Separating the grounds for sensitive signals
Attach that fifth WIRE, from frame of the tone_arm, to the chassis of the PreAmp.
This greatly lowers the electric fields attempting to couple from the tone_arm metal, onto the twisted pairs (Left and Right pairs) inside the tonearm.
The source of these fields? In one case, the Efield trash was from primary_secondary coupling (thru fish_paper) of the tiny transformer powering the turntable motor.