The TA80 is a telephone accessory made by Oticon. There may be some other similar units. It has a microphone and an amplifier powered by a size 675 hearing aid battery. The normal output of the TA80 is electromagnetic radiation provided by a telecoil built inside it. It is attached with an elastic band over the earpiece part of a telephone handset and it picks up sound acoustically. The sound is amplified and sent to the built-in telecoil. A corresponding telecoil in a hearing aid can then receive the signal and the hearing aid converts the signal to sound.
Since it picks up sound acoustically from the handset it can be used on telephones that are not hearing aid compatible (HAC) or on telephones that have weak HAC signal. It can also be used as a handheld microphone and it can be placed close to a speaker on a TV.
In addition to the built-in telecoil, it has a jack for a silhouette which allows two ear listening. One hearing aid picks up the signal from the TA80 held up to the ear, and the other hearing aid gets the signal from the silhouette.
The TA80 works just fine for many people as I have described above. However, some people need more power to blast through electromagnetic interference. They need to keep their hearing aids set to a minimum volume to avoid interference. This project describes a patch cable that will connect the TA80 to an amplifier such as SoundWizard or PocketTalker and a neckloop can be used on these amplifiers.
I use this arrangement mostly on pay phones since the handset cable of a pay phone cannot be unplugged to attach any other kind of device. There are some other ways to connect phones to hearing aids, but for pay phones, I like this approach the best.
The patch cable has a "sub-mini" (2.5mm) two conductor plug on one end
and a "mini" (3.5mm) two conductor plug on the other end. The middle of
the cable has a piece of perf board with a load resistor across (in parallel
with) the signal conductor and the common conductor. It also has a capacitor
to prevent any direct current (DC) flow between the TA80 and the amplifier.
Any physical arrangement will work as long as the diagram is followed.
The load resistor is necessary because the TA80 is expecting a silhouette
with an impedance of about 20 to 30 ohms to be plugged in. A patch cord
without the load resistor will not work since most amplifiers have a much
larger input impedance, usually about 10,000 ohms. See the diagram below.
I used a 4.7 MFD capacitor, one that I happened to have handy. A lower value will probably work. Polarized capacitors should have their + side towards the amplifier.