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Grounding and Cabling Grounding and Cabling for the Small Studiofor the Small Studio
David EtlingerDavid Etlinger
1/17/20061/17/2006
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 don’t count on me to keep you safe
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?
Basic Definitions
Voltage (Potential) – difference in charge between two points (V – Volts)
Current – the flow of electricity (positive to negative) (I – Amperes)
Resistance – a component’s opposition to current flow (R – Ohms)
V = I R
Part IPart I
GroundingGrounding
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
SignalGround
ChassisGround
EarthGround
Two-Prong Outlets
Hot – Black, Red or BlueNeutral - WhiteWhite
Three-Prong Outlets
Hot – Black, Red or BlueNeutral – WhiteWhiteGround – Green
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
Short Circuit with Proper Grounding
Short with Broken Ground
Mics and Electric Guitars with broken grounds are especially dangerous
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
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
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
Noise Source: Two Circuits
Easiest solution: Put both devices on one AC circuit
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
Induced Current
Induced Ground Current
Solution
Induced Current, Cont’d
• 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
Other Sources of Ground Noise
• Internal Power Supply (Induction and Capacitance)– Upgrade or mod the equipment– use balanced cables
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
Part IIPart II
CablingCabling
Balanced Cabling: CMR
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
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
WRONG: Signal Ground
RIGHT: Chassis Ground
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
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
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?!?
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
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
Sidebar: Cable Quality
Cable quality varies substantially
Foil shields: best protection but easily damagedUse in fixed installations
Braided shields: look for maximum coverage
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
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
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
Best Solution: Convert to Balanced
Next Best Solution: Transformers
Audio Isolation Transformers; DI Boxes; Many Preamps; etc.
www.whirlwindusa.com
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 <-> balanced
Most Common: Unbalanced Balanced
• Simply disconnect the shield at one end• TRS: sleeve; XLR: pin 1
Balanced Unbalanced
Balanced Unbalanced
Balanced Unbalanced
Hierarchy of Preference
Balanced --> Balanced• Unbalanced --> Balanced
• Balanced --> Unbalanced
• Unbalanced --> Unbalanced
Chassis-shielded at both ends• Chassis-shielded at one end, other end lifted
• Signal-shielded at one end, other end lifted
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
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
Noise Isolation Procedure
1. Disconnect everything
2. Connect monitors to main I/O; verify good grounding and no noise
3. Connect balanced gear one by one; verify no noise
4. Connect unbalanced gear one by one; modify until noise is acceptable
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
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
Balanced Power
Common-Mode Rejection CancelsGround Noise (just like balanced audio!)
Balanced Power Pros and Cons
ProsEliminates noise from reactive current
(i.e., bad internal design)Isolates from building power
ConsWon’t solve all ground loop problemsPricey!! ($1000 and up)
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 isn’t– Can build an isolator; usually easier solutions
Interfacing with the Computer
• Tricky!!!
• High power load might require a separate AC circuit
• Many different I/O paths
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
Computer I/O Cont’d
• Cable Modem– Ethernet interfaces usually won’t link grounds– USB interfaces can link computer to cable
ground– Use a Cable Isolator to break the coax ground
• Many other I/Os possible
• You’ll need to experiment
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
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
Cabling References
http://www.jensen-transformers.com/apps_wp.html
http://www.rane.com/note110.html
http://www.davidetlinger.comhttp://www.davidetlinger.com
Good luck!!Good luck!!