Grounding and Cabling for the Small Studio

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Grounding and Cabling for the Small Studio

• David Etlinger
• 1/17/2006

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Disclaimers

• NEVER defeat the AC ground!! (round prong)
• AC current can KILL if in doubt, stop and get
  more info
• I am not an electrician so dont count on me to
  keep you safe

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Objectives

• How can we get rid of that annoying hum?!?
• How can we reduce the noise floor through proper
  cabling?
• How can we keep our equipment and ourselves safe
  while doing it?
• Can we do this systematically, not haphazardly?

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Basic Definitions

• Voltage (Potential) difference in charge
  between two points (V Volts)
• Current the flow of electricity (positive to
  negative) (I Amperes)
• Resistance a components opposition to current
  flow (R Ohms)

V I R
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Part I

• Grounding

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Grounding (Earthing)

• Ground (1) Literally the Earth, effectively a
  point with 0 potential and infinite
  charge-holding capacity
• Ground (2) The lowest potential point in a
  circuit, serving as a current return path

Signal Ground
Chassis Ground
Earth Ground
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Two-Prong Outlets
Hot Black, Red or Blue Neutral - White
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Three-Prong Outlets
Hot Black, Red or Blue Neutral White Ground
Green
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Already a Problem

• Many, many home AC circuits are wired improperly
• This can cause noise and shock hazards
• Use a multimeter or 5 Radioshack tester to
  verify every AC outlet

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Short Circuit with Proper Grounding
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Short with Broken Ground
Mics and Electric Guitars with broken grounds are
especially dangerous
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Ground Noise

• All signals are referenced to ground
• Noise (voltage changes) on the ground line create
  noise in the signal
• Since AC oscillates at 60Hz, this often results
  in a 60Hz hum

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Ground Loops

• One point, two paths to ground
• Current requires a closed loop to flow
• Two paths to ground makes a closed loop
• Ground noise is then possible

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Chassis-to-Chassis

• Touching chassis can form an electrical
  connection
• Rackmount rails can also connect chassis
• This can be good or bad, depending on the
  situation

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Noise Source Two Circuits
Easiest solution Put both devices on one AC
circuit
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Noise Source Dirty Ground

• Remember V I R Low resistance means big
  current
• Solve with a power conditioner (Furman, etc.) or
• isolation transformer
• Or put everything on one AC outlet, but watch
  the
• power draw

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Induced Current
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Induced Ground Current
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Solution
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Induced Current, Contd

• Usually, 6 separation is enough to eliminate
  induced current
• Wall-wart transformers have unpredictable fields
  keep them as isolated as possible
• Induced currents cannot be totally eliminated

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Other Sources of Ground Noise

• Internal Power Supply (Induction and Capacitance)
• Upgrade or mod the equipment
• use balanced cables

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Fixing Ground Loops

• Put everything on one AC circuit
• Use a current meter like Kill-a-Watt or Power
  Angel to measure current draw
• Separate Signal cables from AC cables (and esp.
  Wall-wart transformers)
• Try to keep at least 6 between
• Cross at 90 if necessary
• Use Balanced Cables wherever possible

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Part II

• Cabling

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Balanced Cabling CMR
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Balanced Advantages

• High rejection of ground noise
• Also rejects external EMF
• CMR not perfect still wise to minimize ground
  noise
• Proper operation depends on proper ground wiring

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AC vs. Signal Ground

• AC Ground is designed for safety
• Signal Ground is an internal reference for
  circuit paths
• Signal Ground is usually tied to Chassis Ground
  at one point
• Balanced cables should ALWAYS use chassis ground

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WRONG Signal Ground
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RIGHT Chassis Ground
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Problems

• Much old or cheap equipment uses signal ground
• Unfortunately, this can cause noise even with
  balanced connections
• The only choices are upgrading or modding

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Finding Improper Grounding

• Visual Inspection
• Use a multimeter to test for voltage between
  shield and chassis
• Should be very low (ideally 0V)
• But, could tie to both chassis and signal ground

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Fixing Improper Balanced I/O

• Best Cut the trace to signal ground and bond to
  chassis ground
• Easier Disconnect the cable shield at the end
  tied to signal ground
• Breaks any ground loops
• But shield is now an RF antenna
• Can alleviate by bonding shield to chassis
  through a 0.01µF capacitor
• But then why not just mod the equipment?!?

