9
English
3 Applications
LA-202 is a constant current loop ampli-
fier with dynamic compressor . It is used for
setting up an audio induction loop system .
The amplifier is able to transmit audio sig-
nals to hearing aids with a “telephone coil”
and to induction receivers (e . g . LR-202 from
MONACOR) . Wireless audio transmission is
a great advantage of audio induction loop
systems, allowing users total freedom of
movement within the loop .
Audio induction loop systems are used
for various applications, e . g . to provide as-
sistance to people who are hard of hearing
in churches, theatres, cinemas, waiting rooms
and lounges, as interpreting systems, for lec-
tures in museums, exhibitions etc .
In induction loop systems, a constant cur-
rent amplifier will drive an induction loop .
An induction loop consists of a loop of wire
laid into the floor, wall or ceiling . Within this
loop, a magnetic field is created which will
induce a voltage in the induction receiver .
The receiver will reconvert this voltage into an
audio signal . Any number of receivers can be
operated within an induction loop .
In ideal conditions, the induction loop
of LA-202 will be able to cover an area of
200 m
2
. In practice, however, this size may be
reduced due to loss of field strength caused
by metal in ceilings and floors .
4 Setting Up
The loop amplifier is designed for installation
into a rack (482 mm / 19”), but it can also be
placed on a table . To ensure sufficient cooling
of the power amplifier, air must always be
able to flow freely through all air vents .
4.1 Rack installation
Before installing the amplifier into a rack, use
the jumper J2 to enable the metal loss correc-
tion, if required . Open the amplifier for this
purpose (chapter 6 .2) .
For installation into a rack, fasten the two
mounting brackets provided to the sides of
the housing . In the rack, the loop amplifier
requires a height of 2 RS (2 rack spaces =
89 mm) .
To prevent the rack from becoming top-
heavy, insert the amplifier into the lower
section of the rack . The front panel alone is
not sufficient for fixing it safely; additionally
use lateral rails or a bottom plate to secure
the unit .
5 Installation
CAUTION!
Connections must be made by
skilled personnel only . Always switch off the
amplifier before making connections .
Hints
1.
Before installing the induction loop sys-
tem, always use a receiver (e . g . LR-202)
to check the desired place of installation
for magnetic interference which may im-
pair operation or even make operation
impossible . Sources of interference are,
for example, transformers, high-power
cables, fluorescent lamps with conven-
tional ballast and data cables .
2.
Apart from magnetic noise fields, rein-
forced concrete floors or underfloor heat-
ing with copper pipes may cause inter-
ference in induction loop systems . In this
case, the magnetic field will not spread
evenly and in the worst case, operation
of an induction loop system will not be
possible . In case of minor interference due
to reinforced concrete, the control METAL
LOSS CORRECTION (refer to chapter 6 .2)
can be used to match the frequency re-
sponse accordingly .
3.
When laying the induction loop in tubes,
make sure to use plastic tubes . Metal
tubes may strongly affect the magnetic
field of the loop .
5.1 Induction loop
The loop amplifier can be used to set up in-
duction loop systems covering an area of up
to 200 m
2
. The loop is laid around the peri-
meter of the sound reproduction area . The
distance to ear level should be approx . 1 m .
It is strongly recommended to lay the loop at
the same height . For the induction loop, a
basic cable is used .
If local conditions make it impossible to
lay the loop as a rectangle, a special loop
design is required which must be calculated
by an expert .
When the dimensions of the induction loop
have been defined, calculate the cross section
of the cable:
5.1.1 Cable cross section
The resistance of the loop must be 0 .2 – 2 Ω .
When the length of the loop has been meas-
ured, determine the cable cross section . Please
refer to figs . 5 and 6 for the required cross
section of the defined cable length:
0,25
0,5
0,75
1,0
1,25
1,5
1,75
2,0
2,25
2,5
2,75
3,0
25
50
75
100
admissible range
cable cross section [mm
2
]
cable length [m]
max. loop resitance
2Ω
min. loop resistance 0,2
Ω
Fig . 5 Required cable cross section for
the induction loop
cable cross section in mm
2
0.5
0.75
1.0
1.5
2.5
Loop
length
min. at 0.2 Ω
max. at 2.0 Ω
6 m
56 m
9 m
84 m
12 m
110 m
17 m
168 m
28 m
280 m
Fig . 6 Minimum and maximum loop lengths for
certain cable cross sections (copper)
For calculating the loop resistance (R) of a
copper cable, the following formula can be
used:
R = l ×
ρ
cu
= l × 0.01786 Ω × mm
2
A
A
m
A = Kcable cross section in mm
2
l = loop length in m
ρ
cu
= specific resistance of copper
5.1.2 Connection of the induction loop
The loop amplifier must be outside the loop
(figs . 3 and 4) .
1) Twist the cable section between the am-
plifier and the loop .
2) Before connecting the induction loop to
the amplifier, use an ohmmeter to check
and make sure that the loop is not earthed .
3) Connect the cable ends of the loop to the
terminals LOOP OUTPUT (9) .
5.1.3 Operation with two loop amplifiers
If one loop amplifier is not sufficient for the
area required, two loops may be installed
which are driven by one amplifier each . In
this case, the first amplifier will operate as a
master (to which all signal sources are con-
nected) and the second as a slave .
Feed the audio signal from the master
amplifier to the slave amplifier . Fig . 4 shows an
example with adapters from the MONACOR
product range:
1) Connect an adapter 6 .3 mm stereo jack /
2 × RCA inline jack (e . g . NTA-178) to the
jack SLAVE (12) of the master unit . The
output signal to be fed to the second
loop amplifier is available at the tip of the
6 .3 mm plug .
2) When the jack SLAVE is used, the signal
path between the mixer and the power
amplifier will be disconnected . Therefore,
use a Y-cable (e . g . CPR-25 / BL) to be placed
on the adapter NTA-178 to reconnect the
signal path .
3) From the Y-cable, feed the output signal
via an adapter cable 2 × RCA / 6 .3 mm
stereo jack (e . g . MCA-302) to the jack
SLAVE of the slave amplifier . The signal
must be available at the ring of the 6 .3 mm
stereo plug . Therefore, connect the red
RCA plug of the cable MCA-302 to the
Y-cable .
4) For first-time operation, use the controls
MASTER (4) to adjust the current for the
two induction loops separately on the
master amplifier and on the slave amplifier
(chapter 6 .1) .
