Strictly speaking, a crossover separates “full-bandwidth” audio into two or more frequency ranges, and that’s it.
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Every so often, I’ll get a question which indicates that we (the industry) is doing a poor job at explaining the tools we use. The question can take the form of “Is it okay to do this thing if this other thing isn’t a part of the system?” Other such questions may take the form of “Is there one right way to hook this up? Where does it go in the signal chain?”
What questions like these reveal is that there’s gear we talk about as being important – indispensable, even – yet we fail to discuss the fundamental aspects of what that gear does. The equipment in question becomes a kind of magical box or physical spellcasting component, that, if used improperly or neglected, could cause Very Bad Things to happen.
You know, like the “Klaatu, barata, nikto” incantation in “Army of Darkness.” Ash doesn’t know what it does (heck, NOBODY probably knows what it does), but getting it wrong causes Very Bad Things to occur.
“Did you say the words?”
“I said ’em…yeah.”
“Look, I might not have said every single little syllable…”
Cue the army of Deadites marching on the castle.
It seems to me that crossovers are a particularly prime candidate for getting the “black box” treatment. Unlike regular EQ, they aren’t really a device where you go hands-on, twist the knobs while audio is flowing, and hear the results. They’re also quite important for running a system in a sane way. They can indeed be vital to not trashing system components.
But if you don’t understand the whys and wherefores associated with crossovers, you might be unnecessarily anxious over what to do with one. Or without one.
First things first: A crossover is a frequency dividing network. You may even see that terminology used in place of the word “crossover.” What that phrase means is that a crossover is a set of interconnected electronic devices (a network) that acts to separate a single input containing a wide range of audio frequencies into two or more outputs (a signal divider). Each output includes a subset of the original input frequency range (hence the “frequency dividing” designation), and, in standard practice, the frequency range of all the outputs put together should be the same as the input frequency range.
In other words, a crossover takes an input that can potentially contain signals spanning the full-bandwidth of human hearing (or more), and separates that signal into bandwidth-limited outputs. If you took all the outputs and summed them together, you should theoretically be able to recover the original input signal – plus any noise, distortion, phase artifacts, and whatever else that’s a product of the processing.
As an aside, a digital crossover is also – effectively – a frequency dividing network. The difference is that digital processing algorithms are used to simulate the filtering provided by a network of physical, electronic components. The specific methods are different, but the results are functionally the same.
If you want to be strict, a crossover has ONLY one job, and that’s to separate full-range audio into multiple, discrete passbands. (A passband being a filtered range of audio that we expect to be delivered at between unity and -3 dB gain.) Any functionality other than that is not actually part of the crossover domain…which is not to say that additional functionality is “wrong!” Input gain controls, passband output gain controls, special corrective equalization, and other such things are nifty features to have included in the package that contains the crossover. They are not, however, core to what a crossover is.
If you want to get right down to the nitty-gritty, a crossover is a set of well-engineered highpass and lowpass filters. The filters are ideally designed so that they combine with perfect phase and magnitude when adjacent to each other. In a certain sense, you can view a crossover as a highly specialized sort of EQ with a limited use-case.
What Is It Good For? Absolutely Something
Now then. Why would we want to separate full-range audio into multiple, discrete passbands?
There are purely creative reasons to do so, but the most overwhelmingly common reason is utilitarian: Loudspeaker drivers with differing characteristics have frequency ranges that they are best at reproducing. These frequency ranges are smaller than the complete frequency range audible to humans. A properly configured frequency-dividing network allows each loudspeaker driver in a “multi-way” system to receive only the frequency range that it works with optimally.
Beyond just “helping things sound good,” crossovers are very important to the care and feeding of high-frequency drivers. The reason for this is due to one of the classic failure modes of a driver receiving power: Too much power at too low a frequency.
