Crossover Network Basics

The crossover is a fairly complex subject and it is essential that we understand fully what it is and how it works before we make any decisions about how we will channel the signal in a hi-fi system.

Filters

Crossover networks use two types of filters to channel the music signals. A high-pass filter ignores, or passes, frequencies above a predetermined frequency but attenuates, or rolls off, those below it. Conversely, a low-pass filter passes frequencies below the predetermined frequency but attenuates those above it. High- and low-pass filters can also be combined (called "cascading") to form a band-pass filter, which passes only those frequencies (called a "pass-band") that aren't attenuated by the two filters. In each case, the predetermined frequency is called the "crossover point" or "cutoff frequency."

Slopes

The rate at which frequencies are attenuated is called the "crossover slope." Slopes are expressed in dB per octave, and typical figures are 6, 12, and 18 dB per octave, with 12 being the most common. The higher the slope, the faster attenuation occurs.

An example: You wire a high-pass filter with a crossover point of 100 Hz and a slope of 6 dB per octave to a mid-bass. The crossover will gradually filter out signals below 100 Hz so that the level of the signal at 50 Hz (one octave below 100 Hz) is approximately 6 dB less than the level at 100 Hz.

Passive Benefits

There are two basic types of crossovers: passive and active. The most obvious distinction between them is their location in a system's signal path. Passives are placed between amplifiers and speakers, and because they work with amplified signals they are sometimes referred to as "high-level networks." Active, or electronic, crossovers perform their signal-dividing act before the amp, and because they work with non-amplified signals they are often referred to as "low-level networks."

Simplicity and economy are the most compelling reasons to choose the passive variety. Since passives go to work after amplification, it's possible to build a multi-speaker system around a single amplifier; systems using one or more active crossovers require a minimum of two amplifiers. And passive crossovers cost a lot less than their active counterparts.

Passives are also more flexible than actives – when you build them yourself or have them customized for your system, that is. (Some component speaker systems come with passive crossovers, and your local car stereo shop sells pre-made passive devices.) Passives are flexible because you can mix and match the various electrical components that make up their filters to achieve any desired response curve – a feat that only the most sophisticated actives can match.

One thing that actives don't suffer from is "insertion loss." This term is used to describe the amplifier output power that's lost when a passive crossover is inserted between an amp and a speaker. The most significant power loss occurs with low-pass filters, since their key components – inductors (see below) – incorporate many turns of wire that have a small but measurable DC resistance. This resistance will reduce the amount of power delivered to the speaker, but in most cases it is not significant.

Inductors and Capacitors

The key components in any passive crossover are its inductors and capacitors. An inductor (also known as a "coil" or "choke") is simply a coil of wire that is usually wrapped around an iron core. Inductors are used in low-pass filters, since their impedance naturally increases as the frequency of incoming music signals rises. An increase in impedance always causes a decrease in amplifier output, so the result is that high frequencies are attenuated at a rate determined by the filter's slope.

Capacitors consist of interleaved layers of a special foil and an insulating material such as polypropylene or Mylar. They are used in high-pass filters, since their impedance increases as the frequency of an incoming music signal decreases. Again, this increase in impedance lowers the amp's output, and the result is that low frequencies are gradually filtered out.

Every component has a value, which indicates how much resistance the component will present to the amplifier and is largely responsible for determining the crossover point of a specific filter (see "Designing a Network," Impedance, below). An inductor's value is determined by the number of windings of wire around its core and the type of core used (see "Designing a Network," Component Type, below); a capacitor's value is determined by the number of layers of foil and insulating materials it has. The more layers, the higher the capacitance. The values of capacitors are expressed in microfarads (pF), and the values of inductors are expressed in micro-henries (uH), and milli-henries (mH).

Order

A crossover's order is also tied to its slope: First-order devices always have a slope of 6 dB per octave, second-order devices a slope of 12 dB per octave, third-order devices a slope of 18 dB per octave. (See Figures 1 through 3.) Essentially, this physical use of extra inductors and/or capacitors causes higher-order networks to attenuate the signal more sharply.