Konstantin Raudive, amongst other occupations, was an EVP researcher in the 1960's and 70's.
He wanted to devise a method of improving the quality of EVP recordings.
 
  Through experimentation, he found that using a Germanium Diode circuit instead of a microphone plugged into his voice recorder vastly helped improve the quality of these disembodied voices.

  We have found that this Germanium Diode circuit is capable of picking up RF type waves and frequencies associated with modern communications technology in which spirits may use as a conduit to communicate.
 
  You will find that this device will not pick up your own voice or any noise contamination from your recording area. We highly recommend experimenting with this device.

  Simply screw the antenna onto the device (do not overtighten) then plug this receiver into your voice recorder’s MIC input and begin an EVP session.