Active vs Passive

Classification based on components

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

Passive

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 represent 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 resistors combined with reactive components such as capacitors and inductors. 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 boxes use lower quality passive crossovers. Expensive hi-fi speaker systems and receivers use higher quality passive crossovers, to obtain improved sound quality and lower distortion. The same price/quality approach is 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 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 cores, or laminated silicon steel cores, and most are wound with enamelled copper wire.

Some passive networks include devices such as fuses, PTC devices, bulbs or circuit breakers 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 networks) 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.

On the negative side, passive networks may be bulky and cause power loss. They are not only frequency specific, but also impedance specific. This prevents 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."^[2] Alternatively, passive components can be utilised to construct filter circuits before the amplifier. This is called passive line-level crossover.

Active

An active crossover contains active components in its filters. In recent years, the most commonly used active device is an op-amp; active crossovers are operated at levels suited to power amplifier inputs in contrast to passive crossovers which operate after the power amplifier's output, at high current and in some cases high voltage. On the other hand, all circuits with gain introduce noise, and such noise has a deleterious effect when introduced prior to the signal being amplified by the power amplifiers.

Typical usage of an active crossover, though a passive crossover can be positioned similarly before the amplifiers

Active crossovers always require the use of power amplifiers for each output band. Thus a 2-way active crossover needs two amplifiers—one each for the woofer and tweeter. This means that an active crossover based system 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:

• a frequency response independent of the dynamic changes in a driver's electrical characteristics.
• 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, Butterworth, etc.), relative levels, ...
• better isolation of each driver from signals 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.