Fostex ET-RP2.5BL 2.5 mm 4-Pole Balanced Cable for RP Headphones

The ET-RP2.5BL ends in a 2.5 mm four-pole plug, the balanced connector used by an earlier generation of portable players and by several amplifiers still in service. If your source has that jack rather than a 4.4 mm Pentaconn, this is the lead that gets an RP headphone onto its balanced output.

Fostex lists it as an optional cable for the T60RP and the RP headphone series. The supplied unbalanced cable stays in the box for laptops and phones; this one lives with the balanced player.