feed. Ground connections for feed lines often fail either because the connections are corroded or because the connection is not made to a good source of grounding. For example, I've seen instances where the feed line ground connection was made to a water pipe, but that pipe changed from metal to plastic before it entered the ground!
Another common source of Ground Loop current is peculiar to more elaborate Home Theater setups. Such setups may use multiple wall power outlets to power all the gear, and may even have separate power circuits wired to the different outlets to insure sufficient amperage.
For example, speaker power amps often draw a lot of current -- particularly when they turn On. So your Home Theater installer may have run a dedicated power circuit for those to prevent tripping the breaker when things get turned On.
Now, the power coming into your house is what's known as 3-Phase power. We needn't get into the details except to note this means there are TWO different ways to connect any given circuit breaker to that incoming house power: Two separate "bars" a circuit breaker might be attached to.
Normally in house wiring, it makes no difference at all which of the two bars are used. But it is possible -- even common -- that the two bars may be at different "ground potential". What this means is that if you have two outlets connected to opposite bars there is a difference in ground level between those two outlets. If you have gear plugged into both, and interconnected via the cable shields I've discussed above, current can flow simply due to that difference in ground potential.
Your electrical contractor typically doesn't worry about this sort of stuff unless you TELL HIM it's a factor. If you DO tell him, it's easy for him to make sure that ALL the outlets serving your Home Theater are connected using circuits breakers attached to the SAME bar. It's even easy to fix after the fact. The electrician simply moves the circuit breakers inside the breaker box so that all the power outlets serving the Theater are matched.
Now, to tell if THIS is the source of your Ground Loop problem you'd need to (temporarily) plug ALL of your Home Theater gear into the SAME power outlet to see if the problem goes away. This may be tricky as you might have the circuit breaker pop when everything is powered up.
The thing to remember is that you don't NEED everything powered up. Ground Loop garbage current can pass through a given device even when it is powered Off. So just turn on the minimum number of devices necessary to demonstrate the problem. Then try the same set of devices with all plugged into the same outlet. You will also likely find it best to power up your devices one at a time, as their maximum draw on that power outlet will likely happen while they are turning On.
So suppose you've done all the above and you've eliminated the Cable/Satellite TV feed and the power outlets as possible sources of your Ground Loop problem. NOW what?
THIS typically means you have a faulty device in your Theater. Some device is likely "leaking" wall power to its chassis. That's being picked up by the cable shields and transferred to all your other devices!
Now this is actually a dangerous situation. Devices are not SUPPOSED to leak wall power to their chassis. The chassis is supposed to protect you so that you don't get a shock if you touch the device. So this is a problem you definitely want to ferret out.
The usual culprit in such cases is your higher powered gear -- which typically means your speaker Power Amps. But to check this you have to go about testing things in a logical way.
Again, remember that it is the cables BETWEEN devices which are carrying the garbage current. And devices don't even need to be turned On for the current to pass through them to the next device. And also remember that it is usually NOT the case that the newest device you've added to your system is the culprit. It has simply provided the missing link -- that good path back to ground -- which allowed garbage ALREADY PRESENT in your set of gear to flow as Ground Loop current.
SO, disconnect EVERYTHING, and then start re-connecting things, one at a time, in a logical fashion, to try to isolate the source of the problem. Remember that you can use optical (S/PDIF) Digital Audio cables to connect a source device, for example, with no chance of Ground Loop current flowing along that cable -- because it has no electrical conductors.
Also realize you don't need to reconnect ALL the cables you would normally use between any two devices during such testing. Any single, shielded cable will suffice to tie the two devices together as far as Ground Loop current flow.
In tricky cases, you may need to connect several different "minimal" combos of your gear to finally narrow down which device is really causing the problems. This takes some time, but if you approach it in a logical fashion, it won't take all that long.
And it IS worth the time! If your Ground Loop is caused by a faulty piece of gear as just described, it is truly important to find that out, and either get that device fixed or replaced.
A final note: There are devices out there sold as "Hum Blockers" which offer to make your Ground Loop problems magically disappear. These typically work by electrically isolating the cable shields carrying the garbage current.
So, for example, a typical, Cable TV Hum Blocker will be a small gadget you install in-line on the Cable TV feed wire. It consists of a "no gain" transformer which passes the desired Cable TV signal while having no actual electrical connection between the cable shields on either side.
It works, but it has a couple problems. First, that transformer likely doesn't pass ALL the Cable TV frequencies equally well. Meaning you'll now have problems tuning in some channels. But more important, it hasn't actually fixed the underlying problem! Your Cable TV feed is still, likely, not properly grounded where it enters your house.
Fancier (more expensive) devices along this line offer to filter away the Interference present between devices in your system. If you've followed the discussion above, you should know by now that Ground Loop problems are not caused by emissions from one device affecting another device. You CAN have problems if you run power lines too close to signal lines, but these are usually best fixed by being more careful how you lay out your cables.
It's a nuisance finding and eliminating the true source(s) of your Ground Loop problems. But it really is best to take the time to do it right, rather than leaving a POTENTIAL problem still in place -- merely MASKED for now -- which might suddenly become a real problem again the next time you add or replace some devices, or cable things differently.