Diode Radio For Low Impedance Headphones

If you ever look at
construction notes for building old detector type radios the type of
headphones specified always have an impedance of 2 × 2000Ω. Nowadays the
most commonly available headphones have an impedance of 2 × 32 Ω, this
relatively low value makes them unsuitable for such a design. However,
with a bit of crafty transformation these headphones can be used in just
such a design. To adapt them, you will need a transformer taken from a
mains adapter unit, the type that has a switchable output voltage
(3/4.5/6/9/12 V) without the rectifying diodes and capacitor. Using the
different taps of this type of transformer it is possible to optimize
the impedance match.

For the diode radio (any germanium diode is suitable in this design)
the key to success is correct impedance matching so that none of the
received signal energy is lost. The antenna coil on the 10 mm diameter
by 100 mm long ferrite rod is made up of 60 turns with a tap point at
every 10 turns; this is suitable for medium wave reception. If a long
external aerial is used it should be connected to a lower tap point to
reduce its damping effect on the circuit. You can experiment with all
the available tapping points to find the best reception. With such a
simple radio design, the external aerial will have a big influence on its
performance.

Tip:
If your house has metal guttering and rain water pipes, it will be
possible to use these as an aerial, as long as they are not directly
connected to earth. Those who live in the vicinity of a broadcast
transmitter may be able to connect a loudspeaker directly to the output
or if the volume is too low, why not try connecting the active speaker
system from your PC?

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