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Worst Case

• Both input and output tied to signal ground
• No standard solution, but most people connect one
  end of the shield
• Which end is unimportant, but you must make the
  same choice each time
• Might be a very slight benefit to lifting at the
  input side

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One More Problem

• Units with a wall-wart transformer are not
  connected to AC ground
• These units can have balanced I/O
• Solve this by bonding the ungrounded chassis to a
  known grounded chassis

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Sidebar Cable Quality

• Cable quality varies substantially
• Foil shields best protection but easily damaged
• Use in fixed installations
• Braided shields look for maximum coverage

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Some Brands

• Pro Quality Canare, Mogami
• Also good Belden, Gepco, Rapco, Whirlwind
• AVOID HOSA
• Monster Good but way overpriced
• Or make your own pro quality at a budget price
• Connectors Neutrik, Switchcraft

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Unbalanced Cabling

• Two conductors Hot and Neutral
• No CMR no magnetic field rejection
• Often found on semi-pro or consumer gear
• ¼ TS (mono) RCA
• Always keep unbalanced runs as short as possible

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Shield goes to Signal Ground!

• Shield acts as current return path
• Necessary to form a complete circuit
• Not a true shield but does offer some protection

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Best Solution Convert to Balanced
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Next Best Solution Transformers
www.whirlwindusa.com
Audio Isolation Transformers DI Boxes Many
Preamps etc.
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Last Resort Cut the Shield

• N.B. Cutting the shield on a single-wire cable
  will ruin the cable!!
• We are going to cut the shield on a two-wire
  (balanced) cable
• This modified cable can then be used to connect
  unbalanced lt-gt balanced

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Most Common Unbalanced ? Balanced

• Simply disconnect the shield at one end
• TRS sleeve XLR pin 1

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Balanced ? Unbalanced
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Balanced ? Unbalanced
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Balanced ? Unbalanced
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Hierarchy of Preference

• Balanced --gt Balanced
• Unbalanced --gt Balanced
• Balanced --gt Unbalanced
• Unbalanced --gt Unbalanced
• Chassis-shielded at both ends
• Chassis-shielded at one end, other end lifted
• Signal-shielded at one end, other end lifted

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More Info

• If the choice of which end to cut is arbitrary,
  make the same choice each time
• Many possible scenarios
• See the two Rane references for excellent charts
• Also see Jensen whitepapers for a more technical
  discussion

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Summary of Best Practices

• Connect all devices to one AC circuit
• Use balanced I/O whenever possible
• Transformer-Isolate unbalanced lines when
  possible
• Cut shielding at one end if necessary
• Keep signal lines away from AC (esp. wall-warts)
• Cross signal and AC lines at 90 if necessary

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Noise Isolation Procedure

• Disconnect everything
• Connect monitors to main I/O verify good
  grounding and no noise
• Connect balanced gear one by one verify no noise
• Connect unbalanced gear one by one modify until
  noise is acceptable

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Sidebar Advanced Studio Grounding

• Pros
• Can reduce noise floor to commercial-studio
  levels
• Cons
• Complex
• Expensive
• Often a workaround for improper cabling or
  equipment design

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Primary Techniques

• Isolated Ground drive a dedicated ground bar,
  completely separate from the main AC grid
• Star Grounding Use heavy copper wiring to ground
  every device to one central point

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Balanced Power
Common-Mode Rejection Cancels Ground Noise (just
like balanced audio!)
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Balanced Power Pros and Cons

• Pros
• Eliminates noise from reactive current (i.e., bad
  internal design)
• Isolates from building power
• Cons
• Wont solve all ground loop problems
• Pricey!! (1000 and up)

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Digital Audio Cables

• Digital signals themselves should be immune to
  ground noise
• But, the cabling can create ground loops that
  affect other signals
• AES/EBU Isolation transformers prevent loops
• Optical S/PDIF (TOSLink) No electrical
  connection so no loops
• Coaxial S/PDIF Supposedly isolated but some
  cheap equipment isnt
• Can build an isolator usually easier solutions

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Interfacing with the Computer

• Tricky!!!
• High power load might require a separate AC
  circuit
• Many different I/O paths

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Common I/O schemes

• Soundblaster-type cards
• Almost always unbalanced
• Treat like any other unbalanced I/O
• Firewire Interfaces (MOTU 828, etc.)
• Firewire grounds to the computer
• Must then consider the computer part of the
  grounding topology

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Computer I/O Contd

• Cable Modem
• Ethernet interfaces usually wont link grounds
• USB interfaces can link computer to cable ground
• Use a Cable Isolator to break the coax ground
• Many other I/Os possible
• Youll need to experiment

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Final Sidebar Impedance

• Too complex to cover here
• Basically, impedance is like a frequency-dependent
  resistance
• Really good CMR requires I/O circuits with good
  impedance design
• If you want really really low noise, read up on
  impedance

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Grounding References

• http//www.epanorama.net/documents/groundloop/
• http//www.equitech.com/articles/articles.html
• http//www.rane.com/note151.html
• http//www.dself.dsl.pipex.com/ampins/groundloops/
  grndloop.htm

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Cabling References

• http//www.jensen-transformers.com/apps_wp.html
• http//www.rane.com/note110.html

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http//www.davidetlinger.com

• Good luck!!