A ground loop is basically a small electrical current that runs between two grounded components when a third connection is made between the two. In other words, if there is only one path directly back to the ground from any individual electronic component inside of any electronic device (transistor, capacitor, resistor, inductor, integrated circuit, etc.), then there will NOT be a ground loop. BUT when there are multiple paths back to ground, then each path is by definition a ground loop.
Ground loops are commonly found in circuit boards designed by people who do not understand this issue and as a result noise is generated by these multiple-paths back to ground. This noise colors the sound as well as destroying how quiet the device could potentially be. Below is one example of a circuit board with a built-in ground loop as deliberately created by the circuit-board designer.
This is a classic example of incredibly poor circuit board trace design. There are two paths current can travel in this example between point A and point B: one path is on the right side (Path 1) and the other is on the left side (Path 2). While this may appear to be insignificant, it is in fact how such an attitude populates these little buggers in other circuit boards within the device. The issue is that electricity does not flow in a straight line but rather takes whatever path it can find, even if that path is in a circle. So, not only will electricity in this example flow from A-to-B, it will also flow in a circle from B-to-A.
The time it takes for this electricity to flow in this circular loop (A-to-B-to-A) is what destroys the integrity of the signal passing through the trace from A-to-B. What results is an electrical summation of the two signals that both time-smears and amplitude-smears the original signal.
To eliminate the ground loop, all the designer had to do was to cut the trace in the circular path at any point and assure that the remaining trace was wide and thick enough to handle the current through the remaining trace (a big OOPS!). Below shows what this may have looked like to assure that electricity flows only between points A-B.
But how many more ground loops can you find in this same circuit board? Hint: there are nine total. Scroll down once you believe you have found them to see the remaining ground loops.
Answer in picture below.
Although loops 2, 3, 8, and 9 are not technically ground loops, they are still just as destructive since they provide multiple paths in which electricity can flow.
I hope you understand now that there is more going on with ground loops than current wisdom perceives. This blog discusses just the problems on circuit boards but many more potential sources exist. SHAMELESS PROMOTION TO FOLLOW: I have written a series of guides on how to look for these little beasts all throughout your system entitled the "Extreme Audio" series Volumes 1-5 that sells for 99 cents each (a bargain) and this series explains how to get the most sound from your system without resorting to cutting traces on your circuit boards.
Yours for higher fidelity,