Transcript Grounding and Balancing for the Small Studio

Grounding and Cabling
for the Small Studio
David Etlinger
1/17/2006
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=IR
Part I
Grounding
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
Two-Prong Outlets
Hot – Black, Red or Blue
Neutral - White
Three-Prong Outlets
Hot – Black, Red or Blue
Neutral – White
Ground – 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
•
Separate Signal cables from AC cables (and
esp. Wall-wart transformers)
•
•

Use a current meter like Kill-a-Watt or Power Angel to
measure current draw
Try to keep at least 6” between
Cross at 90° if necessary
Use Balanced Cables wherever possible
Part II
Cabling
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
damaged


Use 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
www.whirlwindusa.com
Audio Isolation Transformers; DI Boxes; Many Preamps; etc.
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 twowire (“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.
2.
3.
4.
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
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 Cancels
Ground Noise (just like balanced audio!)
Balanced Power Pros and Cons
Pros
Eliminates noise from reactive current
(i.e., bad internal design)
 Isolates from building power

Cons
Won’t solve all ground loop problems
 Pricey!! ($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/gro
undloop/

http://www.equitech.com/articles/articles.html
http://www.rane.com/note151.html
 http://www.dself.dsl.pipex.com/ampins/gro
undloops/grndloop.htm

Cabling References
http://www.jensentransformers.com/apps_wp.html
 http://www.rane.com/note110.html

http://www.davidetlinger.com
Good luck!!