From this article, it seems one can be made using only the right coil of wire and an ordinary audio amplifier: https://spectrum.ieee.org/a-diy-audio-induction-

From this article, it seems one can be made using only the right coil of wire and an ordinary audio amplifier: https://spectrum.ieee.org/a-diy-audio-induction-loop-for-the-hard-of-hearing https://spectrum.ieee.org/a-diy-audio-induction-loop-for-the...