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Dealing With Ground Loops

A ground loop occurs when two or more AV devices are connected to a common (safety) ground and to each other by signal cables.

What's A Ground Loop?

A ground loop occurs when two or more AV devices are connected to a common (safety) ground and to each other by signal cables. Magnetic induction effects in premises AC wiring create small voltage differences between safety grounds at different physical locations. These voltage differences cause powerline-derived noise current flow in signal cables, which couple the noise into the signals they carry. The coupled noise manifests itself as hums, buzzes, clicks, or pops in audio systems, or so-called "hum bars" that roll upward in video displays.

Fast Facts

  • Ground loops affect roughly 10 percent of AV systems.
  • In video, as little as 10 mV can produce a "hum bar."
  • An earth ground is not required to reduce noise in AV systems.
  • Ground "dummy" devices don't pass a signal but help pinpoint noise when placed by interfaces to identify common-impedance coupling and other issues.

Ground Loops and Unbalanced Audio

Most high-end audio systems and pretty much everything for the home use unbalanced interfaces, making them highly susceptible to ground loops. Leakage current gets into the grounded signal conductor and added to the audio signal. Because both signal and noise take the same current path, you get common impedance coupling, which can cause big problems in an unbalanced interface.

Here are some things you can do to minimize ground loop issues among unbalanced audio interfaces:

Keep cables short. Longer cables increase the common impedance coupling. Coiling excess cable length invites magnetic pickup.

Use cables with heavy-gauge shields. Shield resistance has the biggest effect on audio noise coupling. When cable runs are long, proper shielding is key.

Bundle signal cables. Signal cables between two boxes should always be bundled. If separated, AC magnetic fields will induce a current in the loop area inside the two shields, causing a hum. Bundling AC power cords separately helps reduces their net radiation.

Keep signal and power bundles far apart. Cables or bundles that run parallel will couple most, while those that cross at 90-degree angles will couple the least.

Avoid unnecessary grounds. Perhaps paradoxically, extra grounding of equipment tends to increase system ground noise current rather than reduce it.
Use ground isolators at unbalanced interfaces. Isolators are an ideal solution for common impedance coupling.

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