Philosophy 
FetHead is a high quality, phantom powered in-line preamplifier providing improved sound for ribbon and dynamic microphones. The electronics are housed in a robust metal chassis with a balanced 3-pole female XLR input and a balanced 3-pole male XLR output, rugged enough for use at home, in the studio or on tour. 

Which microphones benefit
In short, we noticed an improvement on all tested dynamic microphones. A Shure SM57 or SM58 for instance, sounds more responsive over a broader frequency range. When mated with a high-end dynamic microphone like a Sennheiser MD441, or Shure SM7 FetHead really shines. Ribbon microphones benefit greatly too because of a much better impedance match, which lessens the load on the microphone, giving it better transient response characteristics. FetHeads high quality, low noise signal amplification also extends the usable range of a ribbon microphone, by making it more suitable for recording softer passages without introducing noise issues. 

Protects the microphone
Some ribbon microphones do not tolerate phantom power, FetHead's circuit uses Phantom power but shields it from its input (and the microphone's output), as a result it offers effective protection to the microphone. 

Low noise
Fethead uses low noise jfets configured as a double class a amplifier. Jfet's are among the most quiet amplifying elements available today. The calculated noise figure of FetHead is lower than we are able to measure.