Grounding Issues and Interfacing Balanced and Unbalanced Signals

What is a Ground Loop?  A ground loop occurs when there is more than one ground connection path between two pieces of equipment. The duplicate ground paths form the equivalent of a loop antenna which very efficiently picks up interference currents.

Can Ground Loops be Eliminated?  Yes – if certain conditions are met:  The method specified by AES48 is to use balanced lines and tie the cable shield to the metal chassis (right where it enters the chassis) at both ends of the cable.  Only by using balanced line interconnects can you guarantee (yes, 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].

An Alternative Solution to Ground Hum  All of the power-consuming devices in a studio are connected to a wall outlet, which is unbalanced power.  Unbalanced power can have noise on the ground caused by switching power supplies found in most computers and digital audio gear.  This ground noise usually shows up as hum in audio gear.

The Balanced Power Answer  Balanced power consists of three wires. The same three wires that are connected to most studio equipment.  Balanced power provides the same common mode rejection we are all familiar with in balanced audio.

Interfacing Balance and Unbalanced Lines  Unbalanced signals are a fact of life in studios, unbalanced inserts on consoles are just one example.  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 Best Way to interconnect Unbalanced with Balanced This is too expensive for practical use when dealing with large amounts of I/O.

T he Next Best Way to interconnect Unbalanced with Balanced  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).

Winning the Wiring Wars - Summary  Use balanced connections whenever possible, with the shield bonded to the metal chassis at both ends.  Transformer isolate all unbalanced connections from balanced connections.  Use special cable assemblies when unbalanced lines cannot be transformer isolated.  Any unbalanced cable must be kept under ten feet (three meters) in length. Lengths longer than this will amplify all the nasty side effects of unbalanced circuitry's ground loops.  When all else fails, lay out $1200 and buy a balanced power transformer.