Because so few people need one. Almost all speakers have entirely adequate crossovers built into them and these incorporate custom features that tailor them for their specific drivers and enclosure(s).
You bring up a good question, why not more use of active crossovers? It solves so many issues and makes for truly superior performance. For anyone who is *truly* serious, active is the only choice, yet for some unknown reason most audiophiles are either unaware or just want plug & play convenience.
As far as selection, there's not enough demand, but what you need is out there. Several ready made ones, as you've mentioned, and also custom made are available.
The problem is that no "off-the-shelf" active crossover is really suitable for any serious loudspeaker. The crossover must be customized to the the specific speaker system in order to account for amplitude/phase issues in and out of each driver's pass-band. This means that a great deal of flexibility is required in the filters and, more critically, more sophistication is required of the user. The former can be bought but the latter demands some learning and experience.
So, in principle, using an active crossover (and not a passive inbuilt one) can afford real advantages but this is most easily realized with speakers that are designed and marketed that way.
Don't most stereo preamps not have a built-in crossover?
Additionally, outputs from subwoofers usually have a high-pass filter that isn't adjustable; I know that on REL subs, this is as high as 120 Hz for line-level outputs. Most people (I would think) don't want to use such a high crossover frequency.
I'll mention that even if subs provide bass management, there are some problems with this arrangement. Suppose you have a tube preamp and stereo subs. The input impedance of sub amps is usually pretty low (~10k ohms), and with 2 amps it works out to 5k ohms. This makes it pretty problematic to run line-level outputs from preamp to sub amps.
In addition, if you wanted to run speaker-level signal through the sub to the mains, the sub's high-pass filter frequency/slope is not adjustable without a hardware change.
It seems like the only solution is an external crossover...
This is my dilemma. I need a preamp with sub out and adjustable high pass crossover. While some have the sub out, most do not have the crossover, where you can employ a highpass to the mains.
Sucks having to choose between only a handful of preamps that have sub out and adjustable crossover: parasound, rogue, emotiva -- not much else.
Well, this little pond is not representative of the audio world at large. Consequently, the volume demand for this type of product is limited. Some reasonable suggestions have been made. The SMS-1 is one but it is limited to only 80Hz for HP. Consider also the Parasound P7 preamp with adjustable crossover. For more complex/sophisticated installations, there are the pro and DIY markets.
I use an NHT X-2 between an ARC LS-25 and an ARC VT 130SE with no discernable (to my ear) impact on the sound of the mains - other than rolling off the bass as intended. At all of $300 (sold direct, on a money back basis) it's an easy call to give it a shot.
BTW, I take the sub out signal from the x-2 and run it into an SMS-1, then on into a pair of Rythmik 12" subs. I do this because, in addition to the flexibility limitations of the SMS-1's high pass section that Kal noted above, I also found it (sometimes) audible (I think) and, therefore (always) distracting (Is that the x-over I'm hearing? no. yes. maybe.). Never happened with the x-2 in the chain.
The set-up took me some time and effort to get right, but it produces really excellent results in a very difficult room.
This is a requirement in my system, the Megaline operates only with an outboard active crossover. The benefit is the amp is looking directly at the driver, no volume control, no inductors, no caps and no resistors. Accomplishing crossover at line level (1 volt or so) is much easier and cleaner than at amp level.
I have tube preamp, tube phono, tube crossover and tube amps. The input impedance of my amps are 150K, so no problem with the tube crossover driving them. The output of several preamps I've auditioned (Ypsilon, Einstein, Aesthetix, Allnic, Audio Research) had no problem operating into my tube crossover.
Some speakers are a natural for outboard crossovers and some would be a nightmare. Which applies to which depends on what the crossover must achieve to make the speaker linear.
The Maggie 20.1 is another speaker that works superb with tube outboard crossover. A friend of mine has this combo and it goes against anything in high end audio, even the kilo buck speakers he's tried it against.
Short answer, with the right speaker and outboard electronic crossover the combo can work miracles.
