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The resistor provides loading for the detector. The crystal earpiece has such a high internal impedance that it is more a capacitive than a resistive load.

The high value resistor in parallel with the earpiece provides a working load for the circuit. Without it the audio signal is largely being shunted by the large internal capacitance of the crystal earpiece.

The resistor is not required if using a high impedance moving coil or moving iron earpiece such as that from an old telephone.

Trust me, if using a crystal earpiece the resistor makes a big difference.




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