Humming/Buzzing from my Audio when using a ProAmp or ProAudio

What causes the humming/buzzing in audio (ProAudio/ProAmp) systems?

In very many cases the issue is usually reported by the user as a humming/buzzing sound on the output of a unit. This is most likely a "ground loop" issue - Audio and video systems have a reference point for their voltages. This is generally referred to as common or ground, although it may not relate to the earth, this reference remains at "zero volts" while other signal voltages "swing" positive (above) and negative (below).

Physically, the common may be a wire, a trace on a printed circuit board, a metal chassis, or virtually anything that conducts electricity. Ideally, it should be a perfect conductor, but in any practical system, it is not. As the complexity and size of the system are increased, the poor conductivity of the common (ground) conductor inevitably causes problems.

Hum and buzz (50Hz/60Hz and its harmonics) occur mostly in unbalanced systems when currents flow in the cable shield connections between different pieces of equipment. Hum and buzz can also occur in balanced systems even though they are generally much less common.

The cable shield currents and ground voltage differences are caused by several mechanisms. The second most common source of hum and buzz is the voltage difference between two safety grounds separated by a large distance or the voltage difference between a safety ground and an "Earth" ground (such as a grounded satellite dish or cable TV source). This problem is usually called a "ground loop". This is the most common one in a severe humming/buzzing problem.

Hum and buzz can also be magnetically induced or capacitively induced directly into signal cables. Or the noise current can leak from mains input through capacitance between the A.C. power transformer primary and secondary windings which causes a portion of the A.C. line voltage will ALWAYS be capacitively coupled directly to the audio circuit ground. This capacitively coupled power line signal will usually contain significant harmonics out to 1MHz or more. These signals will cause currents to flow in the cable shields thus adding this noise directly to the audio signal.

How to get rid of Ground Loop

1. Disconnect each piece of equipment, one at a time, until the buzzing goes away.
2. Re-connect one connection at a time, until the buzz/hum comes back.
3. Once it comes back you can disconnect that last thing you did and use a voltmeter to check voltages between the grounds.
4. If any is present, you must figure out why, based on house wiring, rack connections, etc.
5. Then proceed to eliminate it. 

References

https://www.soundonsound.com/techniques/ground-loops-explained

https://www.ecmweb.com/content/article/20885986/finding-and-fixing-ground-loops-in-av-systems

https://manualzz.com/doc/6665873/ground-loop-problems-and-how-to-get-rid-of-them