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07-21-2004, 09:00 PM
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#1 of 26 
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Member
Location: Katy, TX
Join Date: Aug 1999
Local Time: 02:06 PM
Local Date: 11-18-2008
Posts: 6,501
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I’m sure all DIY speaker builders are familiar with the concept, “the (passive) crossover makes or breaks your DIY speaker.” I’ve always wondered, why even use a passive crossover at all? Why not go active?
A little background:
I had an interesting experience with my speakers a few years ago, a somewhat unique two-way design from the now-defunct Genesis Physics brand. The tweeter has a huge magnet which allows it to run a very low crossover point of about 1200Hz. However, it wasn’t hard high-level, sustained use to burn out the tweeters, and after blowing a couple they gave me a diagram showing how to raise the crossover point to 1800Hz.
Well, immediately I noticed that more than the crossover frequency had changed. The bass sounded different – a little tighter, and the tweeter sounded a little shrill. I eventually traced that problem to a 3-4dB spike centered at about 6kHz.
Another thing: Years earlier I came across a white paper from Harrison Labs (the company that makes those “F-Mod” in-line filters) that said if you took a passive three-way speaker system that was being powered at about 200 watts and switched it to active, you could get the same output levels with a trio of 25-watt amps! Yup – a total of 75 watts active would perform like 200 watts passive!
These two things were enough to convince me that passive crossovers are basically so much crap: They suck out amplifier power and audibly change the sonic characteristics of a driver.
You can see the difference an active system makes at this on-going thread at the Home Theater Talk Forum. The originator of the thread is experimenting with modifying an Adire speaker kit to active, and reports the improvements:- “My friend just came over and we watched the "Brain Damage + Eclipse" music. The MPEG compressed music... was apparently clearer and more open than [with] the regular Kit281 in the next room hooked to my nice CD player and amplifier. I'm starting to become more convinced that the active approach itself is responsible for gains in sound quality that don't have to do with frequency response.”
- “...even considering the slight difference in the crossover, the active speaker still has the basic tone [compared to the un-modified one]... of the other one. It's still a Kit281, just better. It has clearer treble and smoother midrange. The disadvantage is this smoother midrange pushes certain singer's high notes back from a prominence in the soundstage. I appreciate the more pleasant sound and lower distortion, but it's a little less "unrestrained" and I'm going to try and correct it towards more upper midrange excitement, despite maybe a little harshness. It's a defect I don't mind living with.”
- “Each range of sounds is improved in some way. The bass has more "punch" and transparency, the midrange seems smoother (though it still has some of the characteristic AV8 coloration) and the treble is way clearer. In general the speakers maintain the same unrestrained quality I liked so much, but are less distorted sounding than before.”
With reports like this, I personally can’t understand why so few DIYers try active. It seems to me hands-down to be the best way to go. It takes all the mystery and “voodoo” out of coming up with a passive crossover design that works. All you need to do is tweak the crossover point and perhaps the slopes.
Sure, the cost of hardware is a factor, but all the money saved by going the DIY route would cover most of that outlay. So that shouldn’t even be a concern. Why not just bite the bullet and go all the way? It only makes sense, if the desire is to get the best performance for the money. As we’re already seen, you don’t need expensive high-powered amps; new or (even better) used high-quality, low-powered amps sell pretty cheap. Note that the DIYer from the HTT thread is only using an Audio Source amp, not exactly the vanguard for top-flight fidelity.
Once you have the hardware, you can use it forever with any future DIY speaker venture you might want to undertake. Each new project would be virtually “plug and play” – just change the crossover frequency as needed. No endless futzing around trying to come up with an all-new passive crossover design.
So let’s hear it, all you DIY speaker builders. