A ground loop can exist in any system that contains two or more pieces of grounded equipment, whether via power cords or other ground connections.
An isolator is a device that passes signal while interrupting the path for the ground loop current.
They are not filters, and must be inserted in the signal path at a specific location, which is found through troubleshooting to be effective.
A properly installed isolator virtually eliminates hum, buzz, or other noises caused by ground loops.
Many pieces of consumer audio equipment have ungrounded (two-prong) AC plugs. This equipment is normally grounded via signal cables that link it to other equipment that are grounded via three-prong AC plug or another connection such as CATV.
When an isolator is installed in the signal path, these ground paths can be interrupted, leaving ungrounded equipment “floating.” Due to normal leakage currents, the chassis voltage of this equipment can be 50 volts or more with respect to safety ground.
This “common-mode” voltage is impressed between the input and output of an isolator. The noise rejection performance of even the best isolator is compromised because the ground noise is now about 100 times larger than the signal.
As shown in Figure 1, adding a ground connection to the floating device easily solves this problem.
It’s most convenient to simply replace the equipment’s two-prong plug with a three-prong type and add a wire (green preferred) between the safety ground contact of the replacement AC plug and a chassis ground.
To find out if a possible chassis connection point (like a screw) is actually grounded, use an ohmmeter to check for continuity to the outer contact of an RCA connector, which itself can serve as the connection point if necessary. Take note in (again, Figure 1) that the preamplifier is grounded via its signal cable to the power amplifier.
Also note that if the CATV connection is grounded, the added ground to the TV is unnecessary.
