Yesico FL-420ATX

If you've had any experience of customising your PC to keep it quiet, you'll know that the most difficult barrier to silence is the power supply. But finally, after years of waiting, this month two power supplies have come on to the market that are able to run in complete silence.

Our initial fear when we heard about both the Silentmaxx and Yesico supplies was potentially limited power capacity. But that's not the case, with the Yesico rated at 420W and the Silentmaxx a massive 503W. Silentmaxx also produces a 420W model for £151 inc VAT.

For a power supply to be fanless it's got to be well designed and engineered. Although all modern supplies are switch-mode devices - far more efficient than the old-style linear regulators - efficiency of cheap models can still drop to 70 per cent or less. And if a power supply is pumping out 300W of usable power at 70 per cent efficiency, it's generating 90W of waste heat.

But no power supply can ever be 100 per cent efficient - about 85 per cent is the absolute best you can hope for. So the whole thing needs to be engineered with nicely over-specified components and be able to cope with higher-than-usual temperatures: this all costs money, hence the price of these two models. And after as much engineering finesse as possible has been applied, it only remains to throw large amounts of metal at the problem. Hence, both the Yesico and Silentmaxx sport large heatsinks in the place that a normal power supply would have its rear cooling fan air output. The Yesico's is in the circular shape of a fan exhaust to avoid all potential incompatibilities slotting it into a case; the Silentmaxx's is a larger rectangular affair, 100mm wide and 64mm high.

In fact, the Silentmaxx isn't completely silent. There's a three-position switch on the back of the casing that controls a small integrated Papst fan. This allows you to choose between always off, always on (with temperature-dependent speed control) or 'emergency only', when the unit reaches a critical temperature. The fan itself is very quiet indeed, rated at 19dBA, which is lower than standard ambient room level.

The Yesico also has provision for emergency cooling - among its wiring loom you'll find a special 12V molex connector to which you can attach a fan. When the temperature rises above a critical threshold, power is applied to the socket to activate it. The slight drawback with this approach is exactly where and how to mount the emergency fan to cool the unit. Ventilation holes in the casing aren't terribly large: using an additional front case fan in order to draw air through the innards from a distance isn't likely to work too well. The emergency fan basically needs to be mounted right beside the Yesico in order to blow air directly over the metal of the casing. Standard ATX cases don't have any provision for a fan in the right place, so a bit of ingenuity will be needed if you want the peace of mind of having emergency cooling.

We initially tried both units in the test rig we'd set up for the Lian-Li PC-V1100B case. This wasn't really a fair test since the PC-V1100B suffers from overheating problems anyway, but it did highlight a potential problem - at 168mm the Silentmaxx's heavily engineered casing is much longer than standard power supplies, and wouldn't fit in the unconventional Lian-Li. Make sure your tape measure gets an airing to check the external heatsink and the length to avoid problems. The Yesico is of standard length, so should fit anywhere a normal PSU can.