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

Three-Prong Outlets

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 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 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

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 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 undloop/ undloop/ undloops/grndloop.htm undloops/grndloop.htm

Cabling References transformers.com/apps_wp.html transformers.com/apps_wp.html

Good luck!!