Ground loops: how they work and how to deal with them.
A ground-loop is created whenever two or more pieces of mains-powered =
equipment are connected together, so that mains-derived AC flows through =
shields=20
and ground conductors, degrading the noise floor of the system. The =
effect is=20
worst when two or more units are connected through mains ground as well =
as audio=20
cabling, and this situation is what is normally meant by the term =
"ground-loop".=20
However, ground currents can also flow in systems that are not =
galvanically=20
grounded; they are of lower magnitude but can still degrade the noise =
floor, so=20
this scenario is also considered here.
The ground currents may either =
be=20
inherent in the mains supply wiring (see "1: Mains grounding currents") =
or=20
generated by one or more of the pieces of equipment that make up the =
audio=20
system.(see "2: Transformer stray magnetic fields" and "3: Transformer =
stray=20
capacitance")
Once flowing in the ground wiring, these currents will =
give=20
rise to voltage-drops that introduce hum and buzzing noises. This may =
occur=20
either in the audio interconnects, or inside the equipment itself if it =
is not=20
well-designed. See Section 4.=20
Here I have used the word "ground" for conductors and so on, while =
"earth" is=20
reserved for the damp crumbly stuff into which copper rods are thrust. =
HUM INJECTION BY MAINS GROUNDING CURRENTS.
Fig =
1 shows=20
what happens when a so-called "technical ground" like a buried copper =
rod is=20
attached to a grounding system which is already connected to "mains =
ground" at=20
the power distribution board. The latter is mandatory both legally and=20
technically, so one might as well accept this and denote as the =
reference=20
ground. In many cases this "mains ground" is actually the neutral =
conductor,=20
which is only grounded at the remote transformer substation. AB is the =
cable=20
from substation to consumer, which serves many houses from connections =
tapped=20
off along its length. There is substantial current flowing down the N+E=20
conductor, so point B is often 1 Volts rms or more above earth. From B =
onwards,=20
in the internal house wiring, neutral and ground are always separate. =
(In the=20
UK, anyway)
=20
Two pieces of audio equipment are connected to =
this mains=20
wiring at C and D, and joined to each other through an unbalanced cable =
F-G.=20
Then an ill-advised connection is made to earth at D; the 1V rms is now=20
impressed on the path B-C-D, and substantial current is likely to flow =
through=20
it, depending on the total resistance of this path. There will be a =
voltage-drop=20
from C to D, its magnitude depending on what fraction of the total BCDE=20
resistance is made up by the section C-D. The earth wire C-D will be of =
at least=20
1.5 sqmm cross-section, and so the extra connection FG down the audio =
cable is=20
unlikely to reduce the intefering voltage much.
To get a feel for the =
magnitudes involved, take a plausible ground current of 1 Amp. The 1.5 =
sqmm=20
ground conductor will have a resistance of 0.012 Ohms/metre, so if the =
mains=20
sockets at C and D are one metre apart, the voltage C-D will be 12 mV =
rms.=20
Almost all of this will appear between F and G, and will be =
indistinguishable=20
from wanted signal to the input stage of Unit 2, so the hum will be =
severe,=20
probably only 30 dB below the nominal signal level.
The best way to solve this problem is not to =
create it in=20
the first place. If some ground current is unavoidable then the use of =
balanced=20
inputs (or ground-cancel outputs- it is not necessary to use both) =
should give=20
at least 40 dB of rejection at audio frequencies.
Fig 1 also shows a third earthing point, which =
fortunately=20
does not complicate situation. Metal water pipes are bonded to the =
incoming=20
mains ground for safety reasons, and since they are usually electrically =
connected to an incoming water supply current flows through B-W in the =
same way=20
as it does through the copper rod link D-E. This water-pipe current does =
not=20
however flow through C-D and cannot cause a ground-loop problem. It may =
however=20
cause the pipes to generate an AC magnetic field which is picked up by =
other=20
wiring.
HUM INJECTION BY TRANSFORMER STRAY FIELDS.
Fig 2 =
shows a=20
thoroughly bad piece of physical layout which will cause ground currents =
to flow=20
even if the system is correctly grounded to just one point.
=20
Here Unit 1 has an external DC power-supply; this =
makes it=20
possible to use an inexpensive frame-type transformer which will have a =
large=20
stray field. But... note that the wire in the PSU which connects mains =
ground to=20
the outgoing 0V takes a half-turn around the transformer, and =
significant=20
current will be induced into it, which will flow round the loop C-F-G-D, =
and=20
give an unwanted voltage-drop between F and G. In this case reinforcing =
the=20
ground of the audio interconnection is likely to be of some help, as it =
directly=20
reduces the fraction of the total loop voltage which is dropped between =
F and=20
G.
It is difficult to put any magnitudes to this effect because it =
depends on=20
many imponderables such as the build quality of the transformer and the =
exact=20
physical arrangement of the ground cable in the PSU. If this cable is =
rerouted=20
to the dotted position in the diagram, the transformer is no longer =
enclosed in=20
a half-turn, and the effect will be much smaller.
HUM INJECTION BY TRANSFORMER STRAY CAPACITANCE.
It =
seems at=20
first sight that the adoption of Class-II (double-insulated) equipment=20
throughout an audio system will give inherent immunity to ground-loop =
problems.=20
Life is not so simple, though it has to be said that when such problems =
do occur=20
they are likely to be much less severe.
This problem afflicts all =
Class-II=20
equipment to a certain extent.
=20
Fig 3 shows two Class-II units connected together by =
an=20
unbalanced audio cable. The two mains transformers in the units have =
stray=20
capacitance from both live and neutral to the secondary. If these =
capacitances=20
were all identical no current would flow, but in practice they are not, =
so 50 Hz=20
currents are injected into the internal 0V rail and flow through the =
resistance=20
of F-G, adding hum to the signal.
