As DBFan said....the RCA voltage being input into the amplifier. It doesn't actually turn anything "up", all it can do is attenuate the signal. It either allows full RCA voltage to pass through (knob "all the way up"), or as you turn the knob down it will decrease the RCA voltage going to the amp, decreasing the volume. So, you'll never really be able to turn the volume "up" past the level that it would be without the knob being in-line....