Editing Audio crossover (section)

===Classification based on components===
Crossovers can also be classified based on the type of components used.

====Passive====
[[File:Passive Crossover.svg|thumb|right|300px|A passive crossover circuit is often mounted in a [[speaker enclosure]] to split up the amplified signal into a lower-frequency signal range and a higher-frequency signal range.]]

A '''passive crossover''' splits up an audio signal after it is amplified by a single [[power amplifier]], so that the amplified signal can be sent to two or more driver types, each of which cover different frequency ranges. These crossover are made entirely of passive components and circuitry; the term "passive" means that no additional power source is needed for the circuitry. A passive crossover just needs to be connected by wiring to the power amplifier signal. Passive crossovers are usually arranged in a [[Cauer topology]] to achieve a [[Butterworth filter]] effect. Passive filters use [[resistor]]s combined with reactive components such as [[capacitor]]s and [[inductor]]s. Very high-performance passive crossovers are likely to be more expensive than active crossovers since individual components capable of good performance at the high currents and voltages at which speaker systems are driven are hard to make. 

Inexpensive [[consumer electronics]] products, such as budget-priced [[Home theater in a box]] packages and low-cost [[boom box]]es, may use lower quality passive crossovers, often utilizing lower-order filter networks with fewer components. Expensive [[hi-fi]] speaker systems and receivers may use higher quality passive crossovers, to obtain improved sound quality and lower distortion. The same price/quality approach is often used with sound reinforcement system equipment and musical instrument amplifiers and speaker cabinets; a low-priced [[stage monitor]], [[PA speaker]] or bass amplifier speaker cabinet will typically use lower quality, lower priced passive crossovers, whereas high-priced, high-quality cabinets typically will use better quality crossovers. Passive crossovers may use capacitors made from [[polypropylene]], metalized [[polyester]] foil, paper and [[electrolytic]] capacitors technology. Inductors may have air cores, powdered metal cores, [[ferrite core]]s, or laminated [[silicon]] steel cores, and most are wound with enameled [[copper]] wire. 

Some passive networks include devices such as [[Fuse (electrical)|fuses]], PTC devices, bulbs or [[circuit breaker]]s to protect the loudspeaker drivers from accidental overpowering (e.g., from sudden surges or spikes). Modern passive crossovers increasingly incorporate equalization networks (e.g., [[Zobel network]]s) that compensate for the changes in impedance with frequency inherent in virtually all loudspeakers. The issue is complex, as part of the change in impedance is due to acoustic loading changes across a driver's passband.

Two disadvantages of passive networks are that they may be bulky and cause power loss. They are not only frequency specific, but also [[Electrical impedance |impedance]] specific (i.e. their response varies with the electrical load that they are connected to). This prevents their interchangeability with speaker systems of different impedances. Ideal crossover filters, including impedance compensation and equalization networks, can be very difficult to design, as the components interact in complex ways. Crossover design expert [[Siegfried Linkwitz]] said of them that "the only excuse for passive crossovers is their low cost. Their behavior changes with the signal level-dependent dynamics of the drivers. They block the power amplifier from taking maximum control over the voice coil motion. They are a waste of time, if accuracy of reproduction is the goal."<ref name="LinkwitzCrossovers" /> Alternatively, passive components can be utilized to construct filter circuits before the amplifier. This implementation is called a passive line-level crossover.

====Active====
[[File:ThreeWayActiveCrossoverDiagram.png |thumb |right |450px |Implementation schematic of a three-way active crossover network for use with a stereo three-way loudspeaker system.]]

An active crossover contains active components in its filters, such as transistors and operational amplifiers.<ref name="AshleyKaminsky1971" /><ref name="Caldwell2013" /><ref name="Linkwitz1978" /> In recent years, the most commonly used active device is an [[operational amplifier]]. In contrast to passive crossovers, which operate after the power amplifier's output at high [[Electric current |current]] and in some cases high [[voltage]], active crossovers are operated at levels that are suited to power amplifier inputs. On the other hand, all circuits with [[gain (electronics)|gain]] introduce [[noise]], and such noise has a deleterious effect when introduced prior to the signal being amplified by the power amplifiers.

Active crossovers always require the use of power amplifiers for each output band. Thus a 2-way active crossover needs two amplifiers—one for the [[woofer]] and one for the [[tweeter]]. This means that a loudspeaker system that is based on active crossovers will often cost more than a passive-crossover-based system. Despite the cost and complication disadvantages, active crossovers provide the following advantages over passive ones:

[[File:Active Crossover.svg|thumb|right|300px|Typical usage of an active crossover, though a passive crossover can be positioned similarly before the amplifiers.]]

