Hi Friends. Here is a project that took me less time to make than I am spending on making this page! None of this is my idea. I got the idea and information from friends on The RadioBoard. At least a half dozen posters gave me the idea for this project. Here is the thread that inspired me the most. The only thing I can say is that I am one of the first to publish a project like this on the web. Another project like this can be found on this Italian web site. So, please read on and learn how to make a set of homemade headphones.
Sound powered headphones (balanced armature) are generally old and expensive. One has to fight it out with fellow bidders on eBay for their treasure. The military phones are generally more than 50 years old and require adjustments. But they work very well! If you are going to dx, you need the sensitivity that the sound powered phones can provide.
The solution to the age and cost problems are to use a pair of piezo elements.
I found that these are nearly as sensitive as a pair of well adjusted sound powered phones.
I used a set of cheap ear muffs. The piezo elements are some by the number # KBT-44SB-1A. This part has been replaced by the equivalent Mallory PT-4175W. A short piece of wire is needed to go from one side to the other. I used a piece of miniature coax, RG-174A.
The main cord I used is a piece of Belden 8723, which is a two pair shielded cable. The two pair and shielding aren't needed. The best choice for a cord is a real headphone cloth based wire cord, or a rubber covered microphone type cord. The problem with the PVC covered cords is there is audio conductivity through out the cord. When the cord brushes against anything, you can hear the mechanical noise.
First take out the two foam pieces from each side. The two smaller pieces of foam won't be used. You can fit your elements in the phones by placing one larger piece of foam inside, then fitting the element inside. Once the element is inside, turn the element so the ears are facing the narrowest edge of the muff. This will hold the elements inside.
You need to drill three holes. If you use RG-174A coax, the hole will be 7/64 inch in diameter. (0,28 mm) The headphone connecting wire holes are drilled about 3/4 inch (18 mm) above the swivel. The wire is routed through the adjustment slot and over the top of the headband. A little glue will hold the wire in place. The wire is nearly invisible as it goes over the headband.
The main cord hole is drilled close to the edge of the muff, so the wire will naturally hang down.
Now you are ready to make the connections. I placed the elements in series. I observed the polarity of the elements and wired black - red - black - red. I placed 1.5 meg ohm resistors across the elements, so the static charge would drain from the elements. I used that value because I had a lot of them. Any value of 1 meg ohm or higher is fine. Any way of wiring is fine, as long as nothing will touch and everything is out of the way.
A variation is to wire the phone elements in parallel. If you do this, the two resistors can be left out.
Since these are piezo elements, there is no dc resistance across them (ignore the 1.5 meg ohm resistors as they are too high to play a role) If you are using these in a crystal set without an audio matching transformer such as the Bogen T725 or Überformer, then you should place a 12k ohm resistor across the headphone terminals. No resistor is needed if you are using a matching transformer.
The 12k resistor is likely to give you the loudest volume. Unfortunately the selectivity is likely to be poor. If you are willing to give up volume for better selectivity, try a 50k to 150k resistor value. This will reduce the loading on the tank. A pot might be useful. Getting a transformer is the best way.
If these are used in the plate circuit of radio, then a resistor from 1k to about 10k ohm should be used. The value should be such that gives you the best volume.
Important: Use these headphones only on crystal sets or battery operated radios. Using this type of headphone around higher voltages is not considered safe! This is because the B+ voltages will be riding on the cord and elements, which can be a shock hazard.
A set of PiezoPhones can elevate you to the ranks of the master DX'er, and at the same time not be on the path to divorce court. See, you save two ways! I thank the members of the RadioBoard for bringing the proper use of these piezo elements to my attention. Mike Peebles of Peebles Originals has ready built Piezo Headphones with his own creativity available for sale. Good luck with your headphone project! ~ Dave
Exploded View Of The PiezoPhone