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