The GTX680 does do 4K@60Hz via DP1.2 I am using it on my Philips BDM4065UC 40" monitor. There are some quirks with the GTX680 with 4K monitors.- Even if flashed for an Apple boot screen you will not see one @60Hz but will see one @30Hz. The HDMI only does 4K@30Hz
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As the GTX680 does work with DP1.2 to deliver 60Hz@4K I would suspect that the DP-HDMI adapter is the problem.
I have the same monitor - 5770 works just fine with it at 2560x1440 @ 60hz, but 4k @ 30 hz. My GTX780 works just fine with the adapter (4k @ 60hz) on a 4k HDMI 2.0 TV. 5770 doesn't work at all with the adapter. The 680 worked only at 24hz at higher resolutions. I totally believe that the 680 would have driven my 4065 monitor via DP only, but since that wasn't my goal, I didn't test it. Also, my 680 was an unflashed 680 "Classified" version. Not sure if that would make a difference.
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A lot of those adapters will only do 4K@30Hz.
Try this one:
https://www.lindy.co.uk/cables-adap...o-hdmi-4k-60hz-active-adapter-converter-p9448
Or this one:
https://www.scan.co.uk/products/clu...nverter-cable-supports-8bit-4k-uhd-60hz-and-f
The adapter that I got was made by Club3D, and successfully works 4k @ 60hz using my GTX780. The problem I'm having with all this is that my 5770 is outperforming my GTX780 with Pro Tools. I ran a Cinebench R15 test, and the 5770 scored higher than my MacVidCards flashed 780, which makes no sense to me.