It has been my experience that most ground loops are the result of poor quality power supplies, many of which now come in the form of external supplies, also known as "Wall Warts." Instead of a pure DC signal emitted from the power supply, there is often a considerable component of AC ripple. This is because most power supplies no longer us AC filter chokes, which eliminated the problem of AC ripple in a DC circuit.

Additonally, some of the components we now use have differing ground systems. Not all equipment utilizes a negative ground throughout the system. Some have floating grounds, which cause their own set of headaches when it comes to 60hz humm.

Fortunately, most are low voltage systems, therefore, some of these problems can be eliminated, or at least reduced by the use of ground defeater adapter plugs. I have always been a stickler for properly grounding all electrical and electronic devices, however, I encontered a similar problem with my laptop computer's power supply. It was poorly designed, and the 60hz humm it produced when hooked up to my keyboard, amp, and other allied equipment was horendous. By unplugging each device until the humm dissapeared I was able to isolate the offending gear, the laptop computer. A $1.25 ground defeater solved the problem, and after measuring the electrical potential between the laptop and all other pieces of equipment, there was no electrical potential, therefore no concern for electrical shock hazard.

Good luck, and I hope you are able to isolate the remaining hummm or interference as well.

Gary