Practical Induction Loops
The simple form of AFIL using a general purpose amplifier suffers from some disadvantages. The loop driver amplifier requires some additional circuits to overcome these. Using anything other than a correctly designed loop driver amplifier is not only unsatisfactory, but may result in a loop installation that can generate harmonics when driven into distortion, and these will cause radio interference. This must be prevented, both for sound quality and for legal reasons as it is illegal to cause such interference in these circumstances. In Europe, the EMC Directive applies, and it is also illegal to supply or install unsuitable electronic and electrical equipment.
A second factor is that many forms of hearing impairment mean that sound levels must be kept fairly constant. An effective loop driver will have an automatic level control providing a constant loop signal for a wide range of source levels. Meeting this requirement is likely to meet the interference requirement at the same time. To do this, the loop driver should give constant output for at least 30dB input range.
A third problem is the inductance of the loop cable, and its effect upon the higher frequencies of sound. To overcome this, many loop drivers operate as current mode amplifiers instead of voltage mode. By setting the amplifier characteristic between voltage and current mode, the overall performance is optimised for good bandwidth with minimum distortion. There are other options for reducing the effect of cable inductance, including reducing inductance with a multi-core cable where the conductors are connected in parallel.
Structural steel, and other metalwork in buildings can cause a problem by reducing the field strength unevenly across the loop area and causing frequency distortions. In most cases, a solution can be found using combinations of loops with phase shift between them, combined with frequency correction and increased signal strength.
There are many different ways to configure conductive loops to give different patterns of magnetic fields and solve different technical problems such as the presence of metal structures.
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