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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.
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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 = I R
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 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, 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 II Cabling
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
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
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 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
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