Speakers blowing is a problem that we have all encountered at one point or another and one which anyone with a P.A. system should really know about.
There are a few reasons why a speaker would ‘blow’, the most common of which are described in the following text. Although active speakers are less likely to blow, due to the amplification matching stage, they still can if you don’t use them correctly. In any of the following situations, your local PA dealer (if he knows what he’s talking about) should be able to confirm the problem by looking at the voice coil.
Signal Burn is the first process that we’re going to look at. This is caused by the amplifier in use working fine, but the user attempting to use more power than the speakers are capable of dealing with. A knowledgable PA dealer will be able to match the power of an amplifier to your speakers (or vice versa) to stop this happening.
DC Burn (Direct Current Burn) is where the user is misusing either the amplifier stage or the mixing desk output stage. In most cases, this fault is caused by the amplifier ‘clipping’ the input signal due to the user trying to get more out of the amplifier than it will give. This causes an increase in input level, therefore a larger sine wave. If this sine wave is pushed too hard, the amplifier will ‘clip’ (A red light is normally an indicator of this) the top and bottom of the wave and produce a square wave output which your speakers don’t like. At the peak of a sine wave, the voice coil of the speaker is pushed out and therefore, if a square wave is produced (by clipping the amplifier) the voice coil is spending more time out of the gap than it would like. This leads to a poor heat distribution through the magnet which overheats the voice coil and burns it. In the case of DC Burn, you could, as long as your speakers will handle the power, move to a larger output amplifier in which you have more headroom, and less chance of ‘clipping’ the signal. As a loose rule of thumb, in the case of professional speakers, your amplifier should give an RMS output rated higher than the RMS rating of your speakers. In some cases you should double the power to give plenty headroom. If you are unsure, your PA Dealer should be able to confirm this. If budget is a concern, then there are options available that you could use to lessen the chance of your amplifier clipping. These are called limiters, and are available from manufacturers including, Alesis, Behringer and Samson , among others. It must be stressed that these units only work if they are set up correctly and the user doesn’t attempt to get that “little bit more”!