Re: Hearing aids, Inductive loops question

Chris Jones <lugnut808@xxxxxxxxxxxxxxxx> wrote:

Nico Coesel wrote:

martin griffith <mart_in_medina@xxxxxxxxxxx> wrote:

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

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.

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.

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.

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