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The OEM Apple RAM is 1.5V. The Intel Ivy Bridge i5/i7 CPU memory controllers are designed for 1.5V RAM, but will run stable with 1.35V. If you mix 1.35V and 1.5V RAM, it will run at 1.5V, which will not harm the 1.35V RAM, although it's best not to mix voltages.
 
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