Grounding Issues and Grounding Issues and Interfacing Balanced and Interfacing Balanced and

Unbalanced Signals Unbalanced Signals

What is a Ground Loop?What is a Ground Loop? A ground loop occurs when there is more than one A ground loop occurs when there is more than one

ground connection path between two pieces of ground connection path between two pieces of equipment. The duplicate ground paths form the equipment. The duplicate ground paths form the equivalent of a loop antenna which very efficiently picks equivalent of a loop antenna which very efficiently picks up interference currents.up interference currents.

Can Ground Loops be Can Ground Loops be Eliminated?Eliminated?

Yes – if certain conditions are met:Yes – if certain conditions are met: The method specified by AES48 is to use balanced lines The method specified by AES48 is to use balanced lines

and and tie the cable shield to the metal chassis (right where tie the cable shield to the metal chassis (right where it enters the chassis) at both ends of the cableit enters the chassis) at both ends of the cable..

Only by using balanced line interconnects can you Only by using balanced line interconnects can you guarantee guarantee (yes, (yes, guaranteeguarantee) hum-free results. Always use ) hum-free results. Always use twisted pair cable. twisted pair cable.

Chassis tying the shield at each end also Chassis tying the shield at each end also guaranteesguarantees the the best possible protection from RFI [radio frequency best possible protection from RFI [radio frequency interference] and other noises [neon signs, lighting interference] and other noises [neon signs, lighting dimmers].dimmers].

An Alternative Solution to Ground An Alternative Solution to Ground HumHum

All of the power-consuming devices in a studio are connected All of the power-consuming devices in a studio are connected to a wall outlet, which is unbalanced power.to a wall outlet, which is unbalanced power.

Unbalanced power can have noise on the ground caused by Unbalanced power can have noise on the ground caused by switching power supplies found in most computers and digital switching power supplies found in most computers and digital audio gear. audio gear.

This ground noise usually shows up as hum in audio gear. This ground noise usually shows up as hum in audio gear.

The Balanced Power AnswerThe Balanced Power Answer Balanced power consists of three wires. The same three Balanced power consists of three wires. The same three

wires that are connected to most studio equipment. wires that are connected to most studio equipment. Balanced power provides the same common mode Balanced power provides the same common mode

rejection we are all familiar with in balanced audio. rejection we are all familiar with in balanced audio.

Interfacing Balance and Interfacing Balance and Unbalanced LinesUnbalanced Lines

Unbalanced signals are a fact of life in studios, Unbalanced signals are a fact of life in studios, unbalanced inserts on consoles are just one example.unbalanced inserts on consoles are just one example.

Unbalanced equipment when "blindly" connected with Unbalanced equipment when "blindly" connected with fully balanced units starts a pattern of hum and fully balanced units starts a pattern of hum and undesirable operation, requiring extra measures to undesirable operation, requiring extra measures to correct the situation.correct the situation.

The Best Way to interconnect The Best Way to interconnect Unbalanced with BalancedUnbalanced with Balanced

This is too expensive for practical use when dealing with large amounts of I/O.

TThe Next Best Way to interconnect he Next Best Way to interconnect Unbalanced with BalancedUnbalanced with Balanced

If transformer isolation is not an option, special If transformer isolation is not an option, special cable assemblies are a last resort.cable assemblies are a last resort. The key here is to The key here is to prevent the shield currents from flowing into a unit whose prevent the shield currents from flowing into a unit whose grounding scheme creates ground loops (hum) in the grounding scheme creates ground loops (hum) in the audio path (i.e., most audio equipment).audio path (i.e., most audio equipment).

Winning the Wiring Wars - SummaryWinning the Wiring Wars - Summary Use balanced connections whenever possible, with the Use balanced connections whenever possible, with the

shield bonded to the metal chassis at both ends.shield bonded to the metal chassis at both ends. Transformer isolate all unbalanced connections from Transformer isolate all unbalanced connections from

balanced connections.balanced connections. Use special cable assemblies when unbalanced lines Use special cable assemblies when unbalanced lines

cannot be transformer isolated.cannot be transformer isolated. Any unbalanced cable must be kept under ten feet (three Any unbalanced cable must be kept under ten feet (three

meters) in length. Lengths longer than this will amplify meters) in length. Lengths longer than this will amplify all the nasty side effects of unbalanced circuitry's all the nasty side effects of unbalanced circuitry's ground loops.ground loops.

When all else fails, lay out $1200 and buy a balanced When all else fails, lay out $1200 and buy a balanced power transformer.power transformer.