Crossovers as used in Pro Audio Systems
All conventional speaker systems, whether PA, hi-fi or studio monitors, have crossover circuits to ensure that each type of driver is only asked to handle the frequency range for which it was designed. If you have either an active or as passive system with crossovers built in, you may never give them a second thought, but understanding how they work can help you use your system more effectively and may also help you to protect your system from damage.
Lift & Separate
We have seen that, at its simplest, a crossover is a series of passive electrical filter elements, comprising resistors, capacitors and inductors (coils), and these are usually built into the speaker cabinets so that all you need do is feed them full-range audio from a suitable power amplifier. In a three-way speaker system comprising bass, mid and tweeter units, the bass speaker would be fed from a low-pass filter (one that only allows through frequencies below the filter's cut off point) while the tweeter would be fed from a high-pass filter. The filter characteristics are carefully designed so that the bass driver 'hands over' smoothly to the tweeter at the crossover point. A mid-range speaker has both upper and lower limits of frequency handling, so it has to be fed using both high- and low-pass filters to ensure that it receives only mid-range frequencies. (The combination of a low-pass and high-pass filter produces a 'band-pass' response) Because no filter has an abrupt cut off, but rather a slope, the various driver ranges overlap slightly in frequency and the crossover must be designed to provide a smooth transition from one driver to the next.
The filter characteristic of a passive crossover cannot be made particularly steep without wasting power and using a lot of expensive components. (For example, a simple 'resistor and capacitor' filter has a slope of only 6dB per octave, whereas combining a capacitor and inductor can produce a 12dB per octave slope.) To achieve a steeper slope, multiple filter stages have to be cascaded, resulting in a further loss of electrical efficiency. The more dBs per octave, the sharper the response of the filter is said to be (using a 6dB per octave filter, the signal voltage is only halved for every octave you go beyond the cut off point, whereas a 24dB per octave beyond the cut off frequency.) The steeper the filters, the less overlap there will be between drivers handling adjacent frequency bands, which is generally desirable, as too much overlap can lead to phase problems resulting from both drivers trying to reproduce slightly different versions of the signal within the crossover's overlap region.
Unlike is passive counter part; the active crossover splits the audio signal into different frequency bands before it reaches the power amplifiers. Because the signal is still only at line level at this point, the crossovers do not have to handle any significant power, so no large components are used. However, it does mean that separate power amplifiers are needed to feed each frequency range. For example, in a three-way system separate power amplifiers are used to drive the bass, mid and high-frequency loudspeakers.
Sub-Bass in Small PA Systems
Where small mid/high cabinets are augmented by an active sub-bass cabinet (of subwoofer) it is quite common for the bass bin to incorporate a crossover to feed everything above, say 120Hz to the main speakers and everything below that frequency to the sub's own amplifier. The crossovers in the main speakers then divide up their input between the bass/mid-range speaker and the tweeter. As a rule, there is no need to adjust the low-frequency response of the main speakers, as the crossover in the sub takes care of that, a side benefit being that you now have more available headroom (level) in the main speakers for the remaining mid frequencies. However, not all subs handle the filtering for the main speaker - some just pass the main audio through unfiltered, in which case the main speakers may be fitted with low-cut switches instead. Because there are so many possible options with modern PAs, reading the manual lat least once is highly recommended!
Because a loudspeaker has both mechanical and thermal limits beyond which damage occurs, some form of protection system is desirable. In passive systems it is possible to employ simple tricks, such as wiring a properly specified low-voltage light bulb in series with the tweeter (the most vulnerable driver) so that excessive signal heats up the bulb filament, thus causing its electrical resistance to increase, and consequently reducing the amount of signal fed to the tweeter. However, in an active system it is relatively easy to design in level limiters to prevent the amplifiers ever being driven into clipping. Those active speakers with everything built inside the cabinets generally also include thermal monitoring on the amplifiers that reduces the power of shuts it off completely if over-use caused the temperature to become dangerously high. Some manufacturers incorporate a nice variation on this approach, instead of the overall power being reduced, a low-cut filter comes into play to limit the amount of low-frequency energy fed into the amplifiers (which is where most of the energy is in typical pop music) until the temperature returns to normal.
Power amplifiers and loudspeaker systems differ considerable in their specifications and capabilities, and if optimum performance is to be gained from the PA system there are various parameters, which must always be under proper control. The object is to match the input signal as closely as possible to the PA system itself, to give it the opportunity to delivery maximum efficiency, and also to protect against undesirable signal content or levels. In basic terms, this can be achieved by using a crossover, compressor and/or limiter, and an equaliser. These individual functions act as shown in the table below. If all these functions could be combined in a single device, in theory that device would be all we would need, placed after the mixing desk and before the power amps and speakers, to ensure that everything 'downstream' is operating as effectively as it should, and is also protected as far as possible against inappropriate use, or accidents such as mains spikes.
Not too long ago this would have been something of a technical challenge, but with the advent of affordable digital signal processing it no longer represents a problem. Once the audio signal has been converted to digital form it is a relatively easy matter to process it in any way you like - exactly as if you were using software plug-ins to process a recorded signal, but instead with a live signal, and in real time. Where previously there would have been various hardware circuits performing discrete functions, all requiring proper compatibility and correct matching when connected together, and each stage potentially adding unwanted noise, modern equipment enables all the functions to be carried out within the digital domain, and the results depend only on the quality of the algorithms used. These all-in-one packages tend to be called 'system controllers' because they place the entire PA system under their control, and they are the last device in the signal path before the power amps. Even relatively affordable controller systems, now contain a host of useful additional facilities, such as a library of known responses of popular PA system components and the ability to store a room curve or even an entire system set up for when you return to a venue.
Most systems do not require you to know much about what type of crossover they use, but you may be able to use the system more effectively if you have some idea of the implications of passive and active systems, especially where driver protection is concerned. You only need a deeper knowledge if you are using an external configurable electronic crossover, because then you have to set it up to match the requirements of your loudspeakers.
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