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Passive Crossover Networks



Resistors are components that offering a certain resistance to the current flow attenuate the signal that passes through them but don't modify its response. For this reason, as we'll see later, they are used for aligning transducers with different values of sensitivity — the parameter indicating how much louder a speaker plays than another under the same applied electrical tension. Resistors values are expressed in Ohms (Ω), the electric resistance unit. In an electric scheme they are identified by the following symbol and letter:


Inductors look like spools of enamelled copper wire — from here the common name of coils — that can be wound on plate or on ferrite core. Often they may not contain any core at all, in such case it is said that they are wound in air. Unlike resistors, they are reactive components that oppose a very high impedance against high frequency signals, assimilating to a short-circuit while going down toward the lowest frequencies. The transition point is determined by their value, that is expressed in Henrys (H), the inductance unit. For the same inductance, core-inductors require less coils than those wound in air and they are therefore smaller. However they tend to saturate in presence of strong signals, thereby introducing distortions. In an electric scheme inductors are identified by the following symbol and letter:


Reactive, but inverse in comparison to inductors, is also the behavior of capacitors. For this reason both of them are defined as dual elements. Capacitors are measured in Farads (F), the capacitance unit. In an electric scheme they are identified by the following symbol and letter:


It would be wasteful to fully analyze the operation principle of capacitors: it is enough to say here that they are devices able to store electrostatic charges. They are constructed with two electrodes connected to plain conductive plates separate between them by an insulator named dielectric. They are grouped in families according to the type of dielectric used: so we'll have teflon capacitors, polypropylene, polystyrene, polycarbonate, polyester, mica or ceramic. All these materials are characterized by a permeability more or less hard depending on their relative dielectric constant, a number that practically quantifies their insulation ratio. To increase capacitance it will be indispensable to reduce the thickness of the dielectric that separates the plates, but only some materials allow that without risks of perforation, due to their dielectric constant, that's it.

A separate family is represented by electrolytic capacitors, whose dielectric is constituted by an electrolytic gelatinoid solution that, if subjected to polarization and as long as this polarization is maintained, produces a layer of insulating oxide so thin that it allows very elevated capacity values. Electrolytic capacitors are therefore polarized components — that' to say with a positive pole and a negative one — and they can be used only in circuits where the direct component is greater than that alternate since it is just the voltage that allows the formation of the dielectric layer. They are practically irreplaceable in power supply filtering circuits, but they shouldn't be used in crossovers where direct components are pratically absent.

There are for sale however — or they easily can be made — the so-called bipolar capacitors, or non-polar, consisting of two standard electrolytic capacitors with two homonym electrodes connected together: starting from two capacitors with equal capacitance will result a single capacitor of halved capacitance and doubled size, but non polarized and therefore suitable for crossovers. The point is that they are not the ideal items for this type of use and those in polypropylene and polyester must be preferred.

Pondering on the behavior of inductors and capacitors and remembering the definition we've given about crossover filters in the previous lesson, you can see that they are elementary filters by themselves, although from their mutual combination more complexes filters are gotten, as we'll see later on. But first it's important to know those damn filters by name. In fact, according to their function they are distinguished in lowpass, highpass and bandpass