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Operation: Chapter 3
24 RA150/300/500 Reference Manual
ABOUT GROUND LOOPS
A hum or buzz may be introduced into some audio systems by a phenomenon known as a ground loop. This can
occur if a piece of equipment “sees” two or more different paths to ground, as shown below.
Device A
D
evice B
s
hielded
c
able
p
ath 2
p
ath 1
To AC
p
ower
service
One path goes from device A to ground via the
ground terminal of the three-conductor AC power
cord, but A also sees a path to ground through the
shielded cable and AC ground of device B.
Because ground wires have a small amount of
resistance, small amounts of current can flow
through ground and generate a voltage along the
cable shield. This signal may end up getting
induced into the hot conductor.
The loop can also act like an antenna into which
hum is induced, or can even pick up radio
frequencies. Furthermore, many components in a
circuit connect to ground. If that ground is “dirty”
and contains noise, it might get picked up by the
circuit. Ground loops cause the most problems
with high-gain circuits, since massive amplification
of even a couple millivolts of noise can give an
audible signal.
Most ground loop problems can be solved by
plugging all equipment into the same grounded
AC source. However, it is important to make sure
that the AC source is not overloaded and is
properly rated to handle the gear plugged into it.
For really tough cases, you may need to break the
connection that causes the loop condition.
Although some do this by using a ground lifter and
breaking the AC ground, THIS IS A DANGEROUS
OPTION WHICH YOU SHOULD NOT USE
because it sacrifices the safety factor the AC
ground wire provides. In the previous diagram,
a better option would be to interrupt the cable
shield. There are two ways to do this: one is to
simply break the shield at some point, usually by
disconnecting it from ground at one jack. (The
other end should remain connected so that the
shielding properties are retained, even if there is
no direct path for ground.)
The other is to use a balancing transformer or
direct box as mentioned on page 16 “Adapting
the RA150/300/500 to Balanced Lines,” to
provide isolation in the audio line between the
two pieces of gear. Transformers generally have
no ground connection between the input and
output connections.