David J Greaves. University of Cambridge, March 2020.
Under construction ...
Earth loops are ...
There is a lot of nonsence on line about earth loops and audio problems... hopefully this article is completely accurate ...
For instance, John Hull's popular article gives the cause of earth loop problems as "Even though they’re wired to a single breaker box, sharing a common ground, the voltages at those two outlets will be slightly different from one another (say, 115V vs. 122V)." But the line voltage is not relevant.
First a note on safety.
Mains-operated equipment can develop a fault and then deliver a fatal electric shock. Residual current disruptors (RCDs) greatly increase safety, preventing most shocks from being fatal, but they cannot be relied on as an alternative to proper first-level safety mechanisms. The two first-level safety mechanisms are double-insulation or protective earthing. Protective grounding can also be known as bonding and grounding, but we will use the word `ground' differently in this article. The word bonding is used where different conductors are joined together, such as where neutral and earth are connected at a power substation that serves a number of properties, or where gas and water pipes are connected to mains earth at entry to a property. For the purposes of this article, where the ground and the earth are connected to each other inside a piece of equipment, this could be called bonding, and this is what is broken by an earth-lift switch. But probably I won't use the word bonding here.
Any mains equipment that has exposed metal parts and is not marked double-insulated (normally with the concentric box logo) must be earthed. When earthed, the exposed metal parts must be connect to mains earth, as provided on a 3-conductor mains cable. When double insulated, a 2-conductor mains cable is always used and at least two layers of insulation must be present between exposed metal (such as loud speaker terminals or RCA phono socket shields.
In this note I shall use the term earth to denote the mains protective bonding (earth pin on a mains socket/outlet) and the term ground to denote the reference voltage for a power supply or single-ended audio jack. In a floating device these are entirely disconnected within the device. For a double-insulated unit, there is no earth connection. Most other devices join these two together at a carefully-chosen point. An earth lift switch, if present and operated, opens the earth to ground connection.
Within a building, earth loops potentially create audio problems for two reasons: 1/ poor power supply design in one or more pieces of equipment, and 2/ induced current as magnetic fields cross different parts of the loop with different field densities.
Creating an earth connection between different buildings or different parts of a large building typically occurs as part of the mains provisioning or by other piped utilities. No installation should ever create an earth connection between separate consumer units (fuseboards) on the consumer side. This can upset protective ground provisioning and the installed low-power electronics can be ruined by excessive ground currents during lightning storms.
A common mistake in equipment power supply design is to induce a voltage between the ground connection on the audio connectors of the equipment and the protective earth of the mains supply to the equipment. With linear supplies, a heavy mains ripple current occurs in the secondary circuit between the transformer and the smoothing capacitors. This has 100Hz frequency (120Hz in USA) and is called hum. A poor wiring pattern will couple some of this current into the ground-to-earth path. When connected to another component that induces zero or a different voltage in this path (it will tend to be in phase at least) then hum is coupled into single-ended (non-differential) audio signals between the devices. With switched-mode power supplies, it is unlikely that this mistake will be made by the designers, since the hum current is restricted to the primary circuit and is reduced in magnitude by power-factor-correction stages, if present.
Another source of ground-to-earth induced signal that can arise with poor power supply design arises from non-stationary load current. Any device that dynamically changes it electricity consumption is presenting the same challenge as arises from the constantly changing AC mains supply. But the frequency of change is not fixed at the mains hum frequency. Any mix of frequencies is possible and many will fall in the audible band and can thereby cause audio problems. The main example is computers, which generate colloquially-called digital noise. This varies according to CPU load and can include mouse squeak where a sound is heard as the mouse is moved across the screen.
Both forms of ground-to-earth noise are minimised by directly connecting the protective earth to the audio connector ground. On a PC mother board, the audio connector ground is often a physically-separated ground plane that is connected to the main system ground at just one point and possibly through a small resistor or inductor to increase isolation. Standard PC grounding does not follow this pattern, as we shall describe below. Moreover, if external sound cards are being used instead of the audio system on the motherboard, it does not matter anyway.
Consumer HiFi is generally double-insulated. Accordingly, it does not have a protective earth connection. It is thereby free from ground-to-earth problems in standard configurations.
