What are the fundamental differences between active and passive speakers?
The Crossover: Active speaker crossovers are powered circuits consisting of transistors and modern, solid-state electronics. A passive speaker crossover, on the other hand, consists of unpowered capacitors, inductors and resistors
sans transistors and electronics. An active crossover that utlizes electronic components can be designed to be significantly more accurate and neutral than a passive one. An electronic crossover also allows for adjusting the signal in ways that are nearly impossible or impractical in passive crossovers.
The Amplifier: Active systems have one amplifier for each driver. Each amplifier is designed to match its associated driver, taking into account the driver's impedance characteristics, efficiency and frequency response. Amps are optimized for their drivers. A passive design consists of a single amplifier powering all drivers, designed for a wide range of impedances and frequencies, but optimized for none. It is primarily for this reason--the need to be the Jack of All Trades, but Master of None--that outboard amplifiers need to be over-built. Active designs, on the other hand, allow for the design of amps that deliver more precise power to its associated driver. Intermodulation distortion--distortion created by the need to handle vastly different frequencies at the same time--is isolated and vastly reduced in passive designs.
The Sequence of System Stages: In a passive system, the signal travels from the (outboard) amplifier to the crossover to the driver like so:
SIGNAL >>> Amplifier >> ||Crossover > Drivers||
In an active system, the sequence of system stages is different. The signal travels from the crossover to the amplifier to the driver like so:
SIGNAL >>> || Crossover > Amplifier > Driver ||
In this sequence of stages the crossover acts on line-level signals which, coupled with the used of powered transistorized circuits, allows for a significantly more neutral and precise crossover. An electronic crossover operating on line-level signal voltages allows for smaller, less expensive yet more precise and efficient splitting of the signal into its treble/mid-range/bass bands.
In addition, because there is nothing standing between the amp and the driver, the amplifier can more precisely control and damp the driver. The amp is literally hard-wired to the driver, providing seamless integration and vastly improved transient and decay characteristics. Nothing, including long runs of speaker cable, stand in the way between the amp and the driver. Power delivery is immediate, precise and controlled.
Active systems allow for a significantly superior integration and optimization of system stages and components. Many of those optimizations are simply impossible in a passive system. Active speakers are not a fringe, esoteric technology for eccentrics with questionable technical merits or benefits. Quite the opposite, active designs are squarely in the mainstream of professional music making. Active speakers are what most professionals and semi-professionals purchase as a capital expenditure to be used as a tool for commercial, profit-making endeavors. In comparison, passive speakers are largely the purview of hobbyists or "audiophiles" who purchase the speaker as entertainment based on discretionary surplus income. In addition to technological superiority, these market forces are also aligned with active designs in producting high(er) quality equipment and sound. Active monitors are designed to be operated round the clock, and therefore must be built to higher standards.