What is a passive subwoofer?

In the early days, there was no built-in amplifier for driving the speaker unit in the subwoofer. The box only had a passive crossover network responsible for frequency division. This subwoofer is called a passive subwoofer. The so-called "passive" noun is that the subwoofer itself has no kinetic energy and must be driven by an external amplifier. The passive subwoofer music signal is obtained from the speaker output of the latter stage, and itself has a set of signal output terminals for transmitting the music signal that has been split to the two-channel speaker. Since the two-channel speaker and the subwoofer are driven by the same rear stage, the variables of the latter stage are increased, and the driving force is also dispersed. Currently, this subwoofer is quite rare.