Here's what Lehle have to say about the DC-Filter:
The Lehle DC Filter eliminates DC voltage from analog signals. Audio signals containing DC voltage cause ugly switching noise. Now, both line and high-impedance signals are cleaned - but with no loss of sound. The DC Filter is of full stereo type, and thus filters stereo - and, of course, mono - signals. This little aid can even process balanced signals. The Lehle DC Filter is a passive filter, so it needs no power supply.