GROUND LOOPS

Almost all cases of noise in a sound system can be traced directly to ground loops, grounding or lack of a good ground.

Each component of a sound system produces its own ground internally. This ground is usually called the audio signal ground. Connecting devices together with the interconnecting cables can tie the signal grounds of the two units together in one place through the conductors in the cable.

Ground loops occur when the grounds of the two units are also tied together in another place.  This usually happens through  the ground wire in the power cord, or by tying the metal chassis together through the rack rails. Sometimes these ground loops can "come and go" randomly.  It is frequently caused by poor mechanical contact between various pieces of equipment and the rack.  Paint and anodizing, which are pretty common finishes for audio equipment are very poor electrical conductors.   These situations create a circuit through which current may flow in a closed "loop" from one unit’s ground out to a second unit and back to the first. It is not simply the presence of this current that creates the hum—it is when this current flows through a unit’s audio signal ground that creates the hum. In fact, even without a ground loop, a little noise current always flows through every interconnecting cable.  (In reality, it is impossible to eliminate these currents entirely.)

Balanced interconnect was developed to be immune to these noise currents, which can never be entirely eliminated. Unfortunately, many manufacturers of balanced audio equipment design the internal grounding system improperly, thus creating balanced equipment that is not immune to the cabling’s noise currents.

There are those "Professionals" who think connecting unbalanced equipment into "superior" balanced equipment should improve things. It usually doesn't.  Balanced interconnect is not 100% compatible with unbalanced.  There are ways to get around the problem, as we shall see.

THE ABSOLUTE BEST 
"RIGHT WAY" TO DO IT

Use balanced lines and tie the cable shield to the metal chassis (right where it enters the chassis) at both ends of the cable. A balanced line requires three separate conductors, two of which are signal (+ and –) and one shield . The shield serves to guard the sensitive audio lines from interference. Only by using balanced line interconnects can you guarantee hum-free results. Always use twisted pair cable. Chassis tying the shield at each end also guarantees the best possible protection from RFI [radio frequency interference] and other noises [neon signs, lighting dimmers].   Unfortunately, you cannot go out and buy pro audio equipment from several different manufacturers, buy standard off-the-shelf cable assemblies, come home, hook it all up and have it work hum and noise free. Plug and play. Sadly, almost never is this the case, despite the science and rules of noise-free interconnect known and documented for almost 70 years.
Since standard XLR cables come with their shields tied to pin 1 at each end (the shells are not tied, nor need be), this means equipment using 3-pin, XLR-type connectors must tie pin 1 to the chassis (usually called chassis ground) — not the audio signal ground as is most common. Not using signal ground is the most radical departure from common pro-audio practice. Not that there is any argument about its validity. There isn’t. This is the right way to do it. So why doesn’t audio equipment come wired this way? Well, some does, and since the  1993 AES Convention,, more of it does. So why doesn’t everyone do it this way? Because life is messy, some things are hard to change, and there will always be equipment in use that was made before proper grounding practices were in effect.
Unbalanced equipment is another problem.  It is everywhere, easily available and inexpensive. All those RCA and ¼" phone plug connectors found on consumer equipment; effect- loops and insert-points on consoles; signal processing boxes; semi-pro digital and analog tape recorders; computer cards; mixing consoles…
Unbalanced equipment when "blindly" connected with fully balanced units starts a pattern of hum and undesirable operation, requiring extra measures to correct the situation.

THE NEXT BEST "RIGHT WAY" TO DO IT

The quickest, quietest and most foolproof method to connect balanced and unbalanced is to transformer isolate all unbalanced connections. Many manufacturers provide  tools for this task. We sell pre-made transformer isolation boxes from Horizon, which can really help solve these ground loop problems.
The goal of these adaptors is to allow the use of standard cables. With these transformer isolation boxes, modification of cable assemblies is unnecessary. Virtually any two pieces of audio equipment can be successfully interfaced without risk of unwanted hum and noise.
Another way to create the necessary isolation is to use a direct box. Originally named for its use to convert the high impedance, high level output of an electric guitar to the low impedance, low level input of a recording console, it allowed the player to plug "directly" into the console. Now this term is commonly used to describe any box used to convert unbalanced lines to balanced lines.

