How to Choose a Crossover
|AudioControl 6XS 6-channel crossover|
What’s a crossover, and why would I want one?
A crossover is an electronics device that takes a single input signal and creates two or three output signals consisting of separated bands of high-, mid-, and low-range frequencies. The different bands of frequencies feed the different speakers, or “drivers,” in a sound system: tweeters, woofers, and subwoofers.
Think of a crossover network as an audio traffic cop, directing highs to your tweeters, midrange to your woofers, and low bass to your sub.
Without a crossover, a messy, sonic "traffic jam" results. Your midrange and sub duplicate too many of the same frequencies and your sub wastes time trying to put out high notes it wasn't meant to handle. A "fatal pile-up" could also occur, with your tweets being destroyed by some renegade tractor-trailer of a bass note thumping along in the wrong audio lane.
Because they're essential, you'll find crossovers in some form almost any time speakers are present. Crossovers electronically filter the signals so that speakers get only the range of frequencies — high, mid, or low — they’re designed to reproduce. For instance, if your home stereo uses a pair of 2-way bookshelf speakers, it uses 2-way crossovers (inside the speaker boxes). Within each crossover, a high-pass filter blocks the lows but passes the high frequency notes to the tweeter, while a low-pass filter blocks the highs and passes low frequency notes on to the woofer.
|Sound Ordnance P-67C component system: woofers, tweeters, and crossovers|
The crossover “networks” of coaxial, full-range car speakers are usually built into the speakers, and often consist of small electrical components like coils or capacitors. Crossovers for 3-way systems, those systems utilizing tweeters, midrange drivers, and subwoofers, include, besides high- and low-pass filters, “bandpass” filters which play frequencies between two points by utilizing both a high-pass and low-pass in the same filter network. So, for example you could have a midrange driver only playing 100 Hz to 2500 Hz.
Active or passive?
There are two basic kinds of crossovers: active and passive. Passive crossovers don’t need power to filter the signal as desired. Active crossovers require power and ground connections, but give you much more flexibility and fine-tuning control over your music.
|Kicker KX2 2-way active crossover|
A sound system is termed “active” when each driver (tweeter, woofer, sub) has its own channel of amplification. This dramatically increases the available power, dynamic range (softest to loudest sounds), and your control of the system’s tonal response over the whole audio spectrum.
An active crossover gets wired between the receiver and amplifier and cuts out the unwanted frequencies before the amp wastes energy boosting them, so the amp can focus on only the frequencies you want to hear. Active crossovers usually have volume controls on every channel or pair of channels so you can keep all the “voices” of the different drivers in balance. Some active crossovers include other sound-processing features like equalization for further tweaking of the sound to your personal satisfaction.
|Helix P-DSP programmable processor lets you make settings from your laptop (click to enlarge)|
Some digital crossovers use a menu screen, or your laptop, instead of knobs and switches, giving you more filtering options and allowing you to set your crossover points and slopes with much more precision. You can also store different crossover settings in presets, which can be useful for car audio competitors.
The only potential disadvantage of an active crossover is that since it requires +12V, ground, and turn-on connections, it presents more of a challenge to install and set up than a passive crossover. But with a little time and care this shouldn't be a problem, and the rewards and advantages of an active crossover make it clear why you'll find one in virtually every competition-level car audio system. Likewise, stereo systems tuned for high-quality sound will make use of crossovers in order to keep the speakers playing clean and clear.
A passive crossover doesn’t need to get hooked up to a power source to work. There are two kinds of passive crossovers: component crossovers that connect between the amplifier and speakers, and in-line crossovers that fit in between the receiver and the amp.
|Crossover for a Focal Performance PS 130 component system|
Passive component crossovers step into the signal path after your amplification. They’re small networks of capacitors and coils usually installed near the speakers. Component speaker systems come with their crossovers set for optimum performance, and they are simple to install and set up. A full-range signal exits the amplifier and goes to the passive crossover which separates the signal into two parts and sends the high notes to the tweeter and the mid and low notes to the woofer. Most passive component crossovers have optional settings that let you turn down the tweeter some if it seems too loud for the woofer.
Since it is filtering a signal that has already been amplified, a passive crossover wastes power, releasing the unwanted parts of the amplified signal as heat. Also, speakers actually change their impedances when playing which also changes a passive crossover’s crossover point, or frequency response, leading to inconsistent sound definition, especially around the vocal regions. (This is another advantage to using an active crossover, which is unaffected by speaker impedance.)
|Examples of in-line crossovers|
Besides passive crossovers that operate on speaker-level signals and connect between your amp and your speaker components, there are also in-line crossovers that connect before the amplifier. They look like little cylinders with RCA connectors on each end and simply plug into your amplifier’s inputs. In-line crossovers make sure your amplifiers don’t waste energy amplifying signals you don’t want — like high frequencies to a subwoofer amp. Installing an in-line crossover is a great and inexpensive way to sharpen the sounds of your system, especially in a component speaker system.
In-line crossovers each come set to a specific frequency and can’t be adjusted. Another disadvantage of using in-line crossovers is that they react differently to different amplifiers, possibly changing their crossover points unpredictably.
For future upgrades and expansion, go active
If you plan on expanding your system in the future, it's wisest to go with a separate outboard crossover, instead of relying on the ones built into your receiver and amplifier. While these built-in crossovers work well, they don't offer the total system control of an outboard unit. Also, if you ever upgrade your amp, you don't have to give up your crossover.
Tuning your system
Varying your crossover points is one approach to "tuning" your speakers. You can expect this adjustability from just about any active crossover. Setting crossover points also helps define the overall flavor of your system.
Setting your low-pass filter above 100 Hz gives you the type of boom many rap fans are looking for, while pushing it down to 80 Hz tightens up your bass and improves front soundstaging. Because each output channel on an active crossover usually has its own level control, you can even use this component to compensate for varying efficiency or sensitivity ratings among your speakers.
|How a stereo 3-way crossover fits into a system|
Let there be music
Let's look at an example. Take a simple three-way crossover network:
- lowpass filter with a crossover point at 80 Hz;
- highpass filter with a crossover point at 3,000 Hz;
- bandpass filter with a low crossover point at 80 Hz and a high crossover point at 3,000 Hz.
Get Everything You Need
You hop into your ride, slip in a CD and suddenly a hefty dose of unadulterated Dave Matthews Band is headed straight for your speakers. The lowpass cleans up Carter Beauford's kick drum and the low notes on Stefan Lessard's bass, and passes these tones below 80 Hz to your subwoofer system.
Meanwhile, your highpass sends cymbal crashes and acoustic guitar harmonics to your tweeter, while limiting frequencies below 3,000 Hz. And Dave's vocals, Boyd Tinsley's violin, and other sounds between 80 and 3,000 Hz find their way through the bandpass crossover to your midrange drivers.
The crossover assigns the proper frequencies and levels to the various speakers in your vehicle, the pieces of the sonic puzzle fit together perfectly, and DMB sounds righteous. It's all good.