AMPLIFIER AND BIAS FOR THE CRYSTAL RECEIVER
(2003)
Amplifier and bias for the crystal receiver.
The LF amplifier
Much more stations can be received, louder and with a much better quality than with the crystal telephone.
But it is intended for measurements and tuning purposes. When you cannot hear stations, it is impossible to adjust the
antenna tuner.... With the amplifier, weak stations can already be heard when the antenna tuner is not set to maximum.
Adjust the antenna tuner for maximum sensitivity and hope that you can also hear the station with the crystal
telephone, without the LF amplifier.
The amplifier is also a nice extra option for comfortable listening with normal headphones to become familiar with the
Medium Wave. Especially young people never have listened to the Medium Waves!
With the 20 meter long wire and after some experience, I do not need the LF amplifier for tuning adjustments
anymore as there are always stations that can be heard with the crystal telephone. It is only used now for
for comfortable listening with normal headphones and for measurement purposes.
Diagram of the amplifier and bias control for the crystal receiver.
big diagram
Description
The gain of the LF amplifier is not very high, T1 is more an impedance matching circuit, gain is between 1.5 and 10x,
depending on the position of S3, the simple volume control. In the low position, the high input impedance
(680k resistor) hardly loads the detector.
Transistor T2 is the final amplifier and has a gain of 10x. The two earpieces of the headphones are connected in series
instead of in parallel for extra audio output.
Battery life: 1 mA current, one AA cell is good for 2000 hours of use.
The bias circuit is switched on with S1 and the bias voltage is controlled by the 100 k potentiometer.
Bias voltage control
The purpose of the bias voltage is to decrease the negative effect of the turn on voltage of the diode.
The effect of the bias voltage is rather unpredictable, sometimes it has a positive, sometimes a negative
effect on the sensitivity of the receiver. It depends on the tap/top switch, frequency range etc.
My general experience is that the positive effect is negligible in most situations, you can do without it.
But sometimes it had a positive effect on the reception of weak stations and the audio quality of strong stations.
Mostly there was a positive effect when the diode of the receiver was on the tap position and a negative effect
when on the top position.
Inside view
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