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You would still have a ground loop.

Posted by Al Sekela (A) on July 3, 2007 at 15:13:22

In Reply to: Balanced Power = no more ground loop? posted by DC STEVE on July 2, 2007 at 13:03:35:

There is a lot of misinformation posted about this topic. The term "ground" is used for both the AC power safety-earth and the audio equipment zero reference, so this increases the confusion.

Let us consider the simplest possible audio system that can have a ground loop. A CD player is connected to an integrated amp. Both the amp and CD player have three-wire power cords and their chassis are connected to the AC safety-earth. The two pieces are connected by standard RCA cables.

This system has a ground loop as long as both pieces are plugged in to proper AC outlets. The loop consists of the AC safety-earth wires in the power cords and the shields of the RCA cables. The shields of the RCA cables carry the audio zero reference. The audio zero reference is connected to the chassis inside each piece of equipment, and each chassis is connected to the AC safety-earth through the third wires in the power cords. It does not matter whether the equipment is plugged in to standard outlets or the output of a balanced power device, the fact that the two AC safety-earth wires are connected through the outlet guarantees that there is a ground loop.

Any continuous loop of wire can have current induced in it if a magnetic field changes within the area enclosed by the loop. This is Faraday's Law, and was discovered in 1831. It is the foundation of electrical generators and motors. One possible source of hum in a system with a ground loop is a time-varying magnetic field (from, say, a power transformer) that extends into the area enclosed by the ground loop. The current flows through the AC safety-earth wires and the RCA shields. The resistance of the shields converts the current into a voltage that appears to be in series with the audio signal. This voltage is amplified and comes out as the hum.

Another possible source of hum is a voltage difference between the AC safety-earth wires of two separate power circuits into which the equipment is plugged. This was the point of Pooge's objections to my recommendation of separate power circuits. This might be an issue in a large building, where the circuits are routed far apart and there is plenty of opportunity for their enclosed loop to contain time-varying magnetic fields, but IME is not a problem in a house where the dedicated circuits are routed close to each other.

Finally, there can be large currents flowing through the AC safety-earth if there are multiple points of attachment to the earth. The resistance of the wiring converts these currents into a voltage that, again, appears in series with the audio signal and comes out as hum. This is the source of hum in systems with video equipment tied to separate audio gear. The cable TV entrance point is attached to earth per code, and this is usually far away from the point where the AC power neutral and safety-earth wiring are attached to earth. Since the earth is not at a uniform potential, the difference is what drives the currents.

Your unfortunate experience with a misunderstanding with the electrician resulted in two circuits that share a single safety-earth wire. If you have no hum with using one of these circuits, then your ground loop is silent. Adding a balanced power device will not make the loop any more silent than it is already, and will not make it go away.

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