The Phanteks’ PWM Y-Splitter cable allows you to connect two PWM fans to one PWM port for optimal performance and cooling. Only one of the two fan connectors carries the speed control signal, thus there will be no interference when using two PWM fans. And the fully sleeved cable design makes for a clean, professional look inside your computer case.

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Compatible with 4-pin PMW fan connector and 3-pin fan connector
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Only one of the two fans sends its tach signal (the one with full 4-pin connector) and reports its RPM, but the other one will still follow the PWM signal.
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The other connector's tachometric signal pin is missing, which ensures your fan speed monitor won't bounce all over the place from getting two sense signals. This guarantees your motherboard gets an accurate RPM signal.
Pros: Works as advertised. Accommodates 3 or 4 pin fan connections. Only one of the two plugs has the 4th PWM pin, but this is fine as your motherboard or controller only needs to read the speed on one fan to control both. The connectors are solid, and it is nice that most of the cable is sleeved.
Cons: None come to mind.