Why High Impedance Headset for Crystal Radio

Thread Starter

rsteffens2

Joined Nov 30, 2015
2
I am trying to understand the electronics theory behind why a high impedance, high resistance headset is needed for a crystal radio.

Obviously, a crystal radio can only produce extremely low voltage/current. A high resistance headset seems like it should need high voltage/current to overcome that high resistance. I realize this reasoning is wrong, but can someone explain to me why it is wrong. That's what is giving me trouble.

Thanks so much!
Randy
 

alfacliff

Joined Dec 13, 2013
2,458
higher impedance puts less load on the detector, allowing higher Q or quality factor in the tuned circuit. loading the detector causes wide tuning and low sensitivity.
 

AnalogKid

Joined Aug 1, 2013
8,540
For any current, the voltage developed across a load is directly proportional to the resistance of that load (Ohm's Law). So for the very small current available from nothing more than a tuned circuit and a ultra-low Vf rectifier, a high impedance earphone develops the most voltage across it. Couple this with the fact that generally speaking, crystal-based transducers (microphones, piezo beepers, etc.) are high-voltage/low current devices by their very nature, and it looks like a perfect match.

Operationally, a crystal earphone is the exact opposite of a traditional voice-coil speaker or earphone. To make the same acoustic power level, one needs relatively high voltage and low current, and the other low voltage and high current.

ak
 

wayneh

Joined Sep 9, 2010
16,399
I often describe impedance matching as being like choosing a gear on a bicycle. Small tweakings can make a large improvement in power delivered to the load.
 
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