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When using crystal sets as high quality AM/MW audio program
sources,considerable
audio distortion can result from low signal levels,due to issues to do
with the diode turn on voltage barrier or "door",as I call it.I have
found
the use of "bias" an excellent method of overcoming diode detector
distortion
essentially regardless of incoming RF signal level.My method of
applying
bias is done in a rather unique manner by breaking the "dc" path of the
anode of the diode from the feeding tuned circuit with a capacitor,and
applying bias directly to the anode of the diode,through an adjustable
potentiometer arrangement and a 1.5v battery,referenced to circuit
earth.The
potentiometer allows full adjustment of bias from full off to full
on,allowing
for any signal strength situation.
Contrary to what has often been written,I have not really found any
discernable
improvement in sensitivity when using ordinary germanium diodes such as
the 1N34A,with applied bias, however bias does help ordinary silicon
diodes
such as the 1N4148/914 series to perform adequately as
RFdetectors,however,I
have found bias most useful in eliminating diode detector audio
distortion.I
have also found that hot carrier diodes perform best in this
application,having
a particularly low noise audio profile.I have used mainly 1N5711 hot
carrier
diodes with excellent results,but I have also sampled other hot carrier
diodes with excellent results,including diodes called "germanium diode
equivalents" which are apparently variants of hot carrier diodes
(available
as BAT 46's and 48's in this country).IN34a's will work with bias,but
are
not recommended.
In practice,it is a easy matter to simply adjust the potentiometer for
the "cleanest" audio,remembering that at either extreme of the
potentiometers
rotation,detection will simply stop,that is,diode being shorted out at
one end or saturated at the other (full bias supply applied).One note
of
caution,loud clicks and audio transients can be produced and fed to the
audio system under some conditions during adjustment.Conduct all tests
under low volume conditions.The bias circuit can be used with any
"typical"
crystal set circuit.When the crystal set is used with a following audio
amplifier,it may be necessary to interpose an audio preamp to raise the
audio level to a nominal "line level" value,compatible with the line
level
inputs of most audio amps.I use a simple one transistor common emitter
stage using a low noise BC549/550 transistor.A standard opamp non
inverting
audio preamp stage is also very suitable.Don't skimp here,the "biased"
crystal set tuner deserves a high quality audio preamp stage.I also
consider
it quite important to have a defined, reasonably high value resistive
load
(around 47k) on the diode output.A simple potentiometer level control
with
the wiper feeding the following preamp or audio power amp works
nicely.Be
sure to also include a DC "blocking"capacitor in the audio signal
path,to
eliminate any slight DC offset, following the "diode load"
potentiometer.Prepare
to be amazed at the quality of wideband AM audio.
Felix Scerri vk4fuq.
Queensland Australia.
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