A Guide to Limiters in PA Systems
A limiter is essentially a compressor (see the Compressors page) with a very high ratio.
Usually a limiter is a 19″ 1U box with knobs or buttons (and more often than not a couple of LED meters) on the front. However, limiters are often a built-in feature of crossovers and amplifiers, and many compressors include a separate limit function.
It prevents the signal from exceeding a threshold (which may or may not be user-determined). Its main use is protection: it prevents the signal from exceeding the capability of subsequent amplifiers (including preamplifiers). When a signal is too big for an amplifier, the result is ‘clipping’ of the signal. This is undesirable anywhere in the signal path, and potentially catastrophic for loudspeakers if it occurs in the main power amplifiers. A compressor with a ratio of 10:1 or higher is effectively acting as a limiter.
It uses a voltage controlled amplifier (VCA) to substantially reduce the amount of gain the signal receives whenever the signal exceeds a predetermined threshold.
Often the only controls are:
In practice, a compressor with a ratio greater than 10:1 is acting as a limiter, in that for every 10dB increase in the input signal the output will only increase by 1dB.
Some limiters (and most compressors, if you are using a compressor as a limiter) also have an attack/release control. A few limiters also have an optional soft-knee setting, while others are soft-knee by design.
If all else fails, read the manual!
Usually a limiter will be patched in-line between one piece of equipment and the next (e.g. between mixer and crossover, or between crossover and power amplifier). However, limiters can also be used on channel, group or left/right mixer inserts.
Generally, the signal range any professional equipment can accommodate will be stated in its technical specifications (usually at the back of the manual). Set the threshold so that the signal cannot exceed the capability of the next thing it comes to in the signal path. In practice this will mean setting the threshold one or two decibels lower than the maximum the next thing in the signal path can handle. If you don't know (and can't find out) what that is, you'll be fairly safe with 0dB (although you may be dispensing with some headroom). If the next thing in the signal path has a clip indicator, you can use a test tone to find what level of signal causes it to clip (many mixers have a tone generator, but if yours hasn't, test CDs are available from a number of sources). With the signal running, reduce the limiter threshold until the clip light goes out.
If the limiter has an attack control, faster attack will protect your equipment better, while slower attack will be less noticeable. A limiter with fast attack and release can cause a ‘pumping’ effect.
For setting crossover or controller limiters see the Crossovers page.
If your crossover(s) and/or amplifier(s) have built-in limiters, you probably don't need a separate limiter. Otherwise, limiters are a good way to ensure inexperienced users don't destroy loudspeakers. If in doubt, use one, and - if in doubt - set the threshold slightly lower than necessary.
Most compressors can be set up as limiters. Limiters are generally used in-line, so make sure it has balanced inputs & outputs.