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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=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!!