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Hearing aids, Inductive loops question

Discussion in 'Electronic Design' started by martin griffith, Jul 1, 2006.

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  1. My partially deaf neighbour cant afford to install an inductive loop
    in her living room so she can hear the TV through the T position on
    her hearing aids.

    I've offered to see if I can make one, in exchange for home cooked
    food. This seems like a fair exchange

    A bit of googling suggests that a 25way cable, wired as a coil, fitted
    around the ceiling edge will suffice.( approx 3m by 4m)

    Any idea how much drive is required for the loop. I was thinking of a
    741 and a T0220 npn/pnp output pair, run off +- 15V as a starting
    point.

    Any comments, better configuration info, bandwidth limits. etc. would
    be appreciated


    martin
     
  2. Nico Coesel

    Nico Coesel Guest

    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.
     
  3. Mark

    Mark Guest

    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
     
  4. 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
     
  5. Nico Coesel

    Nico Coesel Guest

    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.
     
  6. Chris Jones

    Chris Jones Guest

    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
     
  7. Reg Edwards

    Reg Edwards Guest

    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.
     
  8. Reg Edwards

    Reg Edwards Guest

    ==========================================

    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.
     
  9. Nico Coesel

    Nico Coesel Guest

    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.
     
  10. Kevin White

    Kevin White Guest

    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
     
  11. Nico Coesel

    Nico Coesel Guest

    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.
     
  12. John B

    John B Guest

    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:

    <http://www.applesoundshop.co.uk/acatalog/AVX_Loop_Amplifiers.html>

    They even sell tape to fit under carpets.
     
  13. true, but there is no appreciable budget, it will be all from my junk
    box


    martin
     
  14. Chris Jones

    Chris Jones Guest

    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
     
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