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CONSTANT VOLTAGE (TRANSFORMER-
COUPLED) DISTRIBUTED SYSTEMS A
GENERAL DESCRIPTION
A distributed
speaker system
is one where
many
smaller
speakers
are
used
to
cover
large
areas
for
paging
and
background
music. This type of setup proves most efficient
and controllable, and helps keep down the cost of the overall
system.
In large meeting rooms
or halls with low ceilings,
distributed speaker systems may be the only way to cover an
audience
uniformly.
In large office buildings, hospitals, and
other industrial installations, many small speakers are used in
a similar distributed wiring scheme.
In churches, distributed
systems have been installed with speakers located on the pew
backs so that the sound from individual speakers can be kept
at low levels, thus avoiding problems with excess reverbera-
tion in the hall.
If a large number
of typical
(8-ohm)
speakers were simply
wired in parallel to the SPEAKER
terminals of the amplifier,
the overall
load impedance would be very low (less than an
ohm with more than 8 speakers), and the amp would be over-
loaded. Series wiring would
raise the load impedance to an
acceptable figure, but would subject the entire system to fail
if just one speaker voice coil were to go "open circuit". A
combination of series and parallel wiring would become quite
complex, and would still not permit the installer to easily set
different
sound
levels
for
individual
speakers
covering
different
zones.
The
way
to
overcome
these
difficulties
without buying one power amp for every few speakers is to
use a constant-voltage wiring scheme.
In a constant-voltage system, the amplifier is normally fitted
with
an output voltage step-up transformer.
That transfor-
теге
output then drives one a pair of wires that are run to
many
speakers,
each of which
is fitted with a step-down
transformer.
NOTE:
The
P2075 will deliver up to 35 volts in MONO
mode without a transformer. However, an output transformer
does
prevent
the
amplifier
output
from
'seeing'
the
impedance dips at
low frequencies which occur with many
inexpensive
speaker
transformers.
For
this
reason,
we
recommend
P2075
for use with constant
voltage speaker
systems; this amp
is designed for operation with optional
output transformers.
A typical
small
transformer
mounted
to a typical
ceiling
speaker is rated at b watts (on its highest power tap). Such a
speaker will actually draw 5 watts only when the speaker line
is delivering 70 volts RMS. A 10 watt transformer like the
type found
on
larger ceiling speakers will draw
10 watts
when the audio line is at 70 volts RMS. Such a "70 volt line"
is most commonly used with this type of distributed system.
With the 70 volt line, any number of speakers can be used,
up to the point where the total power that is drawn by all
the speakers and transformers on that line equals the power
output capability of the amplifier driving the line.
When using some output transformers, a 25 volt line can be
run instead of a 70 volt line; such a system uses a center tap
on the same
70 volt transformer,
and simply delivers. less
power to the speakers because less voltage is delivered to the
line. This provides a simple way to scale down the power to
each speaker,
and, if desired, to feed a greater number
of
'speakers, depending on the requirements of the system.
8 ohm
SPKR
FROM POWER
8 ohm
В ohm
SPKR
AMPLIFIER
OUTPUT *
22
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