Many hearing aids can handle direct audio input (DAI), often via a “shoe” that attaches to contacts on the aid itself. This shoe then provides a DAI jack (sometimes called a europlug or eurojack) that cables can be plugged into.
There seems to be a complete lack of cables designed for people with a single hearing aid, so one of my upcoming projects will be to convert a Y-cable that can serve two hearing aids into a joint DAI/earphone cable instead.
The DAI connector has the following pinout:
- short/thick pin: ground
- long/thin pin: VDD (apparently — doesn’t seem to actually be connected to anything on a 3.5mm cable)
- short/thin pin: signal
Remember that a standard 3.5mm audio plug has the following pinout:
- tip: left signal
- inner sheath: right signal
- outer sheath: ground
Next stage: connecting an earphone to the plug on one side of the cable, to see if there are any impedance issues, although the cable I am using (an fmGenie one) is a very low-impedance cable because I have an Oticon hearing aid. Measured with a multimeter, it seems to be around 0?, so I think that’s good enough for me. I believe attenuated cables have a 330k? resistor in series with each signal line (according to Neil Ferguson’s bluetooth hearing aid hack page).
My ultimate goal is to get hold of a DAI jack that I can solder directly to an old earphone, so that I can just plug the spare DAI cable into the earphone directly, almost as if it were a hearing aid itself. Unfortunately, it seems that it is impossible to get hold of such things. According to Connevans, DAI jacks are only custom-made for hearing aid shoes. After some discussion, it seems like I should be able to make one myself though (with some help!) so we shall have to see how that turns out.