From: mcmurtrie@pixelmemory.us   
      
   In article , Randy Yates    
   wrote:   
      
   > Is there ever a reason NOT to use a differential amplifier as   
   > the first stage of a microphone preamp?   
      
   Differencing doesn't have to be in the first stage.  I just has to be   
   performed before common mode noise would be amplified too much.   
      
   In laboratory circuits, sometimes a high impedance voltage follower   
   buffers the input before the differential amp.  The output of that   
   follower also drives a shield on the input wire.  The result is a   
   virtually perfect cable with no capacitance and no insulation losses.   
   (It only works as long as the electrical wavelength of the input is much   
   longer than the wire, as with audio.)   
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