Mikhailtech has take a look at a real monster of a PSU, the 550watt Antec TruePower.

So they developed the "Low Noise Technology", which, like the name implies, is a way to keep the noise created by the PSU to a minimum. Standard ATX power supplies with traditional thermally-controlled variable-speed fans have a linear increase of the fans' RPM based on temperature readings from the PSU, but Antec used a new approach to the temperature controlled cooling to achieve optimum balance between noise reduction and necessary cooling with an advanced temperature response system that outperforms previous solutions. The system constantly monitors the temperature inside the power supply, and then runs the power supply fans at the lowest speed appropriate to load and conditions.

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