8 Advantages of Active Crossovers
Douglas Self

   By Guest Contributor   Categories: Audio EquipmentRecording

The advantages of active crossovers are:

1.)    The over-riding advantage of an active crossover is it offers ultimate freedom of design as virtually any frequency or phase response that can be imagined can be used. The filter slopes of the crossover can be made as steep as required without using large numbers of big and relatively expensive components. Any increase in passive crossover complexity means a significant increase in cost.

2.)    The design of passive crossovers is restricted by the need to keep the loading on the power amplifier within reasonable limits. With an active crossover, correction of the response for each driver is much simpler as it can be undertaken without having to worry about the combined load becoming too low in impedance for the average amplifier.

3.)    The design of passive crossovers is further complicated by the need to keep the power losses in the crossover within reasonable limits. The losses in the resistors and in the inductors (because of their inevitable series resistance) of a passive crossover, especially a complex design employing high-order filters or time delay compensation, can be very serious. In a big sound-reinforcement system the losses would be measured in tens of kilowatts. Not only does this seriously degrade the power efficiency of the overall system by wasting power that could be better applied to the loudspeakers, but it also means that the crossover components have to be able to dissipate a significant amount of heat, and are correspondingly big, heavy, and expensive. It is far better to do the processing at the small-signal level; the power used by even a sophisticated active crossover is trivial.

4.)    If one of the power amplifiers is driven into clipping, that clipping is confined to its own band. Clipping is usually less audible in the bass, so long as there is no intermodulation with high frequencies. It has been stated that an active crossover system can be run 4 dB louder for the same subjective impairment. This is equivalent to more than twice the power, but less than twice the perceived volume, which would require a 10 dB increase in SPL.

5.)    Delays can be added to compensate for differing acoustic centres for the drive units planes quite easily. Passive delay-lines can be built but are prodigal in their use of expensive, lossy, and potentially non-linear inductors, and as a result have high overall losses.

6.)    Tweeters and mid drive units can have resonances outside their normal operating range, which are not well suppressed by a passive crossover because it does not put a very low source impedance across the voice coil. The presence of a series capacitor can greatly reduce the damping of the main resonance [4], and it is also possible for a series capacitor to resonate with the tweeter voice-coil inductance [5], causing an unwanted rise in level above 10 kHz or thereabouts.

7.)    Drivers of very different sensitivities can be used, if they happen to have the best characteristics for the job, without the need for large power-wasting resistances or expensive and potentially non-linear transformers or auto-transformers. If level controls for the drive units are required, these are very straightforward to implement in an infinitely-variable fashion with variable resistors. When passive crossovers are fitted with level controls (typically for the mid unit or tweeter, or both) these have to use tapped auto-transformers or resistor chains, because the power levels are too high for variable resistors, and so control is only possible in discrete steps.

8.)    The distortion of the drive unit itself may be reduced by direct connection to a low impedance amplifier output [6]. It is generally agreed that the current drawn by a moving coil drive unit may be significantly non-linear, so if it is taken from a non-zero impedance, the voltage applied to the drive unit will also be distorted. This may be related to out-of-band tweeter resonance; Jean-Claude Gaertner states that tweeters can have increased distortion below 1 kHz [7]. I do not know for sure, but I strongly suspect, that when drive units are being developed they are driven from amplifiers with effectively zero output impedance, and that linearity is optimised under this condition. Any other approach would mean guessing at the source impedance, which given the number of ways in which it could vary, would be a quite hopeless exercise.

Excerpt from Douglas Self’s The Design of Active Crossovers.


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