Given good equipment design, induced earth current from magnetic fields remains as a separate problem.
The main solution to avoiding induced earth current problems is to control or remove the sources of magnetic fields. Permanent magnets which are stationary, as found in loudspeakers, do not induce earth currents and can be ignored. Changing magnetic fields are produced by rotating motors and generators and heavy current mains equipment where the live and neutral are not twisted around each other or coxaial. Most mains cables are neither coaxial or twisted, but it is easy to change them if really needed. Installed electrical distribution is less easy to change. Note that a shielded mains cable will not generally help. The shielding consists of a thin foil that conducts electricity but not magenetism. It provides good electrostatic shielding but hardly any magnetic shielding. The near-field effect applies where the sender and receiver of electromagnetic radiation are less than a quarter of a wavelength apart. The wavelength of 50Hz mains is thousands of kilometers and hence the coupling of interest is indeed near-field. Non-linear loads, such as the diodes in SMPSUs, distorts the mains waveform and produces harmonics of 50Hz, but this does not stop it being a near-field problem. We are concerned about magnetic coupling, so electrostatic shielding is useless.
The basic approach must be to avoid such cables. It is poor practice to run data and audio cables inside the same trunking as mains cables. Separate trunking should always be used. Having a physical distance between the two sets of trunking is the best approach.
The question sometimes is asked why a loop tends to be worse than an open circuit for induced earth current problems. A varying magnetic field cutting an open loop will induce a corresponding voltage in it, but no current can flow in an open circuit. One answer is that if the mains cable follows roughly the same path as the audio cable(s), the same voltage will be induced in the audio ground as the protective earth and no induced net signal will arise, following the principle of differential signalling. Moreover, the paths do not even have to be the same, they just have to experience roughly the same amount and phase (same vector sum) of coupled noise signal for it to cancel out, which can happen under a variety of circumstances, especially where they run close to each other through a primary patch of magnetic interference. A second answer is that without an earth current, no voltage will be developed across the resistance elements of the earth loop. A closed loop may have one point in its path that has significantly greater resistance than the rest of it, such as at a dirty plug or socket, or as a deliberate approach to reduce earth loop currents by putting a small resistor between ground and earth in a piece of equipment. (Such resistors need to be more than a few ohms and will cause equipment to fail PAT testing, so tend these days to only be used in professional test equipment, such as oscilloscopes, which warrant manual testing.) The high resistance part of the path may or may not be at a critical place in terms of whether earth noise becomes combined with an audio signal that should be noise-free. This explains why and earth lift switch, found on many pieces of equipment, which disconnects signal ground from protective earth, can seem to randomly improve or degrade a noise problem.
Often the high-resistance part of the path is the ground of the audio cable or the mating of the audio connector with a socket at one end or the other. In a properly designed item of audio equipment, a single-ended input should subtract its signal from the ground potential at the ground contact of the audio connector. A single-ended output should generate its signally accordingly. If the loop current develops its voltage between the two end grounds of a single-ended audio connection, there is no hope: the signal will come through loud and clear.
Having multiple audio cables between different components is common. These can generate an earth loop if a simple ring topology exists. More typically, there will be dozens of cables and a complex mesh is generated. A complex mesh is a much better arrangement than a ring. The ring potentially suffers the same induction problems as generated with loops through the protective earth, but, as mentioned, these cables should be kept away from heavy current conductors and the problem is not severe or prone to equipment design faults. The richer the mesh the better. A rich mesh approximates a ground plane, a planar metalic sheet, which is the structure that has the lowest resistance and the least propensity to pick up magnetic induction.
Note that differential audio cables, using balanced TRS or XLR connectors, suffer far less from all forms of noise pickup. More-or-less equal noise is accumulated in both conductors and is cancelled at the receiver by subtraction. The subtraction tends to be accurate to 40dB or so (using 1 percent tolerance resistors) and the coupling in to the two conductors tends to be balanced better than that.
