I-AMP GPT 60-IV User Manual - page 9
~ 9 ~
G p t 6 0 - I v
gpt
DESIGnS
mAnUFActurinG
Excellence Through Engineering
The above example would give:
Cable Type 1, 1.00 mm
2
= 84 x 0.0185 = 1.55 Ohms
Cable Type 2, 0.75 mm
2
= 84 x 0.021 = 1.76 Ohms
Cable Type 3, 0.51 mm
2
= 84 x 0.033 = 2.77 Ohms
Cable Type 1 or Type 2 could be selected since the induction loop resistance is
between 0 Ohms and 1.8 Ohms.
Some installations require the use of a double loop cable on the output. The double loop is easily
implemented with standard figure-8 cables. The double loop increases the field strength for a given output
current by 12dB, however the frequency response can suffer. This configuration also allows for increasing
the overall loop resistance (for small loops) or reducing the current requirement by a factor
of 4.
SETTING THE LOOP OUTPUT CURRENT
The easiest way of setting the loop current is with the “Venue Master” Induction Loop field strength meter.
The recommended field strength, based on Australian Standard 60188.4, “Magnetic Field Strength in
Audio-Frequency Induction Loops for Hearing Aid Purposes”, is 0.1 AMP/Meter average with short term
peaks up to 0.4 Amps/meter.
A field strength meter can also be used to determine the background noise levels that exist in the pick-up
area. Some sources of magnetic noise can be air-conditioning equipment, lighting and lighting dimming
systems.
Often the differing installation requirements make all but the simplest induction loop configurations difficult
to calculate. Testing in the pick-up area may be required before a full installation of the induction loop.
A quick estimate of the current requirement can be made by considering the requirements for a square
induction loop pick-up area.
Average current requirement (AMPS) Length of one side of a square pick-up area ÷ 10
Audio signals have a wide dynamic range and the peak current requirement can be significantly higher than
average. Typically this figure can be taken as around 4 times or 12dB, however this will vary with the input
program material. The output current meter on the GPT 60-IV indicates the peak output current of the