Re: Hearing aids, Inductive loops question

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.

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