DIY Hearing aid T-Loop. Tweaking the resistance of inductance coil

I'm trying to set up a simple hearing aid T Loop circuit to experiment with.

From what I've read so far I'll need an inductance loop powered by an amplifier and the T coil in the hearing aid should pick up its magnetic field.

Surprisingly to me, other than ensuring the magnetic field is strong enough and that the two coils are close enough together it doesn't seem to need any thing else in the way of setting up to tuning in to make it go. Am I wrong? (Mechanical engineer! Already way out of my depth!)

So for an amplifier I planned using something with a standard headphone jack line out. E.g. a radio, MP3 player or sound card. If this doesn't give me enough power I'll switch up a to stereo amplifier.

I understand I need to match the impedance of the amplifier and the inductor being connected to it otherwise I'll overload the amplifier.

My question is: can I use a resistor to bring the T loop inductor up to the required resistance? As I'll need quite a lot of wire to get anything close the 33 Ohms I measured my headphones to be. I assume a small stereo speaker would be even more.

Would the ratio of extra resistance from the resistor to the resistance of the inductor matter much?

My guess is both of these shouldn't matter, as it's not a tuned RF circuit, just a rather large, rather weak electromagnet?

Couple of sources:

The resistance of the loop plus the added resistance needs to present a reasonable load to the amplifier - it is not critical and I would expect that a headphone output could drive 15-20 ohms with no problem.

However the magnetic field created depends upon the total ampere-turns from the loop (product of current and the number of turns). A headphone output will probably not be able to generate enough current for good reception if you only have a few turns.

I think you will need to use an audio amplifier to be able to drive the loop hard enough.

• Thanks for your answer. Is it as simple as measuring the resistance of a speaker that would normally be connected to that amplifier then adding a regular resistor to your induction loop circuit to ensure the loop of wire matches it? – Hades Aug 22 '15 at 23:32
• That should do to make sure you won't damage the amplifier. Most amplifiers are very lenient and will acceptably drive a lower resistance than normal, maybe 1/2 of normal. – Kevin White Aug 22 '15 at 23:35

I set up inductive loop transmitters for music festivals and such. Generally speaking, this is a square or rectangular area about 10 feet by 15 feet or so.

We trench in a multi-core cable and bring the free ends of the cable to a fairly-sizeable audio power amp. Anywhere from 200 Watts on up. Then connect all of the individual conductors in the cable in series so that you wind up with a single pair of leads that is then connected to the output of the power amp.

The cable is picked so that the DC resistance of all of the conductors placed in series is somewhere between 4 to 8 Ohms.

It doesn't take all that much power to drive the loop.

• Helpful, but OPs scale is quite a bit smaller. What's the receiver power/inductance compared to the transmitter coil? 200 W minus the loss in air? (I'm not sure I'm asking the right question) – Passerby Aug 23 '15 at 1:32
• And what is the system for? Monitors? Main speakers? Can you link me to some more info I'd love to learn more. – Passerby Aug 23 '15 at 1:33