The main area of difference is the CPU VRM which is improved for Intel’s Core i9 9900KS with its standard 127W TDP, up from 95W on the Core i9 9900K. While Intel’s i9 9900KS wasn’t released until October 2019, ASRock had already released the Phantom Gaming X revision by July 2019 in anticipation of the release that Intel had communicated to its board partners.
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