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Grounding and Cabling 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.

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Grounding and Cabling for the Small Studio

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  1. Grounding and Cabling for the Small Studio David Etlinger 1/17/2006

  2. 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

  3. 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?

  4. 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

  5. Part I Grounding

  6. 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

  7. Two-Prong Outlets Hot – Black, Red or Blue Neutral - White

  8. Three-Prong Outlets Hot – Black, Red or Blue Neutral – White Ground – Green

  9. 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

  10. Short Circuit with Proper Grounding

  11. Short with Broken Ground Mics and Electric Guitars with broken grounds are especially dangerous

  12. 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

  13. 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

  14. 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

  15. Noise Source: Two Circuits Easiest solution: Put both devices on one AC circuit

  16. 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

  17. Induced Current

  18. Induced Ground Current

  19. Solution

  20. 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

  21. Other Sources of Ground Noise • Internal Power Supply (Induction and Capacitance) • Upgrade or mod the equipment • use balanced cables

  22. 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

  23. Part II Cabling

  24. Balanced Cabling: CMR

  25. 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

  26. 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

  27. WRONG: Signal Ground

  28. RIGHT: Chassis Ground

  29. 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

  30. 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

  31. 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?!?

  32. 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

  33. 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

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

  35. 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

  36. 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

  37. 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

  38. Best Solution: Convert to Balanced

  39. Next Best Solution: Transformers www.whirlwindusa.com Audio Isolation Transformers; DI Boxes; Many Preamps; etc.

  40. 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

  41. Most Common: Unbalanced  Balanced • Simply disconnect the shield at one end • TRS: sleeve; XLR: pin 1

  42. Balanced  Unbalanced

  43. Balanced  Unbalanced

  44. Balanced  Unbalanced

  45. 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

  46. 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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