The crystal earphone, also known as a crystal earpiece, uses a high impedance piezoelectric crystal as its main component. This crystal exhibits the piezoelectric effect, which converts electrical signals into mechanical vibration.
The piezoelectric effect is where the crystal increases or decreases in length depending upon the polarity of the electric field applied to it. It is therefore ideal for us as a transducer, which converts electrical signals into mechanical vibrations.
These devices are more efficient than the voice coil loudspeaker, and are more accurate in the reproduction of sound, because there is no energy loss associated with a voice coil and magnet arrangement. Transducers also require negligible energy to operate in comparison to a loudspeaker.
Electrical signals control the displacement of the piezoelectric ceramic and therefore they are utilised with digital circuits, and laser assemblies, where precision control is of importance.
The Classic Agent Smith Style
It is nice to know that these earpieces still exist and still look the same as always when I used them as a whippersnapper. It reminds me of my granny's hearing aid, but of course, these things are so much more in fashion now thanks to The Matrix.
My Friend Rufus... saw this on my table and straight away, he stuffed it into his ear and did an Agent Smith impersonation. However, their ones had coiled leads, if memory serves.
The Sound Quality
Compared to my vintage Crystal Radio Earphone, the sound was just as clear, but the volume was slightly quiet. This could be because of all the twists in the wire, but it could also be because the piezoelectric material has changed over the years.
The early forms of piezoelectric crystals were Barium Titanate based, however modern forms are metallic oxide based. All modern forms of piezoelectric crystals utilise PZT (Pervoskite Zirconia Titanate), because it is supposed to be more sensitive. The discovery of PZT was of course in Japan.
The piezoelectric effect works in both ways; it can convert electrical signals into mechanical vibration, and it can convert mechanical vibration back into electrical signals. Hence, it operates also as a microphone.
The younger generation may not remember, but the very early crystal earpieces doubled as a microphone, and therefore the manufacturer provided a facility to remove the ear tube. The tube secures by a screw thread and removes like this one. It is also very useful for cleaning as well.
I remember some of the very early reel-to-reel tape recorders using a piezoelectric microphone. The front-end circuit of the amplifier would often be a simple high gain transistor stage.
Opening the Plastic Case
Removing the connector cover reveals two soldered connections.
Here are the soldered connections. If you were really into crystal radios, you might like to unsolder the twisted wire and replace it with a straight pair. Of course, you void your warranty after that!
Here is the front half of the case. I managed to open it to have a look inside. The two halves of the plastic case appear to be stuck with glue. It appears to be the same type used for making the electrical connection shown below.
The aluminium layer bonds to the plastic case around the circumference edge. The purpose of the aluminium layer is to transmit the vibration energy produced by the crystal.
I am not so sure about this bit though. Using glue to secure a wire is something I have not seen before. It might affect reliability, and the ability of the design to operate for a longer period under wider conditions.
Obviously, the copper wire will not solder to aluminium metal, because aluminium forms an oxide layer very quickly when heated. My Soldering Guide for Electronics Students might help here to understand.
Manufacturers normally use a rivet to make the electrical connection, in situations like this. In digital watches, they use a spring clip for both terminals. The spring keeps pressure on the contacts to maintain the electrical connection.
There is always the possibility that the oxygen in the air may get into the glue seal and begin the oxidation process. This could affect its operation in the future, but I will just have to see.
Piezoelectric Transducer Crystal PZT
Here is a photograph of the piezoelectric aluminium plate. It is bonded around the circumference edge to the plastic case. I managed to carefully remove it in one piece.
Here is the inside of the back half of the case.
Here is a photograph showing the piezoelectric crystal in ceramic form. It is a very thin layer of PZT bonded to the aluminium plate.
The most commonly used piezoelectric crystal material is Pervoskite Zirconia Titanate or PZT. A large majority of modern crystals are made of this, and it is very much a standard these days.
Twisted Wire and 3.5 mm Mono Jack Plug
It comes with a 1 m long twisted pair wire, terminated with a 3.5 mm mono jack plug.
Every crystal radio fanatic will probably say that you do not want any sort of coupling between the aerial and the ground as it will reduce the signal strength. On a crystal radio earpiece, you ideally want to keep the two wires wide apart and straight. On my vintage one, they do not use twists in the wire.
Here is a photo of my vintage earphones. The wire is straight and not twisted.
The 3.5 mm Mono Jack Plug was very useful in the old days when you could use the earpiece as an earphone or a microphone. You simply had to jack it into the correct socket of your reel-to-reel tape recorder.
Buy / Price Variation
From what I could tell, there is only one factory in Taiwan that manufactures these; however, the number of outlets in UK and the final price can vary from as much as £14.99 to £1.89.
The cheapest brand new ones were on eBay. I paid £1.89 including postage and they promptly arrived in a nice envelope. I was very pleased with them, considering the price, and they worked on my crystal set straight away. The sound quality was adequate.