PS AUDIO Humbuster III - used with factory packaging. Owner's Manual available on line. Device has several scratches on the bottom underside, and a couple of nicks top side. Item is in good working order. The Humbuster is designed to block DC voltage on the AC line. DC voltage on the AC line can cause mechanical hum to appear in your equipment. Mechanical hum is described as being that noise generated by the power transformer inside a piece of AV equipment. The Humbuster will have little effect on hum and noise heard through the loudspeaker or headphones. In some instances, it is possible that the reduction of mechanical noise in the unit's internal transformer may reduce the magnetic field of the transformer and thereby reduce hum heard through a loudspeaker. Please ask any questions you may have prior to purchase.