Crossovers
Philip Newell, Keith Holland · 2006
The term ‘crossover’ appears to have been originally used to describe the relationship of the filter slopes – crossover filters – as shown in Figure 5.1. In reality, and in many languages other than English, they are better described as frequency dividing networks, or words to that effect, though the name crossover has generally stuck. The fact that no loudspeaker drive unit suitable for music monitoring or serious listening can provide a flat response over the entire musical frequency range requires that the multiple drivers in a system need to be fed by signals which are only appropriate to their designed performance range. The two normal ways to apply these filtered signals are via high level, passive crossovers – where the filter components are placed between the power amplifier and the loudspeaker drive units, or low level active crossovers – where the filters are placed in the line level signal circuits, ahead of the amplifier inputs. In the latter case, each filter output feeds a separate amplifier, which is then directly connected to the corresponding drive unit(s). In some cases, mixtures of the two concepts are applied to one system, such as an active crossover between the bass and mid drivers, and a high level passive crossover between the mid and high frequency drivers, as shown in Figure 5.2. Other forms of crossover also exist, such as simple, low-level passive crossovers, though they are rarely used because the filters can be more precisely tailored when the components are part of the feedback path in an electronic circuit. Mechanical crossovers are another type of filter. These can take the form of aluminium domes in the centre of the cones, which decouple from the main cone at higher frequencies and radiate separately, extending the frequency response above that which could be achieved by the main cone, alone. However, the response tends to be somewhat irregular, but this type of high frequency extension can find use in loudspeakers for music production – as opposed to reproduction – and the technique is extensively used in loudspeakers for musical instrument amplification, such as guitar amplifiers. An example is shown in Figure 5.3. Figure 5.4 shows a ‘parasitic cone’ or ‘whizzer cone’. The concept is generally the same in principle as that of the metal dome – the small cone decouples from the main cone at high frequencies – although the response of the parasitic cone tends to be more controlled, and a flatter frequency response can normally be achieved. Another concept, although not widely used, is inductive coupling, where the high frequency cone is not electrically connected to the amplifier. In fact, the coil can be the single, shorted turn formed by the metal dome itself. The dome and a former are simply placed over the centre pole of the magnet assembly, sharing the same gap as the LF/MF cone assembly. Such inductively coupled transducers, or ICTs, are operated by the modulated magnetic coupling between the ‘coil’ and the magnetic circuit. This type of ‘crossover’ is neither electrical, electronic nor mechanical, but is simply a magnetic-inductive effect.