Handbook for Sound Engineers

System Gain Structure 1223

33.1 Introduction

This chapter is devoted to the explanation and establish-
ment of the proper gain structure of the sound reinforce-
ment system. It has been the author’s experience that
sound systems are rarely producing the optimum perfor-
mance that would be indicated by the specification
sheets of the individual components. Tangible improve-
ments in performance can often be achieved by some
simple adjustments of level controls.
Most technical subjects can be best explained using
ideal relationships, and this one is no exception. The
real world always falls short of the ideal case, but the
ideal can present a model and goal for our efforts. It is
the responsibility of the sound practitioner to form an
understanding of the trade-offs and apparent contradic-
tions through experience and endless hours in the field.
What follows is only an introduction that will benefit
those who supplement it with lab and field work.

33.1.1 Interfaces

An interface exists when two components are to be
interconnected for the purpose of transferring a signal.
One component will be the source (sending) device and
the other the load (receiving) device for the electrical
signal. At least three major topologies exist for inter-
connecting devices, the major difference being which
electrical parameter of the signal that the interface opti-
mizes the passage of—i.e., voltage, current, or power.
This is primarily a function of the ratio between the
source impedance and load impedance. At this point we
will make our first simplification by assuming that the
impedance of these devices is purely resistive with no
appreciable reactive component. This is actually a pretty
accurate assumption for most electronic components in
the signal processing chain.

33.1.1.1 The Matched Interface

A matched interface means that the source and load
impedances are equal. This topology has some admira-
ble attributes:

1. Power transfer is maximized.


2. Reflections from load-to-source are eliminated.

Impedance matching is required when the electrical
wavelengths of the audio signal are shorter than the
interconnecting cable. Examples include antenna
circuits, digital interfaces, and long analog telephone
lines. A drawback of this interface is that power transfer
is optimized at the expense of voltage transfer, and there-

fore the source device might be called on to source

appreciable current. It is also more difficult to split a

signal from one output to multiple inputs, as this upsets

the impedance match. A component that is operated into

a matched impedance is said to be terminated. While the

telephone company must use the matched interface due

to their electrically long lines, the audio industry

departed from the practice many years ago in favor of

the voltage-optimized interface for analog interconnects.

Fig. 33-1 shows a matched interface. It is important

to note that the selection of 600: as the source and

load impedance is arbitrary. It is the impedance ratio

that is of importance, not the actual value used.

33.1.1.2 The Constant Voltage Interface

Most analog sound system components are designed to

operate under constant voltage conditions. This means

that the input impedance of the driven device is at least

ten times higher than the output impedance of the

source device. This mode of operation assures that out-

put voltage of the driving device is relatively indepen-

dent of the presence of the driven device—hence the

term constant voltage, Fig. 33-2. Constant voltage inter-

faces can be used in analog audio systems since the typ-

ical cable length is far shorter than the electrical

wavelength of the signal propagating down the cable.

Figure 33-1. Matched interface.

Figure 33-2. Constant voltage interface.

600 7

Source

Es

E 1

E 2

Load

Interface 600 7

E 1 = Es /2

E 2 = Es /2

E 1 = E 2

100 7

Source

Es

E 1

E 2

Load

Interface 10 k 7 E 2 = Es