Talisman III Dynamic Loop Amplifier Systems (TAL/DLS1/3 & TAL/DLS5/3)
Publication Code TAL-DLS-3_TIG Issue 3
Page 7
4.
CHECK-LIST FOR SYSTEM DESIGN USING TAL/DLS
Refer also to setting up procedure on following pages
4.1
Before starting you will need the following:
a) Tape measure (at least 10m long).
b) Magnetic field strength meter.
c) AFILS receiver (headphones and hearing aid recommended).
d) Tape of continuous speech and tape-player, or a sympathetic assistant with a microphone.
4.2
Using the receiver(s) and field strength meter check the area to be covered for background magnetic noise.
Dimmers, power cabling, motors, etc., which can radiate powerful magnetic noise must be switched off
during this test. No standard currently exists but we recommend that the noise should be at least 26dB
below 0.1A/m and that the character of the noise is not objectionable (buzzes can sound worse than pure
hum).
4.3
Together with the customer decide on the most appropriate positioning for the wire loop and take the
dimensions so that an ‘ideal’ shape can be approximated (square or rectangle, etc.). Note the width
(shortest side of the loop), and calculate in metres the aspect ratio by dividing the width by the length.
4.4
Using the graph, find the width (shortest side) of the loop and move vertically up the graph until an
intersection is made with the appropriate ‘aspect ratio’ line.
4.5
The intersection of the loop width and ‘aspect’ ratio also determine the conductor area of the cable to be
used, taking into account both the current-carrying capacity and cable resistance.
The current demanded by this size of loop can be read – off the vertical axis. If the current indicated
is in excess of 9 Amps then separate loops should be employed, driven by separate amplifiers.
4.6
Temporarily install the loop as close as possible to its final position and test with the taped speech or
microphone and TAL DLS amplifier to confirm design parameters.
Hidden metalwork, etc., may cause reductions in field strength which can be compensated by
adjusting the master control during this test. In extreme cases the installation may prove totally
unsuitable for loop use and an alternative technology offered to the customer (e.g. Infra-Red
systems).
The frequency response should also be checked, losses of high frequencies may be recovered by
the use of the TAL DLS ‘Response’ control.
4.7
Assuming satisfactory test results, permanently install the loop wiring (not in metal conduit or trunking),
signal cabling (which should be routed as far as possible from the loop wire) and a.c. power supply. The
amplifier generates heat and must be located in a position with adequate ventilation.
4.8
Set all ‘input’ front panel gain controls to maximum, the ‘master’ control and ‘response’ control to mid
positions. Set the ‘response’ switch (located on the rear panel) to the required position – 5KHz for hearing-
aid use, 10KHz for other uses.
4.9
Depending on the use of the system, the AGC is set-up in two alternative ways:
4.9.1
AGC and limiting (for hearing aid use). Select the AGC mode by setting the ‘AGC’ switch (located
on the rear panel) to ON.
4.9.2
Limiting only (for non hearing aid use). Select the limiter only mode by setting the ‘AGC’ switch to
OFF (the switch is located on the rear panel).
4.10
The master control should now be adjusted to give the required current drive to the loop, indicated by the
bargraph display on the front panel. The field strength must be checked and the amplifier master control
adjusted if necessary.
4.11
Check the field strength over the entire area to be covered, note any areas of poor coverage and ensure
that hearing aid users are notified of such areas.
