Welllll....they aren't always hardware, but that's a totally different discussion all together - let's just consider hardware only for this discussion...

There are two types of crossovers - passive and active....

Passives are just that, passive - they require no electronics. There are two types of these.

The most common are those downstream of the amplifier. Generally these are used to filter between the tweeter and mid in a component set. They generally have fixed crossover slopes and fixed points. Some of the better ones have switches for phase, tweeter attenuation, notch filters, and even tweeter protection. These components are the most important part that contributes to the overall sound of a comp set. A poorly designed crossover will make a good set of speakers sound like shit. A good crossover can be very expensive too if you use good components. The cheapest version is something like a bass blocker. It's a simple capacitor that acts as a highpass filter. An inductor works as a low pass filter. For those of you with co-ax speakers, take a look at the tweeter, you will typically find a small cylinder - that's your capacitor.

The second type is just before the amplifier - Harrison made the most popular ones - they are called F-Mods - you plug your rca into them, and then the rca shoots out a filtered signal. Same principal - just pre amplification.

Next you have active crossovers - again, three types:
1. software
2. active external hardware
3. on-board active

The first type is generally found in headunits or external processors - such as an Eclipse 8053, Audio Control DQX, or a Alpine H700. Both have software based digital crossovers built into the units. Often these units are digital - they have variable crossover slopes, but a certain set of fixed crossover points.

Then you have an external crossover - what it will do is take input from a source, then split the signal and pop it out the other side cut up how you want them. These come in many different styles and have all kinds of different functionality. These are as basic as a small two way from MTX, up to a four or six channel from PPI (FRX-456) or can be indash like an Audio Control Three.1. I owened a Image Dynamics IDX-24. This unit was a three way crossover that had independent points for left and right. Typically, these are analog units - the good ones have a range of crossover points and are infinitely adjustable by a knob of some sort. They can have adjustable slopes, but that is far more rare. The best allow bandpassing - that means that you take the highs out and the lows out leaving you a certain band of signal passed - hence band pass.

Then you have the built in crossovers on an amplifier. These are typically less flexible hten those that are outboard, but amplifier manufacturers are making them more flexible - such as the JL Slash series or the a/d/s/ amps. These reside in the actual amplifier and are generally sufficient for most systems.

So, in summary:
Outboard Actives:
-Generally more flexible
-Require more cables
-Requires it's own power, amp remote, and ground

On-board Actives:
-Fairly flexible
-No additional cables or real estate

Passives
-Can run multiple drivers on one amplifier channel while playing different frequencies
-Not flexible
-Difficult to design
-Not tailored to your specific application

So, in some cases, a passive does the job very well, in others, you will need the flexibliity of an active. Again, all of this info is very generalized, and there are exceptions to every rule.

 

nice tech manual on how a crossover works, however, since it seems that this guy is fairly new to the stereo electronics world i'm betting this was way over his head. just a guess