Handbook for Sound Engineers

1292 Chapter 34

70 V transformer with a 0.5 dB loss. When this trans-
former delivers 10 W to the loudspeaker, it draws
11.1 W from the power amplifier. This is very important
in selecting the correct amplifier size.

Choose the power level fed to the loudspeaker by
connecting the appropriate primary winding to the 70 V
line. Connect the loudspeaker to the appropriate imped-
ance tap on the transformer secondary. Some manufac-
turers offer packages that include a ceiling loudspeaker
with transformer preinstalled.

34.4.9.2 Designing a 70 V System

Choose the system loudspeakers and transformers. Cal-
culate the power required per loudspeaker as described
in Section 34.3.3.7.1. Then, choose an appropriate 70 V
transformer. Next, calculate the power required by all of
the system loudspeakers (the sum) and choose a power
amplifier that’s big enough to supply this power plus the
amount needed to overcome the loss in the transform-
ers. Here is an equation to help in this final calculation.

(34-33)

where,
PA is the required power amplifier size (minimum size
in watts),

PL is the total power required by all of the loudspeakers
(the sum),
LT is the loss of an individual 70 V transformer in dB (a

positive number).

34.5 System Installation and Commissioning

34.5.1 What Is Commissioning?

After the system has been installed, it must be commis-

sioned. Commissioning involves three general steps.

These are outlined below and followed by a detailed

discussion of selected commissioning topics.

34.5.1.1 Step 1—Test All Components

Test and confirm that all electronic components and all

connections, including microphone and loudspeaker

connectors and patch bays, are correctly wired and in

proper working order. Pay special attention to polarity

and other potential wiring errors.

34.5.1.2 Step 2—Adjust the Electronics

Set up system DSP to its final configuration. Do not

adjust DSP delay, limiter or equalizer settings at this

time. Next, set any loudspeaker DSP to its final configu-

ration. If the loudspeaker DSP includes optimization for

specific models of loudspeaker, implement this optimiza-

tion at this time. Finally, adjust system gains and losses

to minimize hum and noise and optimize head room.

34.5.1.3 Step 3—System Adjustments and Equalization

One at a time, adjust the system power amplifiers to

produce the designed Lp in each audience area. Next,

adjust any digital delays for satellite clusters or

under-balcony loudspeakers. Finally, equalize the sys-

tem as discussed in Section 34.5.2.2.

34.5.1.4 Connectors and Cabling

As simple a subject as this may seem, faulty connectors

and cabling are the source of a majority of sound system

problems. Well-made cabling, of the proper type, with

the right connectors for the job, on the other hand, will

keep a system operating at maximum efficiency with a

minimum of noise pickup.

34.5.1.4.1 General Notes on Cable

A cable is a group of two or more wires, usually in a

single outer (insulating) sheath, designed for a particular

function.

Cables for portable audio systems should always be

made from stranded, not solid, wire. Solid wire cables

Figure 34-53. A typical 70 V transformer. Courtesy Lowell.

PA PL 10

LT

10 ----- -

=