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Copyright
© Austin Hellier 12/02/2001
Diagram 1. Crystal Earphone - an inside view.
How It All Works
Crystal radios are, at best, a compromise when it comes
to tuning (selectivity) and volume (sensitivity) and there are a
number of reasons for this. One of them is what is called 'impedance
mismatch' and this means simply, that some components have a difficult
time working in with other components efficiently. While most receiver
designs will work, some are deifinately better than others!
Matching coils to aerials and tapping the diode into the coil,
are good examples of compromises that have to be made in order to
obtain reception with good results. Another area of impedance mismatch
is in the output stage of the receiver. Crystal Radio designs prefer
a high impedance (hi Z) earphone, so that an adequate signal voltage
from the tuner and detector stages can be developed across it. Many
years ago (when I was young) you could buy hi Z headphones(2000
to 4000R) quite cheaply, as these were often sold by radio shops
and disposals warehouses.
Nowadays, they're quite rare and expensive, but during the 1950's
it was discovered that a cheap and simple alternative could be made
using 'pietzo' crystals. These crystals will vibrate in unison with
an alternating or pulsed signal that is wired across them, and as
you can see in the above diagram, the crystal material is placed
between two plates, just like a capacitor. One of the plates is
a simple contact, while the other one doubles as a diaphragm, which
vibrates along with the pietzo material, in order to generate sound
waves.
The Crystal Earphone is essentially a capacitove device -it has
no direct connection (such as a coil in the older style dynamic
headphones) and can therefore exhibit a fairly high impedance to
the signal fed into it, which is upwards of 10,000R. The Crystal
Earphone is therefore, a very sensitive device - just rubbing the
ends of the connection wires together will bring a crackle in your
ear! Since the earphone acts like a capacitor (charging and discharging
in unison with positive and negative swings of the signal) it can
be affected by interference such as power spikes radiated from electric
motors and inductors (eg fridges and flouro lights) and this is
why you need the 100K resistor across it - to ensure that it discharges
at a fast but steady rate, and to stop strong signals from causing
distortion.
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