Handbook for Sound Engineers

Grounding and Interfacing 1181

32.1 Introduction

Many audio professionals think of system grounding as
a black art. How many times have you heard someone
say that a cable is picking up noise, presumably from
the air like a radio receiver? Even equipment manufac-
turers often don’t have a clue what’s really going on
when there’s a problem. The most basic rules of physics
are routinely overlooked, ignored, or forgotten. As a
result, myth and misinformation have become
epidemic! This chapter is intended to enable sound
engineers to understand and either avoid or solve real-
world noise problems. The electronic system engi-
neering joke that cables are sources of potential trouble
connecting two other sources of potential trouble
contains more truth than humor. Because equipment
ground connections have profound effects on noise
coupling at signal interfaces, we must appreciate how
interfaces actually work as well as when, why, and how
equipment is grounded. Although the subject can’t be
reduced to just a few simple rules, it doesn’t involve
rocket science or complex math either.
For convenience in this chapter, we’ll use the term
noise to mean to signal artifacts that originate from
sources external to the signal path. This includes hum,
buzz, clicks, or pops originating from the power line
and interference originating from radio-frequency
devices. A predictable amount of white noise is inherent
in all electronic devices and must be expected. This
random noise, heard as hiss, will also limit the usable
dynamic range of any audio system, but this is not the
subject of this chapter!
Any signal accumulates noise as it flows through the
equipment and cables in a system. Once it contaminates
a signal, noise is essentially impossible to remove
without altering or degrading the signal. Therefore,
noise and interference must be prevented along the
entire signal path. It might seem trivial to transfer signal
from the output of one audio device to the input of
another but, in terms of noise and interference, signal
interfaces are truly the danger zone! Let’s start with
some basic electronics that apply to interfaces.

32.2 Basic Electronics

Fields can exert invisible forces on objects within them.
In electronics, we’re concerned with electric and
magnetic fields. Almost everyone has seen a demonstra-
tion of iron filings sprinkled on paper used to visualize
the magnetic field between the north and south poles of
a small magnet. A similar electric field exists between
two points having a constant voltage difference between

them. Fields like these, which neither move nor change

in intensity, are called static fields.

If a field, either magnetic or electric, moves in space

or fluctuates in intensity, the other kind of field will be

generated. In other words, a changing electric field will

set up a changing magnetic field or a changing magnetic

field will set up a changing electric field. This interrela-

tionship gives rise to electromagnetic waves, in which

energy is alternately exchanged between electric and

magnetic fields as they travel through space at the speed

of light.

Everything physical is made of atoms whose outer-

most components are electrons. An electron carries a

negative electric charge and is the smallest quantity of

electricity that can exist. Some materials, called conduc-

tors and most commonly metals, allow their outer elec-

trons to move freely from atom to atom. Other

materials, called insulators and most commonly air,

plastic, or glass, are highly resistant to such movement.

This movement of electrons is called current flow.

Current will flow only in a complete circuit consisting

of a connected source and load. Regardless of how

complex the path becomes, all current leaving a

source must return to it!

32.2.1 Circuit Theory

An electric potential or voltage, sometimes called emf

for electromotive force, is required to cause current

flow. It is commonly denoted E (from emf) in equations

and its unit of measure is the volt, abbreviated V. The

resulting rate of current flow is commonly denoted I

(from intensity) in equations and its unit of measure is

the ampere, abbreviated A. How much current will flow

for a given applied voltage is determined by circuit

resistance. Resistance is denoted R in equations and its

unit of measure is the ohm, symbolized :.

Ohm’s Law defines the quantitative relationship

between basic units of voltage, current, and resistance:

which can be rearranged as

For example, a voltage E of 12 V applied across a resis-

tance R of 6: will cause a current flow I of 2 A.

Circuit elements may be connected in series,

parallel, or combinations of both, Figs. 32-1 and 32-2.

EIR= u

R E

I

=---

I E

R

=---