This set up was inspired by one shown to me by my Uncle when I was 11 years old. However I recently found an almost exact copy in a book called THE VOICE OF THE CRYSTAL by H.P. Friedrichs.
All wood is padauk from West Africa in it's natural color. The coil was wound with a small lath using a whole bunch of 42 gauge wire and ended up with 28K ohms. Rather than a steel disk I cut the end out of a styrofoam cup and glued a pea sized steel plate to the center. This vibrating disk is held in place between two sections of PVC which also serves as a sound chamber. Height adjustments for the coil/core above the disk is made with the thumb screws on top of the coil beam. Long springs keep the beam steady. Sound is conducted through the surgical tubing and delivers enough volume to hear local stations
All of my projects are for sale so that I can attempt more of the same. This unique earphone is $100.
This little project was inspired by the book INSTRUMENTS OF AMPLIFICATION by H.P. Friedrichs. Three coils were wound on plastic sewing machine bobbins with two of them being 42 gauge wire and one being 22 gauge (primary). The 42 gauge coils measured almost 2K ohms each and the primary coil was 1.56 ohms. They were mounted on a core of bundled iron wire with their magnetic fields being aligned in the same direction. The wire bundle was then formed around to make an endless loop and the whole thing went inside the PVC cap. Brass posts were already installed and it was a simple thing to connect the coil wires to the proper leads so that the primary coil was positioned in the middle.
I had a test circuit that would only work if connected to a cheap Radio Shack 9 volt amp. However, when this little transformer was applied, any hi impedance headphone would work fine. In fact, the HOMEBREW EARPHONE worked better with this set up than with any other set I have. The secondaries are tapped so that one can use 2 ohms or 4 ohms. While either combination works the 4 ohm produces a much deeper and richer tone.
Very unique. $50