*a frequency response independent of the dynamic changes in a driver's electrical characteristics (e.g. from heating of the voice coil)
*typically, the possibility of an easy way to vary or fine-tune each frequency band to the specific drivers used. Examples would be crossover slope, filter type (e.g., [[Bessel filter |Bessel]], Butterworth, Linkwitz-Riley, etc.), relative levels, etc.
*better isolation of each driver from the signals being handled by other drivers, thus reducing [[intermodulation]] distortion and overdriving
*the power amplifiers are directly connected to the speaker drivers, thereby maximizing amplifier damping control of the speaker voice coil, reducing consequences of dynamic changes in driver electrical characteristics, all of which are likely to improve the transient response of the system
*reduction in power amplifier output requirement. With no energy being lost in passive components, amplifier requirements are reduced considerably (up to 1/2 in some cases), reducing costs, and potentially increasing quality.

====Digital====
Active crossovers can be implemented digitally using a [[digital signal processor]] or other [[microprocessor]].<ref name="WilsonAdamsScott" /> They either use [[Digital data|digital]] approximations to traditional [[Analogue electronics|analog]] circuits, known as [[IIR filter|IIR]] filters ([[Bessel filter|Bessel]], Butterworth, [[Linkwitz-Riley filter|Linkwitz-Riley]] etc.), or they use [[FIR filter|Finite Impulse Response (FIR)]] filters.<ref name="SchuckKlowak1988" /><ref name="wilson1989application" /> IIR filters have many similarities with analog filters and are relatively undemanding of CPU resources; FIR filters on the other hand usually have a higher order and therefore require more resources for similar characteristics. They can be designed and built so that they have a [[linear phase]] response, which is thought desirable by many involved in sound reproduction. There are drawbacks though—in order to achieve linear phase response, a longer delay time is incurred than would be necessary with an IIR or minimum phase FIR filters. IIR filters, which are by nature recursive, have the drawback that, if not carefully designed, they may enter limit cycles, resulting in non-linear distortion.

====Mechanical====
This crossover type is mechanical and uses the properties of the materials in a driver diaphragm to achieve the necessary filtering.<ref name="Cohen1957" /> Such crossovers are commonly found in [[full-range speaker]]s which are designed to cover as much of the audio band as possible. One such is constructed by coupling the cone of the speaker to the voice coil bobbin through a compliant section and directly attaching a small lightweight ''whizzer'' cone to the bobbin. This compliant section serves as a compliant filter, so the main cone is not vibrated at higher frequencies. The whizzer cone responds to all frequencies, but due to its smaller size, it only gives a useful output at higher frequencies,  thereby implementing a mechanical crossover function. Careful selection of materials used for the cone, whizzer and suspension elements determines the crossover frequency and the effectiveness of the crossover. Such mechanical crossovers are complex to design, especially if high fidelity is desired. Computer-aided design has largely replaced the laborious trial and error approach that was historically used. Over several years, the compliance of the materials may change, negatively affecting the frequency response of the speaker. 

A more common approach is to employ the dust cap as a high-frequency radiator. The dust cap radiates low frequencies, moving as part of the main assembly, but due to low mass and reduced damping, radiates increased energy at higher frequencies. As with whizzer cones, careful selection of material, shape and position are required to provide smooth, extended output. High frequency [[Acoustic dispersion |dispersion]] is somewhat different for this approach than for whizzer cones. A related approach is to shape the main cone with such profile, and of such materials, that the neck area remains more rigid, radiating all frequencies, while the outer areas of the cone are selectively decoupled, radiating only at lower frequencies. Cone profiles and materials can be modeled using [[Finite_element_method |finite element analysis]] software and the results are predicted to excellent tolerances.  

Speakers which use these mechanical crossovers have some advantages in sound quality despite the difficulties of designing and manufacturing them and despite the inevitable output limitations. Full-range drivers have a single acoustic center and can have relatively modest phase change across the audio spectrum. For best performance at low frequencies, these drivers require careful enclosure design. Their small size (typically 165 to 200&nbsp;mm) requires considerable cone excursion to reproduce bass effectively. However, the short voice coils, which are necessary for reasonable high-frequency performance, can only move over a limited range. Nevertheless, within these constraints, cost and complications are reduced, as no crossovers are required.