A balanced input or =
ground-cancelling=20
output will remove or render negligible the ill-effects. Reducing the =
resistance=20
of the interconnect ground path is also useful- more so than with other =
types of=20
ground loop, because the ground current is essentially fixed by the =
small stray=20
capacitances, and so halving the resistance F-G will dependably halve =
the=20
interfering voltage. There are limits to how far you can take this- =
while a=20
simple balanced input will give 40dB of rejection at low cost, =
increasing the=20
cross-sectional area of copper in the ground of an audio cable by a =
factor of=20
100 times is not going to be either easy or cheap.
Fig 3 shows =
equipment with=20
metal chassis connected to the 0V; (this is quite acceptable for safety=20
approvals- what counts is the isolation between mains and everything =
else, not=20
between low-voltage circuitry and touchable metalwork) note the chassis=20
connection however has no relevance to the basic effect, which would =
still occur=20
even if the equipment enclosure was completely non-conducting.
The magnitude of ground current varies with the =
details of=20
transformer construction, and increases as the size of the transformer =
grows.=20
Therefore the more power a unit draws, the larger the ground current it =
can=20
sustain. This is why many systems are subjectively hum-free until the =
connection=20
of a powered subwoofer, which is likely to have a larger transformer =
than other=20
components of the system.
Equipment type
| Power consumption
| Ground current=20
|
Turntable, CD, cassette deck
| 20W or less
| 5 uA=20
|
Tuners, amplifiers, small TVs
| 20-100W
| 100 uA=20
|
Big amplifiers, subwoofers, large TVs
| More than 100W
| 1mA |
GROUND CURRENTS INSIDE EQUIPMENT.
Once ground =
currents=20
have been set flowing, they can degrade system performance in two =
locations:=20
outside the system units, by flowing in the interconnect grounds, or =
inside the=20
units, by flowing through internal PCB tracks, etc. The first problem =
can dealt=20
with effectively by the use of balanced inputs, but the internal effects =
of=20
ground currents can be much more severe if the equipment is poorly =
designed.=20
=20
Fig 4 shows the situation. There is, for whatever =
reason,=20
ground current flowing through the ground conductor CD, causing an =
interfering=20
current to flow round the loop CFGD as before. Now, however, the =
internal design=20
of Unit 2 is such that the ground current flowing through FG also flows =
through=20
G-G' before it encounters the ground wire going to point D. G-G' is =
almost=20
certain to be a PCB track with higher resistance than any of the =
cabling, and so=20
the voltage drop across it can be relatively large, and the hum =
performance=20
correspondingly poor. Exactly similiar effects can occur at signal =
outputs; in=20
this case the ground current is flowing through F-F'.
Balanced inputs =
will=20
have no effect on this; they can cancel out the voltage-drop along F-G, =
but if=20
internal hum is introduced further down the internal signal path, there =
is=20
nothing they can do about it.
=20
The correct method of handling this is shown in =
Fig 5. The=20
connection to mains ground is made right where the signal grounds leave =
and=20
enter the units, and are made as solidly as possible. The ground current =
no=20
longer flows through the internal circuitry. It does however still flow =
through=20
the interconnection at FG, so either a balanced input or a =
ground-cancelling=20
output will be required to deal with this.
BALANCED MAINS POWER.
There has been speculation =
in recent=20
times as to whether a balanced mains supply is a good idea. This means =
that=20
instead of Live and Neutral (230V and 0V) you have Live and The Other =
Live=20
(115V-0-115V) created by a centre-tapped transformer with the tap =
connected to=20
Neutral. See Fig 6 below.
=20
It has been suggested that balanced mains has =
miraculous=20
effects on sound quality, makes the sound-stage ten-dimensional, =
etc,etc. This=20
is obviously rubbish. If a piece of gear is that fussy about its mains =
(and I=20
don't believe any such gear exists) then drop it in the river.
If there is severe RFI on the mains, an extra =
transformer in=20
the path may tend to filter it out. However, a proper mains RFI filter =
will=20
almost certainly be more effective- it is designed for the job, after =
all- and=20
will definitely be much cheaper.
Where you might gain a real benefit is in a Class-II =
(ie=20
double-insulated) system with very feeble ground connections. Balanced =
mains=20
would tend to cancel out the ground-currents caused by transformer =
capacitance=20
(see Fig 3 and above for more details on this) and so reduce hum. The=20
effectiveness of this will depend on C1 being equal to C2 in Fig 6 =
above, which=20
is determined by the details of transformer construction in the unit =
being=20
powered. I think that the effect would be small with well-designed =
equipment and=20
reasonably heavy ground conductors in interconnects. Balanced audio =
connections=20
are a much cheaper and better way of handling this problem, but if none =
of the=20
equipment has them then beefing up the ground conductors should give an=20
improvement. If the results are not good enough then as a last resort, =
balanced=20
mains may be worth considering.
Finally, bear in mind that any transformer you add =
must be able=20
to handle the maximum power drawn by the audio system at full throttle. =
This can=20
mean a large and expensive component.
I wouldn't be certain about the whole of Europe, but =
to the=20
best of my knowledge it's the same as the UK, ie not balanced. The =
Neutral line=20
is at earth potential, give or take a Volt, and the Live is 230V above =
this. The=20
3-phase 11kV distribution to substations is often described as =
"balanced" but=20
this just means that the power delivered by each phase is kept as near =
equal as=20
possible for the most efficient use of the cables.
It has often occurred to me that balanced mains =
115V-0-115V=20
would be a lot safer. Since I am one of those people that put their =
hands inside=20
live equipment a lot, I do have a kind of personal interest here...
Words=3D 2087