Many spend thousands of pounds on AV gear and software to play on it, but completely ignore one crucial factor - the mains supply that powers it.

It's compromised by how far you're away from the nearest sub-station, the effects of other electricity-users in your vicinity and even the time of day (the nation's kettles, switched on en-masse to the accompaniment of the EastEnders theme-tune, can actually drag down the voltage). As a result, the mains can be rather 'dirty'. This Chinese-made IXOS XHP420 'black box' acts as a mains filter, removing noise as well as potentially-damaging 'spikes' - some equipment protection also features.

Highs

If the mains voltage exceeds 270 volts, an internal relay isolates your equipment from it.

A circuit breaker 'trips' if too much current (over 10 amps) is drawn - a possibility if your equipment or its mains-wiring is faulty.

A pair of LEDs confirm excessive mains voltage and 'OK' status at a glance.

Compact in size, and thus easily 'hidden'.

Internal construction is of a high standard - particularly noticeable is the filter's massive toroidal inductor.

Lows

Only two mains outlets are offered - and they're not 'standard' 13-amp sockets.

The internal spike-protection components will eventually need replacing, especially if you live in an area of questionable mains quality.

Overt benefits to picture and audio quality depend on the condition of your mains supply, flatscreen TV and other equipment