Low frequencies require large driver displacements to reproduce. This is why you see videos of woofers “pumping” with the bass. More often than not, “large diameter” drivers are capable of very large displacements (front-to-back movement) when compared to “small diameter” drivers. If you try to get a high-frequency horn driver to reproduce 100 Hz at an audible level, you’re very likely to completely wreck the unit. The diaphragm will get smashed into something, or the voice coil will launch out of the gap and never return.
With that being the case, a crossover provides a highpass filter to the small driver which removes that potentially fatal material. If 100 Hz is what we’re talking about, a 24 dB/ octave highpass filter with a corner (-3 dB) frequency of 1500 Hz has a gain reduction of beyond 75 dB…if I did my math correctly. That’s an intensity that’s over 10,000,000 times LOWER than the material in the unity-gain area of the passband, and that’s pretty darn safe.
Where Do You Put This Thing?
With all that established, the implementation questions start to arise. One of the most basic queries is, “where in the signal-chain does the crossover go?”
Crossover come in two basic varieties: Active and passive. Active crossovers require that their components be continually energized by stable voltage from a power supply. Passive crossovers energize their components by way of the fluctuating signal from a power amplifier.
Now, if you’re looking at a piece of rackmount gear that has to be plugged into mains power, you’re looking at an active crossover. However, I mention passive crossovers for the sake of completeness. Hidden inside most multi-way loudspeaker cabinets is a passive crossover that allows the box to be used “full range.” Frequency-dividing does still occur (remember what I said about those horn drivers and low frequency material), but it occurs in an electronic network that’s concealed from view – and sometimes drops out-of-mind as a consequence. Passive crossovers can include the ability to be bypassed, so you must take heed of them!
Anyway, back at the ranch…
The normative signal-chain position of an active crossover unit is to be just preceding the power amplifiers. Yes, the outputs of an active crossover are line-level, so you could theoretically connect other processing between each crossover output and its corresponding amp. Doing so manually, however, is a pretty advanced application. Most folks with physical pieces of outboard gear do all their “interactive” processing before the crossover unit. Doing much after the crossover gets expensive, confusing, and fills a lot of rackspace in a big hurry.
Again, remember that passive crossovers are run POST the power amplifiers (because they need that kind of voltage to operate), and may very well be “stacking” with any active crossover you have in the system. This is not a bad thing at all – it’s actually quite normal – but you should be aware of it. There are lots of PA systems that use an active crossover to get a passband for the subwoofers and a passband for everything else, with the assumption that there will be a passive crossover in the full-range loudspeaker box.
I’m going to refrain from talking about specific crossover settings, because those are so application specific that it’s not worth it.
Various Other Wrinkles
To wrap this up, I want to talk a bit about some of the wider issues that cause headscratching and crossovers to intersect.
One thing to realize is that crossover functionality is increasingly becoming wrapped up with lots of other things. Some folks benignly refer to devices like the Driverack PA+, or the DCX 2496 as “a crossover.” These units, and others, do indeed include frequency-dividing functions. However, they also include lots of other things, like pre AND post crossover EQ, dynamics, time-alignment, and other goodies. If you want to be picky, these “lots of things in one box” products are more accurately referred to as “loudspeaker management” or “system management” or “system controllers.” Because they encapsulate so many virtual processors, the concept of where the actual crossover function occurs can be obscured.
Another issue is that pro-audio is often presented in absolutes when what’s really meant is “normally.” For instance, I do recommend that a person wanting to add subs to a system use a crossover. However, the idea that you have to use a crossover or it just plain won’t work is false. Yes, you can y-split a set of outputs and send full-range signals to both the sub amps and the main amps. The subs will get (and output) a LOT more midrange than in a standard scenario, and so their acoustical output might interact with the mains’ output in a way that’s not all that great. Also, the mains will still be being asked to produce low-frequency content that chews up their headroom. Even so, if you can get it to sound good in your application, then who cares? You’d be better off with a crossover, it’s true, but the system will definitely produce sound, and not blow itself up as long as you’re not being stupid.
The point is that if you know what the crossover does, or should do, then you don’t have to be confused or intimidated by the thing.