3.7 - What are ground loops and how do I avoid them?
Description
This article is from the Audio Professional FAQ, by with numerous contributions by Gabe M. Wiener others.
3.7 - What are ground loops and how do I avoid them?
One of the most difficult troubleshooting tasks for the audio
practitioner is finding the source of hum, buzz and other
interfering signals in the audio signal. Often these are caused
by "ground loops." This unfortunate and inaccurate term (it need
not be in the "ground" path, and the "loop" is not what causes
the problem) is poorly understood by most users of audio
equipment. A better name for this phenomenon is "shared path
coupling" because it happens when two signals share the same
conductor path and couple to each other as a result.
Another semantic problem that should be addressed early on is the
idea that "ground" is one place where all currents go. It's not,
there's nothing special about calling a signal "ground," current
still flows through any path that's available to it.
Referring to the discussion above regarding unbalanced signal
paths, recall that there must be a complete circuit from the
output of some device, through the input of another device and
back to the "return" side of the output if any current is to
flow. Current doesn't flow by itself, it must have a complete
path. If there are multiple paths over which the current might
flow, the current will be divided among them with most of the
current flowing through the path having the least resistance. Any
available path, regardless of the resistance in it, will carry
some of the current, it's not a case of all the current following
the path that has least resistance.
For example, suppose we have two units connected together through
a small piece of coaxial cable, and the units are also connected
together at the wall outlet through their grounded power cords --
the ground pins are connected to the chassis at each end. The
audio signal goes along the center of the coaxial cable, and part
of it might come back along the shield of the coax, but part will
also go through the ground wire of one unit and back through the
ground wire of the other unit. A problem arises when some other
signal is also flowing through this same return path. The other
signal might be another audio signal, video, data, or power. All
of the currents in a wire add together, and the resistance of the
wire causes a voltage to appear in proportion to the current
flowing. All of these voltages add together, so there is a little
bit of the video signal added to the audio, some of the power
signal added to the video, some of the power signal added to the
audio, etc. In rare instances, the "loop" of wire formed by the
intended ground return path and the happenstance lower resistance
return path formed by mounting hardware, power cords, etc. can
form a magnetic pickup as well, so that magnetic fields radiated
by transformers, CRT's, etc. can also induce a current in the
"loop," which makes yet another source of noise voltage.
This shared path coupling is a constant problem with unbalanced
audio systems. Lots of different methods have been tried to get
around the problem, many of them dangerous. Clipping off the
ground leads of equipment so there is no common power line path
between them simply makes any fault or leakage current follow
some other path, back through the signal cable to some equipment
that has a ground -- perhaps through the user's body, if all the
ground pins have been removed. The only general solution to
"ground loop" coupling with unbalanced equipment is to connect
all the chassis together with a very low resistance path (copper
strap or braid, for example), on the principle that since the
resistance is so low, any leakage current will produce a
correspondingly low signal voltage. It may also be effective to
interrupt the ground path of shield conductors over signal wires;
force the return path to go through the designated common strap
while leaving the shield in place only for electrostatic
screening.
With balanced equipment, no current should be flowing in the
shield conductors, and in fact performance should be identical
with the shield left disconnected at one end (preferably the
receiver end). Therefore balanced systems should be impervious to
shared path coupling or "ground loop" problems but in fact they
aren't, because most signals inside a given piece of equipment
are unbalanced, and there are often return paths internal to the
equipment that can be shared with return paths between other
units of equipment connected to it. Especially with mixed
digital, video and audio signals and high gain, high negative
feedback amplifier circuitry, this can be a big problem -- small
currents can create big effects -- and this brings us to the next
question. [David]
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