Ground Loop Isolators, are commonly used when engine noises or alternator whine pollute signal fidelity. While they do not solve every possible cause of noise, ground loop isolators are useful when troubleshooting proves that the cause of interference is related to the signal cables or the ground circuits they create.

What They Aren't

Although sometimes referred to as "RCA noise filters", their effectiveness is not the result of filtering in the classical sense. The term is not entirely inappropriate, however, as undesirable artifacts of its operation do act like a true filter - especially at low bass frequencies where the physics of their operation slumps. This effect results in the suppression of bass with the characteristics of a high pass filter.

Why They Work

Ground loop isolators are effective because they manage the grounds of all RCA cables connected to them in a way that allows only signal current to flow in each of their ground conductors. Since the only ground current they permit is equal, but opposite to the signal conductor current, the ground loop isolator achieves a current balance in each of the cables connected to it. As a result, no other extraneous or unbalanced ground loop currents unrelated to the signal flow, and so all possible ground loops are effectively broken.

Current balanced lines can be as effective in canceling vehicle noise as the balanced line systems widely used in professional audio equipment. But current balanced lines offer the additional benefit of eliminating any possible ground loop current in those lines —which can be a source of interference and distortion in related mobile electronics equipment.

Ground loop isolators achieve this balance of currents with a transformer, which is a component based on the physics of magnetic fields. They use the audio signal to create a magnetic field on the input side of the transformer with an inductor coil, and then sense these magnetic fields on the output side of the transformer with a second, magnetically coupled, inductor coil. The signal is thereby relayed by the magnetic field, instead of a wire that would otherwise conduct extraneous and noisy ground currents.

Location & Direction

While the balance of currents created by the transformer cancels all common mode noises, including the interference due to magnetic fields that cross the signal cables, the audio magnetic field sensing coil of the transformer is unfortunately subject to sensing the noisy magnetic field too. Placing the ground loop isolator in areas of the vehicle where interference magnetic fields are weakest will help minimize this problem. Locations in the rear of the vehicle are usually preferred because locations furthest from the cable between battery and alternator are usually subject to smaller interference magnetic fields. Changes in the physical orientation of the device, in addition to its placement, can also help. Depending on the relative direction of the magnetic field interference, the simple rotation of the ground loop isolator may reduce this problem significantly.