Crystal Radio
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crystal set



High Quality AM Audio Source
by
Felix Scerri

 


VC1  Variable Capacitor
VR1  Variable Resistor  47k ohm
VR2  Variable Resistor  1 meg.  ohm
C1 Capacitor 1 or 2 uf  polyester
C2 Capacitor 180 pf  ceramic bypass
C3 Capacitor 2 uf  polyester
D1 Diode  1N5711
B1 Battery  1.5 volts  AA size


            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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