Circuit Diagram
Description
This is basically a crystal radio with an audio amplifier which is
fairly sensitive and receives several strong stations in the Los Angeles
area with a minimal 15 foot antenna. Longer antennas will provide a
stronger signal but the selectivity will be worse and strong stations
may be heard in the background of weaker ones. Using a long wire
antenna, the selectivity can be improved by connecting it to one of the
taps on the coil instead of the junction of the capacitor and coil. Some
connection to ground is required but I found that standing outside on a
concrete slab and just allowing the long headphone leads to lay on the
concrete was sufficient to listen to the local news station (KNX 1070).
The inductor was wound with 200 turns of #28 enameled copper wire on a
7/8 diameter, 4 inch length of PVC pipe, which yields about 220 uH. The
inductor was wound with taps every 20 turns so the diode and antenna
connections could be selected for best results which turned out to be 60
turns from the antenna end for the diode. The diode should be a
germanium (1N34A type) for best results, but silicon diodes will also
work if the signal is strong enough. The carrier frequency is removed
from the rectified signal at the cathode of the diode by the 300 pF cap
and the audio frequency is passed by the 0.1uF capacitor to the
non-inverting input of the first op-amp which functions as a high
impedance buffer stage. The second op-amp stage increases the voltage
level about 50 times and is DC coupled to the first through the 10K
resistor. If the pairs of 100K and 1 Meg resistors are not close in
value (1%) you may need to either use closer matched values or add a
capacitor in series with the 10K resistor to keep the DC voltage at the
transistor emitter between 3 and 6 volts. Another approach would be to
reduce the overall gain with a smaller feedback resistor (470K). High
impedance headphones will probably work best, but walkman stereo type
headphones will also work. Circuit draws about 10 mA from a 9 volt
source. Germanium diodes (1N34A) types are available from Radio Shack,
#276-1123.
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