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:
http://www.mreilly.com/WebPages/loop/loop.html
http://www.marcspages.co.uk/tech/deafloop.htm
Best Answer
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.