I N S TA L L A TI O N I N S T R U C TI O N S
4.34
Ground Loops
a) A GROUND LOOP CAUSES 60 CYCLE HUM
A ground loop exists when there is more than one ground path between two pieces of elec-
tronic equipment. The two ground paths form a large loop antenna which picks up noise cur-
rents, particularly 60 cycle AC. The resistance in the ground paths converts these currents into
fluctuating noise voltage differentials between the two pieces of equipment. If there is an audio
signal connection between these two pieces of equipment then this noise is added to the sig-
nal voltage.
AUDIO
PRODUCT 1
AUDIO
PRODUCT 2
SHIELD
GROUND PATH #1 THROUGH SHIELD
GROUND PATH #2 THROUGH AC GROUND
MAIN AC
POWER
BOX
EARTH
GROUND
CHASSIS
GROUND
EARTH
GROUND
SIGNA;L
GROUND
The most common
Ground Loop is caused
by shield grounds con-
nected at both ends and
the 3rd wire AC safety
ground
b) REMOVE THE GROUND LOOP BY BREAKING THE LOOP (Full Ground Lift)
To stop the 60 cycle hum, one of the ground paths must be removed. The typical method
to remove the second ground path is to have the shield grounded on one end only.
c) PARTIAL GROUND LIFT
Sometimes it is possible to insert a 50-100 ohm resistor between the shield and ground.
This will allow the ground to remain connected for audio signal flow but increase the imped-
ance to break the ground loop. This may reduce the problem but will not fix it.
d) EQUIPMENT WITH A GROUND LIFT SWITCH
In some equipment, the ground is part of the connector assembly and a switch is provided
to disconnect the ground. If no switch is provided, then a standard cable may need to be
modified or a custom cable built with modifications to the shield ground.
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