Hearing aids, Inductive loops question

I've been involved in the hearing loop business several years ago and designed an amplifier for that purpose to be used for big installations (churches, conference rooms, etc) several years ago. I know a few basics on this subject and also know there is a lot of nonsense floating around (even Phillips got it all wrong in the past).

You can use a standard stereo amplifier to drive a loop. It is best to use thin wire, like ordinary telephone cable. If you use a stereo amplifier, you can make 2 parallel loops to get twice the field strength. Never use multiple turns since this will cause serious deterioration of the high frequencies. Keep in mind that the amplifier will run hot since nearly all power will be dissipated in the amplifier. Look for an amplifier which has a thermal shutdown.

The ceiling may not be the best position for the loop.

the folks over at rec.audio.pro may be able to help you..

My GUESS is it would take at least a several Watt ampllifier...

Mark

well, RAP does have a couple of experts, but there is generally more chaff than wheat there, I'll go with what Nico suggested.

I'll probably scale it down to cover the sofa area to start off with

martin

Definitely no. Using thin wire causes the loop to have some resistance so the amplifier is not completely shorted. An extra resistor can be added, but not more than 1 Ohm. The typical inductance of a induction loop is somewhere between 100uH to 200uH (depending on the amount of steel object in the room and walls). This means that the worst case impedance around 5kHz is 6.3 Ohms. If you insert an 8 Ohms series resistor, you'll lose more than 6dB in field strength. If you create multiple turns, the impedance of the loop goes up with the square of the number of turns, hence you'll lose the high frequencies very rapidly.

When driving induction loops, the amplifier is used as a current source. Most amplifiers will drive a 1 Ohm load perfectly, but they won't reach their rated power output.

Depending on the amplifier, it may be desirable to put a power resistor in series with the loop to bring the load of the amplifier up to 4 Ohms or 8 Ohms, whatever the amplifier is designed to drive. This will probably mean you'd have to turn up the gain of the amplifier a bit, but should prevent the amplifier from being damaged. Also if there is more resistance in the circuit, then it could be worth trying multiple turns again because the time constant will be L/R, so it you increase R, then you may be able to increase L also without undue loss of audio frequency response. This could help in getting a loud enough signal.

Chris

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.

"Reg Edwards" wrote

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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.

The load resistance of an amplifier is dictated by its power supply voltage and cooling capacity. Driving low impedance loads is usually no problem as long as you don't crank the volume up all the way.

This is true for very small loops (around a sofa or coffe table) where the impedance of the loop is very small. It won't work for a loop in a modest living room.

Nico Coesel wrote: ...

...

Do they put high-frequency de-emphasis in the hearing aid? If you maintain a constant flux density as the frequency rises you will get a rising amplitude response at the receiver.

A simpistic model of the transmit and receive windings would be a transformer - if you drive a transformer with constant voltage as the frequency rises you will get a constant voltage out.

kevin

I don't know exactly, but I suppose they do. I do know the current versus frequency response is supposed to be flat in order to keep the magnetic field within limits.

martin griffith scrobe on the papyrus:

It depends on how much spare time you've got, but you might do better to buy the amplifier and cable here:

They even sell tape to fit under carpets.

true, but there is no appreciable budget, it will be all from my junk box

martin

The only caution I would add, (apart from not turning up the volume control too high and exceeding the current capability of the output transistors) is that if the amplifier is direct-coupled (no output ac-coupling capacitor) then a nearly purely inductive load with very low DC resistance might cause significant direct current to flow due to any DC offset in the amplifier. If, as you suggest, you use thin wire and the wire is long enough (which is equivalent to adding series resistance), then it may well not be a problem.

Also, when I mentioned using multiple turns, I meant maybe two or three instead of one. For some very small rooms this may well be beneficial.

Chris