Active vs. Passive...

The diagram below describes how most audio systems are set up in your typical car:



This is called a PASSIVE setup, as characterised by the passive crossovers found in our 2 way component speaker systems (those that have both tweeters and woofers)

Some HU models, mostly the higher end ones (Example - Pioneer 6750 and ABOVE, Alpine 98xx models..and so on) do not need a pre amplifier to run as they already have their pre-out voltages sorted out. If your head unit doesn't have this feature built in, then you NEED a pre amplifier for signal/voltage amplification purposes.

There are a few advantages of your typical passive setup:

Cheaper to setup and install. You only need 2 amplifier channels to run the front component speakers (midbass and tweeter) as the passive crossover splits them and the power up.

The most easiest to upgrade from nothing or our crappy stock system.
SQ can be above average standard immediately

Disadvantages of the passive setup:

There is NO or very little flexibility in tuning the system except for the EQ
Power from the amp is wasted in the passive crossover.

Mids often don't have enough power as a result, while the tweeters are too loud.

Phase instability and non-coherency in the crossover frequencies.
It is very difficult to achieve a good sound stage with as well.

Advantages of the Active setup:

Ability to fine tune the exact amount of crossover filtering
Superb and optimum control of individual speakers and tweeters (gain, crossover freq, phase)

You can use the time-alignment function in your HU's DSP to delay the correct amount time (in microseconds), giving an optimum soundstage and stereo imaging

Overall phase-coherency in the drivers arriving at the listener's ear

Full power is delivered to the speakers

Disadvantages of the Active setup:

Expensive - You might need 1 dedicated amplifier channel for EVERY speaker.

Multichannel amps are used to provide individual optimized bandlimited signals to the corresponding speakers

Complicated tuning that is time-intensive

Higher processing power involved in the processing of low-level RCA or digital signals.

Credits: Amery Reuben