These units are used to broadcast local audio, such as from a public address system or local microphone. They accomplish this by producing a modulated magnetic field that a hearing aid is capable of picking up.
Not many controls on this bit of equipment. A bi-colour LED for status indications, a microphone, external audio input, charging input & a power switch.
Popping the cover off reveals a small lead-acid battery, 2.1Ah at 12v. This is used when the loop is unplugged.
Here’s the main PCB, which takes care of the audio & battery charging. The inductive loop itself is just visible as the tape-covered wire bundle around the edge of the casing.
Here’s the input section of the main PCB. The microphone input is handled by a SSM2166 front-end preamplifier from Analog Devices.
This audio is then fed into a TDA2003 10W Mono Power Amplifier IC, which directly drives the induction coil as if it were a speaker. Any suitable receiving coil & amplifier can then receive the signal & change it back into audio.
