Speck Electronics MicPre 5.0 - Manual (Page 34)

Given the same circumstances, if the AC safety ground has been
defeated with a ground lift or the AC service is incorrectly wired,
the equipment's chassis and quite possibly everything attached in
that rack would be "live" with 120 volts.
In an electronics context, an earth provides a path for unwanted
EMI noise to be carried away from your audio equipment. If you
disable your earth with a ground lift or do not have a reliable earth
connection, the unwanted noise (EMI or RFI), will find an
electrical path of least resistance. That will most likely be your
audio equipment and would result in unwanted buzzes or hums.
In order for any audio signal to get from “Point A” to “Point B”
requires a cable with a minimum of 2 conductors. One conductor
is the hot, or high, or whatever you are familiar with; the other
conductor is the ground or common. Additionally, all audio wires
must be protected from environmental occurrences such as EMI
(Electro Magnetic Interference) and RFI (Radio Frequency
Interference) with an outer shield. An outer shield protects the 2
inner conductors from outside interference, and prevents that cable
from inducing its signal onto adjacent audio cables.
One common misconception is that the shield of a cable should act
as the common. This may be acceptable for guitar cords or semi-
professional applications, but not for professional applications. The
audio signals must be carried only by the 2 inner conductors and
the shield must act only to cover these 2 conductors without
transmitting the signal from one location to another. It is
recommended that the shield be attached to the common (ground)
at one connector's end, and the shield not be connected at the other
connector's end. It is recommended that all shields be connected at
the mixer end, and the shields not be connected at the other ends
(synths, effects, power amps, etc.).
If a patchbay is utilized in your mixing system, the rules for
shielding change. With a patchbay, normally all shields are
connected at the patchbay jacks, and not connected at the mixer or
external audio equipment.
Proper Grounding
and shielding
Audio earth
Chapter 4 Wiring and Other 28