Technical Notes - Quest Engineering QTA 6060M User Manual

Technical Notes

Constant Voltage Distributed
Speaker Systems Demystified
In a typical paging and background music
speaker installation, quantity loudspeakers are
placed across a single amplifier in a parallel
wiring configuration (see Fig 1.).
Each ceiling speaker will contain a small
transformer and you will notice that the
connection block near the transformer will
have a common terminal (C or earth), and a
number of wattage terminals.
Connect the wattage setting terminal for the
desired acoustic output (volume) level you
need from each individual speaker.
This will be a 100V setting in Europe and
most of Asia and 70V in the USA. (See the
explanation below)
100V Line Public Address System
Fig. 1
Often some speakers need to be set at
different volume output levels, and the
calculations involved in determining the actual
load impedance at the amplifier's output
can be quite involved but there is a simple
technique for not overloading the amplifier
As for the amplifiers, there are two common
standards, 100 Volt line in Europe and Asia
and 70 Volt line on the American continent.
For the purpose of simple calculations, we will
use the European standard for this exercise.
On the rear of a 70/100V amplifier you will
find an output terminal strip (see Fig 2). This
terminal strip will contain a number of +
voltage outputs (70V 100V) and a terminal at
one end for the negative return wire (COM).
A low impedance terminal will be for
8-ohm speaker installations and is under
no circumstances to be connected to a
transformer type speaker system.
Installation practice
Step 1 .
connect the stripe/coloured wire to the 100V
terminal and the uncoloured wire to the COM
on the amplifier terminal strip.
Step 2 .
(uncoloured) to the Com/EARTH connection
on the ceiling speaker transformer and the
other + (strip) wire to the required voltage
taping.
Step 3 .
and connect the plain wire to the parallel COM
connection and the other + (stripe) wire to the
parallel output from the transformer terminal
block and connect it in the same way to the
next ceiling speaker.
Step 4 .
ceiling speakers are connected in a parallel
Remote Speaker
connection with all the COM connections
with 100V Line
Transformers
and all the wattage connections following a
parallel connection.
Common problems with ceiling
speaker installations
System is distorting. Check for the following
causes:
1. Too many speakers set to too high a power
tapping for the power of the amplifier
Solution: Calculate the total power
draw and re connect the speakers to a
lower the wattage tapping. (See below)
Some cheap ceiling speakers have
incorrect taping labels. A 10 watt speaker
may really be drawing 15 or 20 watts.
The only way to test this is with an
impedance meter on a single speaker. A
QTA 6000 Series
User Manual
Take a piece of figure-8 cable,
Connect the other end of the wire
Take a second piece of figure-8 cable
Continue this process until all the
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QTA 6060M/6120M/6250M
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User Manual