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Name: Anna
Status: Student
Grade: 12+
Location: CA
Country: United States
Date: Fall 2009


Question:
Hi, Some audio devices have a connection for an 'grounded' cable. But when we do connect one, there comes a stronger humming noise from the speakers, rather then less. A friend told us that there is no such thing as 'clean ground link' in populated areas, because there are many 'signals' lead to it from all kinds of sources. Is there a way to get some kind of 'clean ground link', or other way to get rid of the 'signal' created in the metal casing by electromagnetism?



Replies:
Hi Anna,

Your problem is related to electrical noise in the environment, but connecting additional ground connections usually makes the problem worse, not better.

Sixty Hertz power line noise such as you describe typically is induced in low- signal-level connections such as between a turntable, or CD player, or tape machine, and their inputs to the stereo amplifier. A turntable connection (if you have one) to the amplifier is especially susceptible since its signal level is much lower than that from CD or tape players. Speaker connections are not susceptible to humming, because they are so-called "low impedance" devices, and not sensitive to pickup.

The humming is usually worse when you add additional ground connections, because now there are two or more ground paths for the same "ground return" signal to flow between the CD player (or record player, or tape player) and the amplifier input. One path is along the ground wire inside the connection cable, and the second path is the additional ground connection you are supplying.

When this happens, "ground loop" is formed, and the 60 Hz power line noise is induced into the loop. The induced current flows in a continuous loop, first along one grounding connection, then back along the second one. The humming you hear is the result of the 60 Hz noise being induced into the "ground loop" and constantly circulating around the loop. This is picked up by your amplifier, then amplified and sent to your speakers.

The golden rule for connecting grounds is that if a ground to "earth" is used, it must be a so-called "single point ground connection". That means, of all the various devices that are connected together (amplifier, CD player, Turntable, etc), only ONE (the amplifier) can be connected to a separate ground. This prevents electrical "noise" from being induced into ground loops. You should also note that most amplifiers already make an "earth" ground connection via their AC plug's 3rd ground pin, so adding separate grounds is pointless, and generally makes the humming worse. All other devices such as CD players, turntables, or tape decks, are normally grounded to the amplifier via the ground wire in their connection cables. Adding separate ground connections for them is counterproductive since this will form ground loops (more than one ground connection for the same ground return signal), and hence more noise.

Hope this helps.... it is a difficult concept to grasp.

Regards,

Bob Wilson


Hi Anna

The thing you are describing is called a ground loop, and it comes from having multiple points, physically separated from each other, to which ground wires are attached. Because there are multiple points, there is the likelihood that there will be an induced current between those points. That manifests itself as a hum that gets amplified by the high gains of audio circuitry. In professional sound installations, a single ground point or ground bus is used to terminate cable shields. In addition, low level signals, such as those from microphones, are balanced through two wires that act in a push- pull manner, surrounded by a shield. You can try swapping locations of power cords, or lifting the ground on all but one audio cable. Sometimes a stereo pair will create a ground loop, and breaking the shield connection on one side of one of the channels will eliminate the problem. For serious issues, balanced inputs through transformers, such as those used with microphone connections, will also work. Hope this helps.

Bob Froehlich


Anna

One end of the cable is connected to the audio device, But where do you connect the other end of the cable to?

If you live high in a multi-story building, try making a good connection to a metal plumbing pipe. By a good connection I mean metal-to-metal with no intervening paint or dirt.

Be careful about this: Do not even begin to try this if you are unfamiliar with working with electricity: In a typical wall socket, there are three holes. The round hole is in the center, below and between two slot-shaped holes. This center round hole is the ground of your electrical system. Try to connect your ground cable to the round center hole (without sticking it in the two slot holes - this is dangerous, please be careful.)

Ground connections are pretty clean of signals provided they are actually a ground. Sometimes electricians make a mistake and connect the wrong wire to the ground cable, but the properly connected ground cable should be pretty clean.

One time on a ship, the electricity generated by a crane brake was misconnected to the ship's ground, and when the crane brake was activated it blew out all of the ship's computers.

Sometimes if you have a long ground wire, and it is not connected to a proper ground, it can pick up radio signals from the air.

Some adjoining electronic equipment may be radiating signals into your audio device. Unplug other electronic devices in the vicinity of your audio device to see if you can discover the offending piece of equipment.

But you know, if the hum does not appear until you connect the ground cable, then why connect it. Just leave it as a "floating ground."

Sincere regards,
Mike Stewart



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