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.
- 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.
Ground Loops and Video
Video represents an unbalanced interface with no inherent noise reduction. Because the shield of a video cable can be a return path for video signal and powerline ground current, and because the shield impedance is therefore shared by the signal and ground current, common impedance coupling is introduced.
In such situations, a voltage drop appears along the cable. Roughly half the voltage drop is added to the video signal, so the goal is to reduce that drop. Shorter cables and lower shield resistance can reduce coupling impedance. But so can putting a device in the signal path that has high common-mode impedance. Among the choices:
Common-mode chokes–aka hum suppressors, ground loop isolators, etc.–are commonly used to fight video hum bars, in part because they handle wide bandwidth (100 MHz to 1 GHz), so they work for HDTV (see "Ensure the Bandwidth You Need"). CM chokes introduce a high impedance into the ground loop to "choke" ground voltage differences and reduce hum.
Isolation transformers, which convert the video signal to an AC magnetic field then create a replica signal, are like the audio transformer for unbalanced audio interfaces. Loop current is reduced to almost nothing and hum is controlled even in situations where ground voltages differ greatly and/or you require long cable runs. One drawback: limited bandwidth (10 Hz to 10 MHz), meaning they're best suited to Composite or S-video.
Isolation amplifiers require power, unlike CM chokes and isolation transformers. This means they can have multiple outputs and adjustable gain, but you need to ensure they support the bandwidth you require for your video application.
Deciding which device to use depends on bandwidth required, the ground voltage difference in the loop, and the length and type of cable. Ground noise rejection decreases as cable length increases, and some voltage differences are too much for the device you're considering to handle.
Ensure the Bandwidth You Need
When choosing a device to put in a video signal path in order to eliminate ground loops, make sure your choice supports the video bandwidth you require.
Source: adapted in part from "understanding, finding, and eliminating av ground loops," a presentation by bill whitlock, president of jensen transformers (www.jensen-transformers.com), fellow of the audio engineering society, and senior member of the institute of electrical and electronic engineers.
A clamp-on ammeter can be a quick way to locate ground loop currents. It's able to measure low currents at 1 mA or better. Just clamp the meter around a system's cables at various locations to figure out the path and extent of ground loop current. A good one can be had for around $250.
Words of Caution
Also, never, ever try to solve a ground loop problem by removing the grounding pin from a cord connecting the device to the outlet. Doing so risks electrocution.