This is a modified, brand-new MDR CD900ST, addressing its weaknesses in bass response, left/right balance, resolution, and adding re-cabling and folding capabilities. The modifications are all based on the information provided on the Umbrella Company website. Specifically: * 3-core to 4-core cable conversion * 4-core wiring is used up to the point of plugging into the equipment to minimize common impedance. * Air vent hole drilling (pinhole) * Left/right adjustment of the driver unit angle * Dummy cable installation inside the right side * Left/right Perfect Symmetry Mod * Driver unit tuning The above are the tuning recommendations from Umbrella Company. In addition to these: * Re-cabling * Dummy terminals & wiring installed in the right unit to balance the left and right sides (Ground separation wiring on the headphone side) ※ Re-cabling involves adding a plug inside the unit, which creates a significant left/right difference in the internal space. Therefore, for re-cabling, it is essential to install a similar dummy in the right unit to balance the sound. * Folding modification * MDR7506 parts are not available individually, so they were sourced from relatively good condition used items → The head section has been replaced. This makes it convenient to carry. * Upgrade of the left/right connecting wires to OYAIDE HPC-28-2U V2 * Solder: OYAIDE SS-47 * Plugs: Highest-grade silver rhodium-plated * OYAIDE P-3.5R * OYAIDE P-3.5/4SR * Ear pads: New 7506 ear pads (Determined to be the best after comparison) As a result, the weaknesses of the standard MDR CD900ST have been eliminated, and the resolution has been significantly improved. Also, the folding modification makes it compact, making storage and carrying convenient.