I was looking at throwing out some scrap office telephones at work but we are not able to dump electronic waste in to our refuse. So I stripped them down to recover the plastics and was left with a pile of electronics. All good stuff , Jolly Useful Necessary Kit (JUNK) - switches, electret microphone inserts, medium impedance earpieces and other goodies like piezo sounders. Seeing the sounders took me back to an article I saw a couple of years ago where an enterprising SWL had fashioned a couple of these sounders into a pair of high impedance headphones.
Now from time to time I like to build a crystal sets just to remind myself why I got interested in radio, trouble is the only devices I have for listening on is one of those uncomfortable crystal earpieces, salvaged from an old hearing aid. So I decided to build a pair of high impedance phones using a cheap pair of ear-defenders and two of the piezo sounders.
The sounders have a very high impedance, too high for normal use, so I wired a 43k Ohm resistor in parallel with each sounder and then placed a sounder each defender ear-cup. I wired them with the cable salvaged from one of the phones in the same configuration as a stereo headset, terminated in a miniature stereo jack plug. This enables the inserts to be wired in series or parallel, in parallel the impedance of 21k Ohms is just about right for crystal set use.
Any sounder can be used, from the type you may find in musical greetings cards to the old trim-phones - bigger the sounder the better the low frequency response.
I'm very pleased with the result, my new headphones are comfortable and have a nice frequency response for communication use, together with a couple of useful resonances for peaking Morse signals - particularly good when used in conjunction with one of my simple valve regen receivers.
Selection of salvaged piezo sounders
Piezo sounders installed in cup including
original sound deadening foam behind each insert
Finished phones including salvaged curly wire
Link to original article
https://drive.google.com/file/d/1hubS5vzhiqMuaMFrqDx2c5GLJDHal8lT/view?usp=share_link
