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's circuit uses 48v phantom power but shields it from the microphone, so the microphone is protected.
"I have a Tascam DR40 and am about to get a Rode Lavalier (condenser) mic. Would the Fethead Phantom be the right choice?"
A FetHead Phantom can be used to get extra amplification, it has an xlr input. Does the Lavalier mic operate on 48V Phantom power and does it have an XLR male connector? In this case it should be no problem to use it together with a FetHead Phantom
"What is the difference between the regular FetHead and the FetHead Phantom?"
A regular FetHead is designed for dynamic microphones and ribbon microphones, it has a relatively high impedance of 22k which allows the microphone the microphone to operate at "no-load" condition. A FetHead Phantom is designed for for Condenser microphones or other Phantom powered devices, it passes Phantom Power to the microphone.