Ground Loops - Eliminating System Hum and Buzz
You've just connected your system and there's a buzz or hum that won't go away. You're running your gear through power conditioners and you're beating your head against the wall trying to figure out what's up. Congratulations - you've just entered The Ground Loop Zone..
Several weeks ago I was pulling my hair out after I installed a new component into Reference System 3 for review. It was an amplifier that came with a three-prong power cable. Immediately after placing the amp in my system a very noticeable 60Hz hum starting pouring from my speakers.
If this has happened to you the chances are it's a ground loop between your Cable TV and another component in your system (like an amplifier or powered subwoofer). Now, how do you solve it? First it helps to define exactly what a ground loop is and how it may affect our home theater system.
Editorial Note on Ground Loops
When two or more devices are connected to a common ground through different paths, ground path noise, or a ground loop can occur. Thus, a system grounded at two different points, with a potential difference between the two grounds can cause unwanted noise voltage in the circuit paths. Currents flow through these multiple paths and develop voltages which can cause damage, noise or 50Hz/60Hz hum in audio or video equipment. The ground loop can be eliminated in one of two ways:
- Remove one of the ground paths, thus converting the system to a single point ground.
- Isolate one of the ground paths with an isolation transformer, common mode choke, optical coupler, balanced circuitry, or frequency selective grounding.
The most practical and usually most cost effective method for consumer audio applications is to use an isolation transformer. An isolation transformer is a device which, in the case of cable signals, allows all the desired signals to pass freely, while interrupting ground continuity, hence breaking ground loops. By using an isolation transformer, the ground noise voltage will now appear between the transformer windings and not circuit input. The noise coupling is primarily a function of parasitic capacitance between the transformer windings and can be reduced by placing a shield between the windings. This is an effective method to implement assuming the transformer has sufficient bandwidth, isn't too costly or bulky, and a direct DC signal path is not required for the application.
Diagnosing and Troubleshooting the Problem
To accurately determine the correct solution to a problem you first have to find and isolate it. For example, if you simply start flailing away, swapping gear and cables and everything all at once, you may never know what actually caused (or fixed) the problem. In addition, you may end up making more and more work since you are expending energy in areas that don't have any effect on the problem at hand.
Start simple. Troubleshooting ground loops involves taking things in order and checking a few basic, common elements to see if the problem is simple, or complex. For example, if adjusting the volume on your processor/receiver does not alter the hum level, then the problem must be occurring after that point. It if occurred prior, then the receiver/processor would typically raise the overall level of noise. Make sense?
Work in the following methodical manner:
- Start with the processor receiver to determine if the hum/buzz is source-related or due to a ground loop occurring after the amplification stage
- Note any recent changes to the system that brought on this problem. Chances are, you can more easily isolate a problem if it just starting with the addition of a new piece of equipment.
- What can you do quickly and easily to isolate or identify the problem and point to the proper solution (i.e. unplugging the cable from the wall to see if the Cable TV is the source of a ground loop.)
One other test to eliminate your receiver or processor is to see if the hum changes based on what input you have selected (DVD player, Cable TV, etc.) Does the hum change or go away when selecting a different input? No? Then your problem is occurring at a later stage in the system (most likely a ground loop caused by the addition of an amplifier or powered subwoofer with a 3-prong power cable.)
The final test is to unplug your Cable TV cable from the wall. Does the hum go away? It did in the case of Reference System 3. Eureka ! There must be a ground loop in the system involving the Cable TV line.
Other Common Causes of Hum and Buzz
While this article addresses a very common ground loop problem, realize that there are plenty of ways in which system buzz and hum can enter into your home theater setup.
Common Problem #1: Check to see if you have a heavy power cord or an outlet in the wall that is worn out and will not grip. If the hot/neutral/ground prongs on the plug the ground are making intermittent or light contact with the tang on the inside of the outlet, it can cause a hum through the system. The best solution for this is to replace the outlet with an industrial version, available at Home Depot for about $4. The industrial outlets have better gripping and will hold power cables more securely. If you are installing a front projection system ceiling mount this outlet is a must.Common Problem #2: Check the polarity of the outlet - it may be wired backwards. You can get a polarity checker at Home Depot for about $5. This is one of the first things you may want to check if unplugging the cable box does not remove the hum (and in some cases, the reverse polarity may still be the culprit.)
Common Problem #3: Light dimmers, flourescent lamps, and other appliances that share the same circuit or common ground with your home theater equipment can cause hums.
Fixing the Problem
There are at least two practical ways to fix the problem of a ground loop in your system. Once you know the problem is related to the Cable TV ground and the amplifier ground, as it is in this case (and lots of cases) you can lift the Earth ground on either device at the line level. I find it much easier to lift the ground on the Cable TV line than on 5.1 multi-channel inputs going into the amplifier!
NEVER use a three prong to two prong AC adapter to fix a ground loop problem. These devices are meant to provide a safety ground (via the cover plate screw to a grounded outlet) in the event a three prong plug is used with a two prong outlet. It is always best to lift the ground safely at line levels.
Editorial Note About Signal Ground Lift Method
One can try to use a ground lift in situations where two grounded pieces of equipments with unbalanced connections experience ground loop related humming problems. Ground lifting in unbalanced connections works effectively only when both pieces of equipment are properly grounded to same point. In some cases the humming problem may become worse if a ground lift is used. Thus, this so called "fix" should be employed with extreme caution and usually only as a temporary solution. If the related equipment is properly grounded, simply lifting signal ground between equipment, may cause enormous amounts of humming and potentially damage the input amplifier of the receiving equipment because of the flowing stray currents on the ungrounded equipment. A better method of employing ground lift is to modify the cable to include an AC path between grounds, or a small capacitor. This will reduce the possibility of the ground lifted cable to pick up RF interference, but can also cause frequency response variations depending on capacitor size and equipment source impedance. Because of this, we feel the best solution for solving unbalanced connection ground loops is using audio line isolation transformer.
Use a Cable TV Ground Isolator
The most common, and easiest, solution is to add a Cable TV ground isolator. Jensen Transformers has been around for over 30 years and makes one of the best (MSRP $59.95) as it has a flat frequency response from 2MHz - 1300 MHz, spanning the VHF/FM/UHF/CATV spectrum. Why is this important? Well for starters if you plan on using digital cable, a cable modem, or on-demand services you better not get a cheap RF filter from the local electronics store as it will most likely filter out more than you bargained for. In addition, we respect a company who measures their products and is willing to post a frequency response performance graph to back up their claims. Getting a flat frequency response from 2MHz - 1300MHz ensures no loss of signal quality and a great result.
Cheap Solutions that Sound Too Good to Be True - Probably Are
One of the most widely popular methods of eliminating ground loops is to take a 75-ohm to 300-ohm converter, connected to a second matching transformer with two screw terminals on the 300 ohm side, and putting it inline with your cable TV feed. While this might break the ground loop, unless you merely subscribe to basic cable and don't give a hoot about signal quality we'd recommend avoiding this bit of McGuyver magic.
Are We There Yet?
In a word - yes. For around 80% of ground loop problems this is the cure. For those other problems there are other solutions, but we wanted to cover this often-encountered scenario and outline some quick and easy solutions to getting yourself out of the ground loop blues. If you have a ground loop caused by your cable TV run , don't walk, and pick up one of these beauties - you won't regret it.