I found a couple of DIY pages and I will post the content here. This first one is easy, but doesn’t have many “turns” so I don’t know how powerful it would be.
"I use a neckloop to listen to an MP3 player while at work (actually, I listen to the FM radio on it more than the music)
Points I’ve found:
1: The loop contains a couple of resistors that sum the two audio channels into one. (i.e. you wont get stereo from a neckloop)
2: Compared to the ‘normal’ ear buds these players are designed to be used with, a neckloop needs a lot more audio level applied to it, so the battery runs out faster.
3: Being able to listen to just the radio or music - no background noise at all - while at work is great! (You may also be able to listen to both the T-coil and normal sounds, if the aid has provision for it). You dont need to pay that for a neckloop, you can make your own.
A meter or so of flexible telephone cord (more conductors the better),
3.5 stereo plug,
(2) 1/8th watt small-value resistors (I use 12 ohms).
Cut a length of the cable long enough to go over the head, plus a bit. Note that making it longer is no advantage, it would just place the loop further away from the t-coil in the aid.
Strip and tin all wires in both ends.
Connect the wires to form a coil, i.e. if it has 4 conductors of red, green, white & blue, then you might connect:
Red (at one end) to green.
Connect other end of green to white.
Connect other end of white to blue
You would now have one red and one blue end left over.
Connect one of these to the common conductor of the plug.
Solder a resistor to each of the tip and ring conductors of the plug.
Solder the other ends of the resistors together and connect to the other
Naturally, you’ll want to insulate all theses connections from each other with heat-shrink tube.
Realizing you have left off the cover for the plug, you now de-solder all the above and feed the wires through it then re-solder [grin] Plug in and test."