Ground loop problems and how to get rid of them
Written and copyright by Tomi Engdahl 1997-2006
NOTE: The information presented here is believed to be correct and is made available here by the author. The author of this document is not liable for any effect this information or any use thereof may have.
The documents have been used and recommended by many people and are belived to be accurate. So accurate that they have also been featured as GB AUDIO Ground loops DATA SHEET on their web pages (with permission from me).
Basics
The dilemma is that solving "noise" problems is an art within itself. Since it doesn't come up every day, we all have limited practical experience. This has spawned an industry for those who are now specialize in solving noise problems.
A good power distribution system is essential for proper operation of audio system. Professional audio systems just doesn't work well with normal extension cords running hundreds of feet to a stage. Besides the power feed the good grounding of the whole system is essential.
Ground loop is a condition where an unintended connection to ground is made through an interfering electrical conductor. Generally ground loop connection exists when an electrical system is connected through more than one way to the electrical ground.
When two or more devices are connected to a common ground through different paths, a ground loop occurs. Currents flow through these multiple paths and develop voltages which can cause damage, noise or 50Hz/60Hz hum in audio or video equipment. To prevent ground loops, all signal grounds need to go to one common point and when two grounding points cannot be avoided, one side must isolate the signal and grounds from the other.
The bottom line is that a perfect "quiet" ground does not exist. The basics of all noise problems on the grounding system boils down to what is objectionable current. With the exception of hospital systems, the definition is vague at best. The standard electrical grounding system throughout the building isn't designed to have current constantly flowing through it--and yet it does, you cannot stop it. The reason a ground will not and never be perfectly noise free is that the grounding electrode conductor is nothing more than a long wire from point A to point B. And the longer the wire the more noise it will pick up.
Sound and video people are referring to the type of noisy ground with term like ground loops: current running the equipment grounding conductor, metal within the building, and grounding electrode conductor. Use of any of today's standard 120-volt or 230-volt single-phase AC systems mean potential problems for audio equipment. Computer guys have the same problem in their line of work and so forth.
Usually ground loops are an after-the-fact type of problem in which the end-user blames the installer, the installer blames the manufacturer, and actually nobody is at fault. Neither the manufacturer nor the installer can usually predict where a loop will occur. Only after the system is installed can it be determined if a problem will exist.
Ground loop problems can be corrected and avoided. It is important for the dealer, isntallee and the end user to be aware that this problem can occur. It is a good idea to design the system to avoid most obvious source of this kind of problems, and then be prepared still to face some problems when starting to use the system. A ground loop problem may occur at several points in the system, and each occurrence of the problem must be corrected individually.
Why grounding is so important ?
Earthing of electrical systems is required for a number of reasons, principally to ensure the safety of people near the system and to prevent damage to the system itself in the event of a fault. The function of the protective conductor, or earth, is to provide a low resistance path for fault current so that the circuit protective devices operate rapidly to disconnect the supply.
The NEC, National Electrical Code defines a ground as: "a conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth." When talking about grounding it is actually two different subjects, earth grounding and equipment grounding. Earth grounding is an intentional connection from a circuit conductor usually the neutral to a ground electrode placed in the earth. Equipment grounding is to ensure that operating equipment within a structure is properly grounded. These two grounding systems are required to be kept separate except for a connection between the two systems to prevent differences in potential from a possible flashover from a lightning strike. The purpose of a ground besides the protection of people plants and equipment is to provide a safe path for the dissipation of Fault Currents, Lightning Strikes, Static Discharges, EMI and RFI signals and Interference.
Improper grounding can create a lethal hazard. Correct grounding is essential for correct operation and safety of electrical equipments. Grounding can solve many problems, but it can also cause new ones. One of the most common problem is called "ground loop".
What causes the humming in audio systems ?
