Tom,
Am radio signals are modulated on both sides, these two sides are
effectly out of phase with each other and thus cancle out. So the diode is thereto remove one side of the signal. This gives a rsdio frequency signal that isdi
modulated on one side at an audio rate. A small capacitor is often put across
the head phones to filter out the radio frequence part. Another approach is to
use a crystal head phone that uses the piezoelectric effect to filer out the
high frequencies, and is very sensitive to audio signals.
Gary