The down side is the crossover design must absolutely and PRECISELY match the requirement of the speakers drivers and a minimum of four to six mono amps (or two to four stereo amps) to function. The benefit is astonishingly low distortion, ultra high speed and control of drivers and wide dynamic range without compression.
Required sound pressure level is not as critical as with a passive crossover. Some speakers have a threshold that must be breached before dynamics and realism come into play and frequently that window is shallow, with high sound pressure levels becoming blurred as you push the envelope.
These issues are greatly reduced by direct drive to the speaker elements and upper limit if SPL is astonishing. There are benefits from active outboard crossover but it is a LOT of work to get it perfect.
Another alternative is the Bryston 10B sub. It has fairly flexible xo frequency and slope settings. Maybe a little expensive for a sub crossover, but a very solid and good sounding piece (I use the full range version). By adding a crossover at line level for the sub you will achieve many of the benefits Albert Porter has described.
Amen brother. I also think Shadorne would agree as he has had active speakers for quite awhile.
In an attempt to get the most out of my modest system of Magnepan 1.6's and older Arcam electronics I gutted the 1.6's of the passive crossovers, rewired them, and inserted a Bryston 10B. The change was truly transformational in all the ways you describe. A lot of hard work (and apprehension for someone who had no experience in this kind of thing), but worth it.
In an attempt to get the most out of my modest system of Magneplanar 1.6's and older Arcam electronics I gutted the 1.6's of the passive crossovers, rewired them, and inserted a Bryston 10B. The change was truly transformational in all the ways you describe. A lot of hard work (and apprehension for someone who had no experience in this kind of thing), but worth it.
A good friend who is a reviewer for Absolute Sound owns Maggies and like you, went all active on crossover. He hears very well and I respect his opinion.
Several well respected, super expensive speakers have passed through his listening room and the Maggie held their ground. I don't doubt you are getting the results you claim, active done right solves problems we didn't know we had, until after the fact.
It is still a modest system, and not "perfect" by any stretch, but it is amazing the difference going active made. More control, more transparency, more headroom for the little amps.
You said one thing that I think especially pertains to Magnepans;
"Some speakers have a threshold that must be breached before dynamics and realism come into play and frequently that window is shallow, with high sound pressure levels becoming blurred as you push the envelope."
The speakers are in a large room (30' x 60' irregular) and you could hear the strain as I would try to push the SPL's to the point where the music came to life with passive speaker level crossovers. Night and day difference going line-level active.
I need to give credit to the guys over on the "Planar Assylum" for documenting how to go about it. It is not a plug and play thing with the 1.6's. Your descriptions as you went through the development of the crossover for the Mega Lines was also an inspiration.
The MG 1.6 crossover is a simple HI/LO divider and has none of the extra elements that Kal refers to. It is, therefore appropriate to consider for biamping. However, the stock crossover does have different break frequencies and slopes for the HI and LO sections. If this is not duplicated performance may be degraded (or, perhaps, improved).
Nice to talk to you again. You're correct about Maggies and the assymetrical slopes and frequencies. The Bryston gives just enough adjustability to pretty closely match the stock settings and experiment a little with different combinations. I dabbled a little with measurements to let me "visualize" what I was hearing. Two observations:
You can make different combinations of high and low pass slope settings (with frequency adjustments) measure the same, but the presentation is different. I am no "golden ear", but it is really obvious to me.
"If this is not duplicated performance may be degraded (or, perhaps, improved)."
I actually prefer symmetrical 6db slopes (1st order) with the active crossover. The sound has more "clarity" (for lack of a better term). For my room and tastes, I prefer a different setting from "stock".
Stilljd...Experimentation is always interesting. I have not biamped my three MG1.6s unless you count the Subwoofer/mains as biamping. I did rebuild the passive crossovers using AWG10 air coils that are externally mounted. Because my Maggies will not have to handle SW frequencies I figured that the woofer panel could go a bit higher. My Woofer/Tweeter crossover break is about 250 Hz higher than stock. By the way, this saved a few bucks on the three AWG10 inductors, and the tweeter capacitors. I am well satisfied with the result.