Why High Impedance Headset for Crystal Radio

Discussion in 'General Electronics Chat' started by rsteffens2, Nov 30, 2015.

  1. rsteffens2

    Thread Starter New Member

    Nov 30, 2015
    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!
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
    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.
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    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.

    rsteffens2 likes this.
  4. crutschow


    Mar 14, 2008
    The crystal signal source has a high resistance in operation.
    The Maximum Power Transfer Theorem states that for maximum power transfer, the load resistance should be the same as the source resistance, thus the use of the high resistance headset.
  5. rsteffens2

    Thread Starter New Member

    Nov 30, 2015
    Thank you very much for your thoughtful responses. Thank really helps clear things up.
  6. wayneh


    Sep 9, 2010
    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.