Handbook for Sound Engineers

Grounding and Interfacing 1193

differences can be developed between connection points
to it.^18 Therefore, it should come as no surprise that IT
and RF engineers prefer mesh grounding techniques
while audio engineers prefer star grounding techniques.
At power and audio frequencies, a so-called ground
loop allows noise and signal currents to mix in a
common wire. Single-point grounding avoids this by
steering signal currents and noise currents in indepen-
dent paths. But at ultrasonic and radio frequencies,
noise currents tend to bypass wires because they look
like inductors and tend to flow instead in unintended
paths consisting of parasitic capacitances. This makes
star grounding essentially useless in controlling
high-frequency interference in practical systems. Mesh
grounding does a better job of controlling
high-frequency interference, but since many ground
loops are formed, low-frequency noise can easily
contaminate signals. For audio systems, sometimes
even inside audio equipment, there is clearly a conflict.
This conflict can be resolved by the hybrid
grounding scheme. Capacitors can be used to create
multiple high-frequency ground connections while
allowing audio-frequency currents to take a path deter-
mined by the directly wired connection. Thus, the
ground system behaves as a star system at low frequen-
cies and a mesh system at high frequencies.^19 This tech-
nique of combining ground plane and star grounding is
quite practical at the physical dimensions of a circuit
board or an entire piece of equipment. At the system
level the same conflict exists regarding grounding of
audio cable shields. Ideally, at low frequencies, a shield
should be grounded at one end only, but for maximum
immunity to RF interference it should be grounded at
both ends (and even intermediate points, if possible).
This situation can be resolved by grounding one end
directly and the other end through a small capacitor.^20
The shield grounding issue will be discussed further in
Section 32.5.2.

32.4.4 Grounding and System Noise

Most real-world systems consist of at least two devices

that are powered by utility ac power. These power line

connections unavoidably cause significant currents to

flow in ground conductors and signal interconnect

cables throughout a system. Properly wired, fully

Code-compliant premises ac wiring generates small

ground voltage differences and leakage currents. They

are harmless from a safety viewpoint but potentiality

disastrous from a system noise viewpoint. Some engi-

neers have a strong urge to reduce these unwanted

voltage differences by shorting them out with a large

conductor. The results are most often disappointing.^21

Other engineers think that system noise can be

improved experimentally by simply finding a better or

quieter ground. They hold a fanciful notion that noise

current can somehow be skillfully directed to an earth

ground, where it will disappear forever!^22 In reality,

since the earth has resistance just like any other

conductor, earth ground connections are not at zero

volts with respect to each other or any other mystical or

absolute reference point.

32.4.4.1 Power Line Noise

The power line normally consists of a broad spectrum

of harmonics and noise in addition to the pure 60 Hz

sine wave voltage. The noise is created by power

supplies in electronic equipment, fluorescent lights,

light dimmers, and intermittent or sparking loads such

as switches, relays, or brush-type motors (i.e., blenders,

shavers, etc.). Fig. 32-19 shows how sudden changes in

load current, zero to full in about a microsecond for a

triac light dimmer in this case, generate bursts of

high-frequency noise on the power line 120 times per

second. Even an ordinary light switch will briefly arc

internally as it is switched off and its contacts open,

generating a single similar burst. This noise contains

significant energy to at least 1 MHz that is launched

into the power wiring. The wiring behaves like a

complex set of misterminated transmission lines gone

berserk, causing the energy to reflect back and forth

throughout the premises wiring until it is eventually

absorbed or radiated. Power line noise can couple into

signal paths in several ways, usually depending on

whether the equipment uses two-prong or three-prong

(grounding) ac power connections.

Figure 32-19. Light dimmer noise.