https://www.trhf.no/ror/Eliminating_Ground_Loops.html

Key Takeaways

Through grounding, the safety of the user and the equipment or circuit can be ensured.

Ground loops can cause many issues.

A ground loop acts as an antenna that picks up electromagnetic energy, causing noise interference that disrupts the quality of the signal.

Have you ever heard an irritating hum coming from a sound system? In most cases, this noise is from the AC power lines entering the audio equipment. A ground loop is formed when pieces of equipment are all connected to a common ground through different paths in a sound system. The ground loop creates a path for the current to flow between different equipment grounds and back. The current flowing through the audio equipment ground causes humming.

In this article, we will explore what a ground loop is and how it can be eliminated.
The Need for Grounding

Ungrounded devices or circuits pose issues of large static charges. In an ungrounded system, large static charges can build up due to insulation leakage. There is a fairly high chance of getting a shock when touching these circuits. On the availability of a conductor at a lower potential, there is a good chance for high static charges to discharge, causing thousands of ampere currents to flow, which can damage the system. Through grounding, the safety of the user and the equipment can be ensured.
Ideal Ground

In electrical circuits, current flows only if there is a closed path. A return path must exist for the current to flow back to the source. The return path is provided by a common point, called ground, in circuits.

Ideally, there is no resistance or parasitic capacitance in ground connections. The circuit components are connected to the ground and are assumed to have the same potential. In ideal cases, the absence of potential difference in the ground prevents the current flow. However, this is not the case in reality.

Typically the common ground potential is only true in books or models. Practically, non-idealities such as resistance and parasitics of the wires that lead to the difference in the ground potential exist. The non-idealities cause the difference in ground potentials and circulating current flow.

When it comes to grounds, I am of the opinion that most people simply do NOT pay close enough attention. It is really not enough to simply look at a piece of equipment and try to decide on the ground scheme, you really need a “system grounding philosophy” that applies not only to equipment you build, but everything you use. The thing about the fight between the “direct ground” camp and the “parallel cap/resistor camp” is that the actual grounding philosophy is never discussed. The cap and resistor separation between chassis and signal grounds was almost an absolute requirement with two wire ac systems to achieve a safe design. Today it, more often that not, leads to funny ground loops and background noise.

This note, originally written in 1985, continues to be one of our most useful references. It’s popularity stems from the continual and perpetual difficulty of hooking up audio equipment without suffering through all sorts of bizarre noises, hums, buzzes, whistles, etc.– not to mention the extreme financial, physical and psychological price. As technology progresses it is inevitable that electronic equipment and its wiring should be subject to constant improvement. Many things have improved in the audio industry since 1985, but unfortunately wiring isn’t one of them. However, finally the Audio Engineering Society (AES) has issued a standards document for interconnection of pro audio equipment. It is AES48, titled “AES48-2005: AES standard on interconnections — Grounding and EMC practices — Shields of connectors in audio equipment containing active circuitry.”

Rane’s policy is to accommodate rather than dictate. However, this document contains suggestions for external wiring changes that should ideally only be implemented by trained technical personnel. Safety regulations require that all original grounding means provided from the factory be left intact for safe operation. No guarantee of responsibility for incidental or consequential damages can be provided. (In other words, don’t modify cables, or try your own version of grounding unless you really understand exactly what type of output and input you have to connect.)