i noticed a slight humming comming from my phono stage when my record player had the arm lifted off a record. i unplugged the tt from my amp as well as its earth and the humming is still there. i have to turn the amp volume up quite high to hear the humming but its in both chanels ( the amp and phono stage is dual mono design ) and its on mm and mc positions. itds sounds like huuuuummmmmm then it moves up a notch and sounds HHHUUUUUUMMMMM then it gets quieter as if someones switching on and off a loudness switch. is this normal? do i need a seperate phono stage?