The purpose of a crossover is to limit the frequencies that will
pass through the device, and then attenuate those frequencies
which are undesirable at a fixed rate. Follow all that? let me
give you an example.
In the system I am designing for my Probe, I am planning on using 4 speakers. 2 bi-ampable co-axial drivers, and a pair of subwoofers. All the speakers have particular ranges in which they operate effectively and efficiently. The tweeters will run fine from 3500 Hz up, and the subs will sound best from about 100 Hz down. This means that I must divide the signals going to each speaker from the entire reproducible audio spectrum. I am assuming a minimum range of 20Hz to 20 kHz from my Clarion Head unit, through my equalizer, and through the amplifiers. Therefore, the operating ranges for each speaker are quite clear. The subs will play from 20 Hz to 100 Hz, the midrange from 100 Hz to 3500 Hz, and the tweeters from 3500 Hz up to 20 kHz. The points that we must be concerned with are 100 Hz and 3500 Hz. The other cutoff frequencies are an inherent limitation of audio equipment, CD's, etc.
There are two ways to accomplish this system design. It
can be done with active (electronic) crossovers, or passive crossovers.
Active crossovers work on the signal coming from your head unit,
going to your amplifier. Passive crossovers are in the wires between
your amplifier and your speakers. What are the advantages? Using
an electronic crossover will only allow the range of frequencies
you choose to be amplified by the amplifier, and no others, and
you can adjust the level of those speakers using the crossover
or amplifier gain. The passive crossovers simply filter out the
frequencies you don't desire from the already amplified signal.
A system using electronic crossovers will be more flexible, and
will likely be more efficient than one using passive components,
as we are only amplifying the signals we desire. Electronic crossovers
are also much easier to adjust, while with passive components,
we must replace parts to change frequencies.
Slopes: What are the advantages of steeper slope cross-overs? There are 3. Reduced speaker localization (harder to hear the location of the speaker). Reduced overlap (The range of frequencies in which two speakers are playing the same music). And increased power handling (a steeper slope on the bottom end (low frequency cutoff) will reduce the amount of motion that is caused by these low frequencies. Example, 90 Hz high-pass at 24 dB / octave on an MB Quart 5 1/4" mid, works great!
One disadvantage of using a lot of electronic crossovers is that each speaker will require a dedicated amplifier channel. But, you will have complete control over all levels directly from the amps or crossovers.
What am I going to do?
I have 2 options. In my previous system, and in my father's old system, we used four amplifier channels, and then used custom built passive crossovers for the division between the midrange and tweeter. This is quite effective. In my Probe, I am planning on using an amplifier channel for each of the speakers in the car. I was talking to Dave Gumienny at Precision Power Inc. (1-800-62-POWER) this morning, and we are planning on using their brand new AX-606.2 amplifier. It is a six channel @ 50 Watts per channel amplifier with built in crossovers. The crossovers can be configure for low-mid-high or low-high-high. The front 4 channels will double their power from 4 ohms to 2 ohms, and the rear 2 channels will double their power from 4 - 2 ohms, and again from 2 - 1 ohm. Thus producing 400 watts into a 2 ohm mono load. Should be enough for me! And quite simple to install. I need only 2 sets of RCA's, one from the head unit to my crossover, and one from the crossover to the amp. Phoenix Gold will be the cables of choice for this project. But, we will have to see what happens over time.