To avoid the possibility of amplitude distortion it is best to load an

amplifier with a resistance of about the same value as what the

amplifier is designed for.

So a resistance of roughly 8 ohms can be connected in series with the

low impedance loop.

The resistance has a beneficial effect - it levels the frequency

response. It allows more turns to be used in the loop without loss at

the higher audio frequencies.

The loop thinks it is being fed from a constant current source with an

internal resistance of 8 ohms. This is considerably higher than the

impedance of the loop itself even when it has several turns. It is

this constant current characteristic which maintains the frequency

response.

The amplifier is also happy because it thinks it is driving an 8 ohm

loudspeaker.

The smaller the room area to be covered, the greater the number of

turns allowed. The number of turns can be increased until the loop

impedance is several ohms at the higher audio frequencies, say 4 ohms

at 6 kHz if the amplifier has been designed for an 8 ohm loudspeaker.

But it is very non-critical.

The loop impedance is that of its inductance which can be calculated

prior to installation. or measured afterwards.

Reasonable hi-fidelity can be expected.

But performance ultimately depends on the sensitivity of the pick-up

receiver and on the level of noise, interference and 50-60 Hz mains

hum and its harmonics.

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I have made a simple calculation.

For a room 5 metres (about 16 feet) square, and an amplifier with a

resistor of 8 ohms in series with the loop, two turns of 14 AWG wire

will do very nicely.

The inductive reactance of two turns on a 5 metre square loop at 6 kHz

is 4.8 ohms.

The amplifier will think it is driving a loudspeaker of impedance 9.3

ohms and will be quite happy.

The power output required from the amplifier will depend on the

sensitivity of the pick-up receiver and the background noise level.

With a smaller room, 3 turns could be used without undue loss in the

high frequency audio response. This would reduce the power required

from the amplifier.

With a large room, 20 metres (60 feet) square, only one turn could be

used for high audio quality and a high-power amplifier with an 8 ohm

series resistor would be needed.