Review on Noctua NF A4X20 FLX Premium 40X20Mm by David Cronin

The fans are quiet, pump some air and the extra ports are nice. The "anti-vibration" mounts on the corners of the fan are just rubber pads glued to low-quality glue. Despite careful assembly, both pads of the two fans we bought fell out. In addition, they are of high quality and a good product. 4 stars for cheap pads and somewhat misleading product description. The product description states that they are useful to replace fans in network equipment. Be careful when choosing the type of fan used in your device. They were purchased to replace the fans in an HP ProCurve 2824 (J4903a). Unfortunately, this switch uses PWM fans with a stuck rotor sensor (VHigh=failed VLow=pass). The Switch is now completely silent, unfortunately this is due to the fans not spinning. When swapping out fans, you need to find out if the factory fans are PWM, 12V direct, 5V direct, or some other voltage. You must then also determine if the device is checking for a PRM signal or a locked rotor. If it shows RPM, you need a fan with RPM/RPM signal. If it is a locked rotor test only, you must also determine if the unit detects a spinning rotor by high or low voltage on the signal line. With this information you can select a replacement fan.