FetHead Filter is a Fethead version with a built-in 6dB/octave high pass filter with a corner frequency of 220Hz (other cutoff frequencies available on request). It is specially designed for microphones which exhibit an excessive proximity effect / low frequency response, like long motor ribbon microphones. The components which make up the filter are not directly in the signal path but are applied to the components which set the gain of FetHead Filters amplifier, thus making it frequency dependant. This way the sonic character which makes a FetHead attractive to the ear is preserved.
Protects the microphone
Some ribbon microphones do not tolerate phantom power, FetHead Filter's circuit uses Phantom power, it shields it from the microphone and offers effective protection.