Audio and video systems need a reference point for their voltages. Generally referred to as common or ground, although it may not be actually connected with the earth, this reference remains at "zero volts" while other signal voltages "swing" positive (above) and negative (below) it. Physically, the common may be a wire, a trace on a printed-circuit board, a metal chassis, virtually anything that conducts electricity. Ideally it should be a perfect conductor, but in any practical system it is not. As the complexity and size of the system is increased, the imperfect conductivity of the common (ground) conductor inevitably causes problems.
Hum and buzz (50Hz/60Hz and it's harmonics) occur in unbalanced systems when currents flow in the cable shield connections between different pieces of equipment. Hum and buzz can also occur balanced systems even though they are generally much more
The cable shield currents and ground voltage differences are caused by several mechanisms. The second most common source of hum and buzz is the voltage difference between two safety grounds separated by a large distance or the voltage difference between a safety ground and an "Earth" ground (such as a grounded satellite dish or cable TV source). This problem is usually called "ground loop". This is the most common one in severe humming problems.
Hum and buzz can also be magnetically induced or capacitively induced directly into signal cables. Or the noise current can leak from mains input through capacitance between the A.C. power transformer primary and secondary windings which causes that a portion of the A.C. line voltage will ALWAYS be capacitively coupled directly to audio circuit ground. This capacitivly coupled power line signal will usually contain significant harmonics out to 1MHz or more. These signals will cause currents to flow in the cable shields thus adding this noise directly to the audio signal.
Why grounding without problems is do difficult ?
Virtually all data and broadcast construction projects run into problems of grounding. These problems occur primarily because there is a conflict between issues of safety (ground- ing to prevent electrical shock) and electronic noise reduction (using "ground" as an electronic "dump" for noise and inter- ference.) These two uses are often not compatible and can sometimes be in direct conflict with one another. The ultimate purpose of good groundind scheme is he preservation and adherence to the safety aspects while obtaining the maximum noise reduction possible. That is not usually an easy task to do.
Why ground loop is a problem ?
Ground loops are a mystery to many people. Even college-trained electronic engineers may not know what ground loops actually are. Engineers have either concentrated on power distribution (for the electric company) or on equipment that happens to plug in to the power distribution system. Not much thought has been given to power distribution and equipment as a single entity where ground loops arise.
Ground loops are the most common cause of AC line frequency hum in sound systems. Ground loops can be geenrally identified by a low hum (60Hz in the US, 50Hz in Europe) through the sound system. A ground loop in the power or video signal occurs when some components in the same system are receiving its power from a different ground than other components, or the ground potential between two pieces of equipment is not identical.
Ground loop is a common problem when connecting multiple audio-visual system components together, there is a good change of making a nasty ground loops. Ground loops commonly cause humming noise to audio signals and interference bars to picture. Ground loop makes the system sensitive to pick up interference from mains wiring which can lead to erratic operation of the quipments or even damages to the equipments. Some articles claim that wiring and grounding problems account for up to 80 percent of all power quality related problems related with sensitive electronic equipments like audio/video systems.
The audio/video and power industries have each designed their systems and equipment independently. As a result, there's a degree of incompatibility. Usually which is pwefectly adequate in power distribution and operation safety sense is not good enough for AV-systems. Ground loop interference problem is a consequence of this.
Always when operating with grounding issues remember that there is no absolute ground. There is a certain amount of resistance to electrical current between all grounding points. This resistance can change with humidity, temperature, connected equipment and many other variables. No matter how small, the resistance can always allow an electrical voltage to exist across it when there is any current flowing between those grounding points (and there is some current flowing almost always).
Audio-frequency groundloop problems are typically in the low millivolt range, so it does not have to be much interference in grounding system to cause problems in audio systems.
Remeber that there is no absolute ground. There is a certain amount of resistance to electrical current between all grounding points. This resistance can change with humidity, temperature, connected equipment and many other variables. No matter how small, the resistance can always allow an electrical voltage to exist across it. The ground wires between wall sockets and power company transformers are not perfect conductors and neither is the shield of your coaxial video cable. If they were, ground loops would not be a problem. Effects of ground loop in video pictures are in the form of a black shadow bar across the screen or as tearing in the top corner of a picture. This is caused by different earth potentials in a system.