It is common to use ferrite rings around cables to alleviate noise problems. Such a ring acts as a common-mode choke or ballun (balanced-to-unbalanced transformer). It essentially makes a break in the cable that still preserves the voltage difference between the conductors of the cable. This would be perfect for breaking earth loops in principle. But the inductance of commonly-used rings is insufficient to make any difference at mains frequencies. Such rings only make a difference at radio frequencies. They help stop the cable from acting as a transmitting antenna for RF noise generated inside a device such as a lap top computer. To make a difference at mains frequencies, the ring would need to be sufficiently large for the audio cable to be looped through it many hundreds of times (I should do the calculation!)
A standard PC power supply joins mains earth to chassis earth and to the ground (OV) rail. A second connection to chassis and ground occurs with a metal motherboard enclosure where the shielded connectors mate with the motherboard backplate. This creates a small earth loop with two current paths between the PSU and the motherboard. A third and fourth path are made up from the CPU supply cable and the SSD supply cable. These paths can possibly degrade the performance of the on-board audio, which is singled ended. But most professional audio will instead use external sound cards or sound cards with differential (balanced) signals.
An external audio device using a galvanically-isolated (transformer-coupled) physical layer does not form an earth loop hazard. Ethernet (using unshielded twisted pair) is transformer isolated, so no earth loop problem can arise. S/P-DIF and ADAT on plastic fibre does not conduct at all, so no earth loop problem can arise.
Shielded Ethernet cables can cause a ground loop, but are often not needed. There are three reasons for using shielded Ethernet cables: 1/ to keep noise out, 2/ to keep noise in, and 3/ because it was readily available and/or perceived as being unilaterally better. The second and third reasons are spurious in general, whereas the first reason only applies in heavy-duty industrial situations such as where arc welders or Saturn-V release clamp solenoids are operating.
Firewire, USB and S/P-DIF use copper and can form ground loops. S/P-DIF is dc-balanced and hence easily transformer isolated (or even a pair of capacitors can be used: e.g 10nF in series with earth and ground) as shown in the above figure. USB has several DC aspects to its protocol and cannot pass through such isolators. Fortunately, external sound cards normally use differential (balanced) audio connections which reject ground noise. But this fails if a balanced output is converted to single-ended by grounding one side of it at the input to an external component that is single-ended and earthed. Or if feeding a domestic, stereo HiFi amplifier which is ungrounded but requires two balanced outputs to become unbalanced. See separate article: Mixerton HiFi Amplifier Balanced Input Addition.
The above figure shows basic earthing structures for an external sound card connected to a single-ended HiFi amp. The differential output must be converted to single-ended. Audio transformers are the best way to do this, since they will break any earth loop. But without resorting to transformers, the two cold outputs need to be wired together and to the amplifier ground. The amplifier may not be earthed it if is double-insulated. On the other hand, it might be earthed or other equipment also connected may earth its ground. The earth of the balanced outputs from the soundcard can also be connected to the amplifier earth. Whether this improves or degrades audio performance will vary.
For studio-quality audio use, there is an earth loop hazard ...
... uses capacitive isolation ... sufficently wideband transformer ... DC signalling on USB ...
DI boxes are commonly used to break grounds loops. Many are fitted with an earth lift switch that selects whether the loop is broken or not (broken when switch is open), but having the switch closed generally has no benefit.
A passive DI box uses an audio transformer to provide galvanic isolation. An active DI box contains powered electronics and does not need to have a transformer, although some active DI boxes still use the transformer for the isolation and the electronics just for amplification.
There are two forms of active ground loop breaking: baseband and HF.
A ground lift isolator could contain any of the mechanisms found in a DI box to provide the isolation. The same principles and techniques apply.
The Hosa isolator, pictured above, offers a simple alternative solution for breaking the ground loop. This principle only works for balanced connections. This device just disconnects the pin 1, the ground signal. The presence of the rest of the ground loop is then sufficient to keep the two ends at a sufficiently close potential that common-mode input range is not exceeded ... ... The illustrated device, being end-to-end metal bodied, connects the metalic parts of the XLR connectors at each end together. This will defeat the ground isolation if both sides also connect pin 1 of the XLR to the metal body, which some do. See elsewhere in this article.
This material is provided in good faith, but you use it at your own risk. I will deny any liability for any losses or infringements that may arise.