Handbook for Sound Engineers

System Gain Structure 1225

simple analog voltmeter can measure the rms value of a

sine wave. Complex waveforms require more sophisti-

cated instruments to yield their effective value.

The peak voltage of the waveform must pass through
the sound system component without being clipped. It is
the parameter of interest when establishing the gain struc-
ture of the system. The crest factor of the signal deter-
mines the energy content and therefore the power
produced by the amplifier and delivered to the loud-
speaker. It is of interest when considering the heat that the
loudspeaker must dissipate. Additionally, the rms voltage
is the signal parameter that relates most closely to the
loudness of the signal as perceived by a human listener.
Since the goal of an audio system is to reproduce
appropriate waveforms for a given application, these
waveform principles have universal application for all
parts of the sound system.

33.3 Gain Structure

The following are general terms of gain structure begin-
ning with how it applies to an individual piece of elec-
tronic equipment. It matters little whether the
equipment is a mixer, equalizer, amplifier, or other
active system component. By active we mean that the
device has a power supply for its internal active cir-
cuitry. This can be as simple as an internal battery or
two, or as complex as an internal or external ac
line-powered supply. The power supply voltages estab-
lish the maximum amplitude that a waveform can take
on as it passes through the component, Fig. 33-4. In
audio equipment, most power supplies form a bipolar
set of rails—a fixed positive and negative zero fre-
quency (dc) voltage that the waveform is developed
between. The value of the rail voltage determines the
peak amplitude that the waveform can take on. Exceed-
ing this peak value will cause deformation of the wave,
commonly known as clipping. We will proceed under
the assumption that the rails are fixed, and indeed they

are for most signal processing devices. Some power

amplifier topologies use multivalued or fluctuating rails.

The principles are the same, but we will not consider

such devices here.

Under a no input signal condition, all audio compo-

nents will still emit a residual output signal. Thermal

noise is generated at the molecular level and is present

at the output of all system components whether active

or passive. The level of the thermal noise determines the

noise floor of the component. In practice, other factors

can also make a contribution to the residual noise of an

electronic device. An undersized power transformer or

poor shielding can elevate some frequencies above the

broadband thermal noise floor, Fig. 33-5. Equipment

designers try to minimize thermal noise by component

selection and careful design, but it can never be elimi-

nated. We must accept the fact that it exists. Part of the

reason for establishing the proper gain structure of a

system is to render the effects of thermal noise insignifi-

cant. The thermal noise floor is affected by the settings

of the component’s level controls. While a low noise

floor can be achieved with all controls set at minimum,

this is not realistic, as we cannot operate it that way. The

controls should be set at a point appropriate for the

operation of the device. A good starting point is a

setting that produces the same voltage at the device

output that is present at the device input, often called

unity by audio practitioners. Level controls placed at

their 0 dB setting generally produce this condition, and

represent a good starting point for setting up a system.

Knowledge of the supply rails and noise floor estab-

lishes a dynamic range for the device—the difference in

level between the highest possible undistorted peak and

the lowest level that the signal can take on without

being buried in the noise. The dynamic range is what

can happen when a signal is passed through a device. It

Figure 33-4. Crest factors for sine waves and speech

waves.

0 VA

rms

Sine waveform Speech waveform

+ Rail

Crest Factor = 10log E^ Rail

peak /Erms

Peak

Figure 33-5. Thermal and spectral noise.