FetHead Filter is a Fethead version with a built-in 6dB/octave high pass filter with a corner frequency of 220Hz.
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.
FetHead Filter can be distinguished by the type indication ring and green markers below the TritonAudio label.