THE LAST BEST "RIGHT WAY" TO DO IT

If transformer isolation is not an option, special cable assemblies are a last resort. The key here is to prevent the shield currents from flowing into a unit whose grounding scheme creates ground loops (hum) in the audio path (i.e., most audio equipment).
It is true that connecting both ends of the shield is theoretically the best way to interconnect equipment, although this assumes the interconnected equipment is internally grounded properly. Since most equipment is not internally grounded properly, connecting both ends of the shield  often  creates noisy interconnections.
A common solution to these noisy hum and buzz problems involves disconnecting one end of the shield, even though one can not buy off-the-shelf cables with the shield disconnected at one end. The best end to disconnect is a matter of personal preference and should be religiously obeyed; choose inputs or outputs and always lift the side you choose. If one end of the shield is disconnected, the noisy hum current stops flowing and away goes the hum — but only at low frequencies. A one-end-only shield connection increases the possibility of high frequency (radio) interference since the shield may act as an antenna. Many reduce this potential RF interference by providing an RF path through a small capacitor (0.1 or 0.01 microfarad ceramic disc) connected from the lifted end of the shield to the chassis. The fact that many modern day installers still follow this one-end-only rule with consistent success indicates this and other acceptable solutions to RF issues exist, though the increasing use of digital and wireless technology greatly increases the possibility of future RF problems.
If you’ve truly isolated your hum problem to a specific unit, chances are, even though the documentation indicates proper chassis grounded shields, the suspect unit is not internally grounded properly.

GROUND LIFTS

Many units come equipped with ground lift switches. In only a few cases can it be shown that a ground lift switch improves ground related noise. (Has a ground lift switch ever really worked for you?) In reality, the presence of a ground lift switch greatly reduces a unit’s ability to be "properly" grounded and therefore immune to ground loop hums and buzzes. Ground lifts are simply another Band-Aid ® to try in case of grounding problems. It is, however, true that an entire system of properly grounded equipment, without ground lift switches, is guaranteed  to be hum free. The problem is most equipment is not internally grounded properly. Most equipment is built to conform to UL (Underwriter's Labs) specifications.  These specs seem to work well for toaster ovens and electric drills, but frequently clash with our desire for hum free audio.  Given proper design, the two requirements of safety and low noise can co-exist.
Most units with ground lifts are shipped so the unit is "grounded" — meaning the chassis is connected to audio signal ground. (This should be the best and is the "safest" position for a ground lift switch.) If after hooking up your system it exhibits excessive hum or buzzing, there is an incompatibility somewhere in the system’s grounding configuration. In addition to these special cable assemblies that may help, here are some more things to try:

FLOATING, PSEUDO, AND QUASI-BALANCING

During inspection, you may run across a ¼" output called floating unbalanced, sometimes also called psuedo-balanced or quasi-balanced. They typically use a three circuit  1/4" phone plug, also known as a "TRS Plug.  There are three contacts on the plug: the "Tip", the "Ring" and the "Sleeve."   In most, but not all, the Tip is considered to be positive and the sleeve is negative in a balanced circuit.  The sleeve is almost always ground. You should consult the owners manual of your equipment before you assume anything about the hook up of TRS equipment to find out which pin is which and if it is really balanced.

In this  quasi-balanced configuration, the sleeve of the output stage is not connected inside the unit and the ring is connected (usually through a small resistor) to the audio signal ground. This allows the tip and ring to "appear" as an equal impedance, not-quite balanced output stage, even though the output circuitry is unbalanced.

Floating unbalanced often works to drive either a balanced or unbalanced input, depending if a TS or TRS standard cable is plugged into it. When it hums, a special cable is required.

SUMMARY

If you are unable to do things correctly  by using. use fully balanced wiring with shields tied to the chassis at the point of entry, or transformer isolate all unbalanced signals from balanced signals, then there is no guarantee that a hum free interconnect can be achieved, nor is there a definite scheme that will assure noise free operation in all configurations.

WINNING THE WIRING WARS

The following chart can be used to custom make special "Telescoped Ground" cables which may be helpful to eliminate a ground loop in some situations.