Filtering is very important in a system, a complex filtering is not necessarily synonymous with quality. It is possible to combine active filter and passive filter in one system.

There are two types of passive filters:
- High Pass: the frequencies are removed below. For example a high pass at 2000 Hz passes frequencies above 2000 Hz,
- lowpass: frequencies above is removed from. For example a low pass at 2000 Hz passes frequencies below 2000 Hz

The 2-way crossover is comprised of a qualified filter module for "high pass" (high-pass filter) and a qualified filter module of "going down" (low pass filter). The cutoff frequencies of the two filters modules are generally identical.
3-way passive filters (and more) are composed of multiple filter modules "high pass" and "low-pass", the medium channel (center) composed of a high-pass and low-pass is called " bandpass "(band-pass filter). The filter module "band-pass" is nothing more than the combination of a filter "High Pass" filter and a "low pass".Moreover, the operational amplifiers generate noise. Historically, the first discrete filters ROE (+ RC circuit operational amplifier) appeared in the mid sixties. It should nevertheless expect the seventies, with the first commercial integrated circuit (HIC and DIP) to be revealed the economic interest of this type of filter.