U must mean a regular line driver. It amplifies the RCA output if you have a weak output from the HU. Some can amplify it up to 8v on the RCAs. Im planing on getting one in a few weeks. They also have built in Clip LEDs.
yea, you can adjust your output voltage on the RCAs I need one, cause my HU voltage is only 1.5v! Ive seen some put out 10v on the output. Put most amps cant take that much. I think mine takes up to 8v