Monacor LA-75 Bedienungsanleitung Seite 9

Inductive sound reproduction systems are used for
various applications, e. g. as an aid for persons hard of
hearing in churches, theatres, cinemas, waiting rooms,
meeting rooms, as interpreting installations, for lec-
tures in museums, exhibitions, etc.
The LA-75 has two channels to which units with a
line output (e. g. CD player, cassette recorder, mixer)
and microphones may be connected. The channel
INPUT 1 is equipped with an optional talkover function
which attenuates the volume of the other channel by
approx. 40 dB in case of an announcement. Via an
alarm input a siren sound can be released. The loop
amplifier may be used as a mixing amplifier or be inte-
grated into a PA system.
3.1 Operational principle
An induction loop consists of a flexible cable laid into
the floor, the wall, or into the ceiling. Within this loop, a
magnetic field is generated which will induce a voltage
in the induction receiver. The receiver will reconvert
this voltage into an audio signal. Any number of receiv-
ers can be operated within an induction loop.
The maximum size of an induction loop may be
reduced due to field strength losses which are caused
by metal in ceilings and floors. Small losses can be
compensated with the metal loss correction.
In modern inductive sound reproducing systems, an
induction loop is triggered with a constant current
amplifier. As to the sound, constant current amplifiers
are considerably superior to constant voltage ampli-
fiers used in older systems as their power is not re-
duced at higher frequencies which are especially
important for the speech intelligibility.
4 Installing the Loop Amplifier and the
Induction Loop
Place the loop amplifier so that the air will be able to
move freely through all ventilation slots in order to
ensure a sufficient cooling of the power amplifier.
CAUTION
All connections should only be made by qualified per-
sonnel and in any case with the amplifier switched off.
Notes
1. Prior to installing the sound reproduction system
check in any case if magnetic interference occurs at
the intended place. This may impair the operation or
even make the operation impossible. Interference is
caused e. g. by transformers, high power cables,
fluorescent lamps with a conventional ballast, and
data cables.
2. Besides magnetic noise fields, interference in
inductive sound reproduction system may also
occur due to reinforced concrete floors or a floor
heating with copper pipes. In this case, too, the
magnetic field does not spread equally and in the
extreme case the use of an inductive sound repro-
duction system is not possible. If the interference
caused by the reinforced concrete is not too exten-
sive, the frequency response may be adapted
accordingly by means of the metal loss correction
(⇒ chapter 5.3).
3. When laying the induction loop in tubes, these must
be made of plastic as metal tubes will strongly
impair the magnetic field of the loop.
4.1 Induction loop
The LA-75 allows to realize inductive sound reproduc-
tion systems for an area of up to 75 m
at the edge of the sound reproduction area. The
distance to the ear level or the receiver should be
approx. 1 to 2 m. It should be avoided to lay the loop at
different heights. A simple flexible cable with a single
winding serves as an induction loop.
If the local conditions do not allow to lay the loop as
a rectangle, a special loop design is required which
must be calculated by an expert.
After the dimensions of the induction loop have
been defined, calculate the cross section of the cable:
4.1.1 Cable cross section
The resistance of the loop must be in a range between
0.2 Ω and 0.5 Ω. After the cable length of the loop has
been measured, determine the cable cross section.
The required cross section for the defined cable length
can be taken from figs. 5 and 6:
100
75
50
25
0.25 0.5 0.75 1.0
cable cross section [mm
Fig. 5 Required cable cross section for the induction loop
min. at 0.2 Ω
cable
max. at 0.5 Ω
length 14 m
Fig. 6 Minimum and maximum cable lengths at certain cable
cross sections
For the calculation of the cable resistance R (material:
copper) also the following formula may be used:
L L
R = – × ρ = – × 0.0175 ––––––––
cu
A A
A = cable cross section in mm
L = cable length in m
ρ
= specific resistance of copper
cu
0.0175 Ω × mm
2
2 . The loop is laid
admissible range
1.25 1.5 1.75 2.0 2.25 2.5
]
2
cable cross section in mm
0.5 0.75 1 1.5
6 m 9 m 11 m 17 m
21 m 29 m 43 m
Ω × mm
2
m
2
/m
GB
2.75 3.0
2
2.5
29 m
71 m
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