Ground Loop

What is a ground loop? I understand that they can be caused by loose connections between components. What else can cause them? What are the effects? I have been told that humming from my power amplifier can be a symptom of a ground loop. True/False? Can ground loops cause damage to equpment?
In audio electronics a "ground loop" exists when different parts of the circuitry, say preamp and power amp, are separately grounded, creating a loop. This means that the signal reference ground may be different in the two places, and the usual result is hum.

For airplanes, a "ground loop" is a maneuver you do if you are about to run off the end of a runway. Basically you tip the plane so that one wingtip touches the ground. This makes the plane spin around and come to a stop quickly, usually with minimal damage, like a scuffed-up wingtip.
Some amps have a ground switch. Check it out.
Sthomas, do you mean a ground lift switch or a hum pot?

If the hum is from the amp and not the speakers, it is probably a noisy ac transformer. If it is from the speakers and you have no hum with no interconnects attached to the amp, it is a ground loop 99 times in a hundred. One method for dealing with a ground loop is to have only one path to ground by ungrounding everything except one component, usually the line stage. There are other more complex solutions.
Ground loop hum
Here is a great article on the subject by Charles Hansen
Thanks for the input. Eldaford, how did you know I was studying to become a pilot?
Eldartford knows all.
Connect all chasses together, so that you have one electrical point, use large guage wire. Now use one path to earth from the chassis - i.e. one wire. This should eliminate a ground loop problem.
Timrhu...I have been described by fellow workers as a walking encylopedia of useless information. It makes life interesting.
Musicnoise: Please excuse the question but how do I connect the chasses together? Do I run the cable through the venting slots, tape the wire down to each component, use any method I can think of? When you say to run the wire to earth, what would be a good example of earth to connect to?
Mordenmail, you could use screws in the chassis connected to a spade. The best ground you would probably have is your wall plug ground. You could connect the final wire to the ground plug on a male ac plug. Neither the hot or the neutral should have any connection to them.

I suspect that you would have to lift the ac grounds of all of your ac cords or you would introduce a third path to ground and worse hum. I have never tried this solution.
in reponse to your question about connecting chasses - loosen a chassis screw on each item and connect via spade connectors. Earth ground would ideally be a ground rod - but the street side of the water pipe works fine or for that matter the electrical panel ground bus. The idea is to have only one path to ground. Make sure that all your chasses are connected to the ground pin of your ac socket (where your power cord plugs into the item). Cheat the ground pin on all power cords except one. That way you have one path to ground for all items. The only difference in resistance being the short high guage wire connecting the chassis - tighten the screws and this should be in the range of less than 50 milliohms. Just remember, if you later move your equipment around - uncheat the grounds. The only reason this is still safe is that you still have all chasses connected to ground via the interconnecting wires.
Note - by chassis I am referring to the metal case of each piece of equipment. Do not stick anything through the vents, to not connect anything to any component, do not insert anything into the interior of the equipment. Doing so is dangerous and will very likely end up with a "smokey" flavor to the music - i.e. you will fry the equipment. The connection I am referring to (to the chassis) is to a point on the case that you could touch by picking the unit up or moving it around - i.e. on the case.
TBG / Musicnoise,

Thank you for the remedial tutorial. It is appreciated. Thanks to Jea48 for the article.
07-20-08: Musicnoise
Connect all chasses together, so that you have one electrical point, use large guage wire. Now use one path to earth from the chassis - i.e. one wire. This should eliminate a ground loop problem.
Problem is if a piece of equipment introduces a ever so slight ground fault current in the signal ground to equipment ground I believe the ground loop hum will still be present and will still travel through the ics.

With your added chassis ground the ics are still running parallel with it.

The offending piece of equipment or ic needs to be identified and corrected....... Jmho.
Mordenmail - it is highly unlikely that you have anything wrong with any of your components. The solution of connecting all your chasses and having one wire to ground addresses the most common cause of ground loop hum - either one of your chassis is not really connected to the third prong of the ac connector or the ground from one of your outlets is loose at some point - likely through other connections at other outlets where it is looped through. If that doesn't work the next step is to disconnect any cable from cable television, the internet, or an antenna if you have such connected anywhere in your audio system. This is another common cause of ground loop problems, the shield of the cable is at a different "ground" than your household wiring ground. After that, the place to look would be the connection between signal ground and chassis ground inside each piece of equipment. This is likely made via a wire from the pcb or a metal connection where the pcb is mounted to the standoffs to the chassis. Simply tighten the screw or nut that makes the connection. After that, the cause would be a cold solder joint from one signal ground path to the wire or connection to the chassis for the pcb. Grounding the shields of balanced interconnects at more than one point can also cause the problem. One work around if you cannot find the problem is to use isolation transformers between the interconnects for the audio signal - but this would not be the place to begin. Remember, we are talking about a ground loop hum (sounds like 60 Hz) so the problem is caused by coupled ac currents - this is not unusual, you will have inductive and capacitive coupling of ac currents because you have current carrying wires and conductive plates all over your system in any system - for example normally operating transformers. Ground loop problems can be hard to track down because there is almost never anything wrong with any of the individual components - it is the different paths to ground that causes the problem - you end up with a resistance where there should not be a resistance and hence you develop a 60 hz signal - that signal gets transferred between the signal carrying wires connecting your components, gets amplified and you hear coming from your speakers. The way to troubleshoot this problem is to address the most common causes first and in the most cost effective manner. Let me know what you find.
Time to work...
Update: I've decided to revamp my system. Replacing Denon AVR 2803 with a B&K AVR 507. May take Adcom GFA 7000 completely out of the equation. Hoping ground loop problem will resolve itself following changes.

Musicnoise: Thank you for your extensive explanation. Thanks again to all. I learned a lot from the process. Will be back on this thread if unable to resolve problem if it persists.
Here is a excellant paper on hum and buzz in a unbalanced interconnect system; google jensen an-004;written by Bill Whitlock.
08-16-08: Rleff
Here is a excellant paper on hum and buzz in a unbalanced interconnect system; google jensen an-004;written by Bill Whitlock.
Bill Whitlock