Handbook for Sound Engineers

Sound System Design 1291

34.4.8 Pads and Transformers

A pad is a resistor circuit that reduces the output level
from a source device to make it level compatible with a
load device (also see Chapter 23). In the past, external
pads were used to reduce the level from high-output
microphones to make them compatible with normal
microphone input circuits. Now, most mixers include
“trim” controls to compensate for low- or high-level
microphones.
Pads were also used to convert older high-level
devices to be compatible with low-level devices. Today,
most professional equipment has +4 dBu compatible
input and output levels. In addition, passive compo-
nents, like passive equalizers, are no longer in common
usage. As a result, pads are usually only needed for the
occasional connection of a professional device into a
semipro or hi-fi device.
Transformers (also see Chapter 13) are devices that
can be used to connect devices with unlike impedances
and levels. For example, a hi-Z to lo-Z microphone
transformer converts the high (voltage) level and high
impedance of a high-impedance microphone to the low
(voltage) level and low impedance of a low-impedance
microphone input. Transformers can also be used to
connect an unbalanced source to a balanced line. For
example, a transformer could convert the unbalanced
output of a consumer CD player to a balanced line for
connection to a professional mixing console.
Loudspeaker transformers are used for 70 V loud-
speakers as described in Section 34.4.8. Power ampli-
fiers sometimes include transformers to convert the
output of a conventional power amplifier to 70 V usage.
Transformers are level and impedance sensitive.
That is, a microphone hi-Z to lo-Z transformer cannot
be used for line-level impedance conversion. (It would
distort.) Neither can a line-level transformer be used for
microphone-level conversions. (It would also distort,
although in a different manner.) Thus, when selecting
transformers, needs must be defined in terms of both the
impedance ratio desired and the level of the devices that
will be connected to the transformer.

34.4.9 70.7 Volt /100 Volt System Design

A 70.7 V (referred to as 70 V) or 100 V loudspeaker
system, as shown in Fig. 34-52, allows relatively long
loudspeaker lines while minimizing I^2 R line losses.
70 V or 100 V distribution also allows multiple loud-
speakers to be connected to a single power amplifier
without the need for complex series-parallel connec-
tions. For these reasons, 70 V or 100 V systems are

commonly used for distributed ceiling loudspeaker sys-

tems and for any system where loudspeaker lines must

be relatively long, 75 to 100 ft or more. Variations on

this concept include 25 V distribution, which is some-

times used for school intercom systems, and 140 V dis-

tribution, which is sometimes used outside the United

States. For brevity, the remainder of this discussion will

use the term 70 V to refer to all of these systems. With

the exception of local electrical codes, which may limit

usage, design principles are the same for any of these

systems.

34.4.9.1 70 V Transformers

Fig. 34-53 shows a typical 70 V loudspeaker trans-

former. Choose a transformer that supports the imped-

ance of the selected loudspeaker and has appropriate

power taps for the application. Also choose a trans-

former with performance specifications that are appro-

priate for the application. In particular, pay attention to

the transformer’s frequency response, its distortion, and

its dB loss figure. Low-cost 70 V transformers will have

poor low-frequency performance, higher distortion

(especially at low frequencies), and a dB loss of 1.5 dB

or more. These specifications may be suitable for

low-level paging and background music. Higher-cost

70 V transformers will have improved low-frequency

response, reduced distortion at low frequencies and a

dB loss of less than 1.5 dB. Use these for higher-perfor-

mance applications.

Note that the dB loss specification is usually given in

such a way that the transformer delivers its rated power

to the loudspeaker but draws slightly more from the

power amplifier. As an example, consider a high-quality

Figure 34-52. A 70.7 volt loudspeaker system. Courtesy

Rane.

8 7

8 7

Amp 70.7 V @

full power

Power taps

(loudness)

Transformer coupled

constant voltage

distribution system

In