If you're running a subwoofer, you’ll need an amplifier. That’s because subwoofers require plenty of amplifier power – certainly more than a standard head unit can provide. A budget subwoofer amplifier should meet these criteria:

This amplifier allows its outputs to be bridged (for much higher power into a single channel) and runs a 20A fuse (which is indicative of decent audio output capability). It's important that you look for these features when selecting a secondhand amplifier.

(1) The amplifier should have a bridging facility – ie, an option to allow the outputs of both stereo channels to be summed (or bridged) to provide more power into a single channel. Most amplifiers are two-channel (ie, stereo) but a subwoofer amplifier need only be mono – bridging a stereo amplifier makes use of both channels to give a higher-power mono output (theoretically four times the power output of a single channel).

Not all car stereo amplifiers are designed for bridging, so you need to find an amplifier that’s capable of this function. This is usually indicated by labelling adjacent to the speaker terminals, which indicates that the output of two channels can be used to drive one speaker (see photo). Don’t attempt to bridge the output of an amplifier that’s not designed for bridging, as this will destroy the output stages.

(2) The amplifier should preferably feature an in-built crossover. You don’t want to feed high frequencies to a subwoofer and if the amplifier has an inbuilt low-pass filter, this saves you the extra cost of buying other bits. In addition, it’s preferable that the filter crossover frequency be adjustable, so that you can choose the optimum point at which the subwoofer begins to do its stuff.