Ground Loop
A ground loop is basically what happens when two separate devices (A and B) are connected to ground via different paths and then also connected to each other by another path, creating a loop. When a ground loop is created, current may flow in unanticipated directions. The current may flow to ground via the device’s own ground path or it may flow first to the other device before going to ground due to the difference of electrical potential between devices. This unintended current flow causes system noise or interference to be transmitted to nearby devices.
Ground loops are very evident on audio equipment as it can cause humming or buzzing sounds which, typically, you do not want to have in your audio signal.
One example where a ground loop can happen is in a simple setup of a laptop computer and an AC powered speaker. When the speaker is used by the laptop, connected through an audio cable, while both the laptop and speaker are plugged into the AC mains, the speaker may produce a buzzing sound. This issue may be solved by unplugging the laptop charger from the mains, which breaks the ground loop. This scenario may or may not happen depending on the grounding of one's home or establishment.
Other effects of ground loops are interference on data transmission, RF interference and power supply noise.
Solutions to ground loops
When you happen to encounter a ground loop on your devices, don’t worry. Ground loops are easy to eliminate once you understand them.
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1. Use a single AC plug
You can eliminate most ground loop problems by just plugging your devices into one AC source with the same ground. By doing so, you reduce the difference in electrical potential between grounds of the devices.
2. Ground lift
One of the easiest ways to eliminate a ground loop is to cut the ground path from one device to the other. By doing so, it breaks the loop created in the ground. Most audio devices nowadays have a ground lift switch, so that one can easily cut a ground path from one device. If ground lift switches are not available, you can simply break or disconnect the ground shield at one end of the cables used between devices.
3. Audio Isolation Transformer
This device is inserted along the audio path. It contains a transformer which isolates the audio ground and signal between the source and the output. A 1:1 transformer is usually used but other transformers could also be used to boost the level of the output.
4. Power Isolation Transformer
You can also eliminate a ground loop by isolating direct connection from a device to AC ground by using a power isolation transformer. It is similar to the audio isolation transformer, but the isolation is inserted in the power side not in the audio output side.