Converting 5v PWM Noctua NF-A8 to 12v
The PWM output of the mainboard is usually a open drain configuration and it works with 3.3&nbsp;V or 5&nbsp;V, so it should work with the lower voltage fans as well.

The TACH signal is also a open drain configuration inside the fan. The mainboard however will have a pull-up to 12&nbsp;V. I haven't found the specified maximum current for motherboards on that pin, but I guess it will be okay for the 5&nbsp;V fan as well.

Noctua has a PWM specification which tells the maximum current is the same for 12&nbsp;V and 5&nbsp;V fans, so it should work fine (the current being independent because the pull-up is on the 12&nbsp;V line of the motherboard).

You don't actually need a separate step down converter. 5&nbsp;V is readily available inside your PC already - the red wires from an ATX power supply carry 5&nbsp;V.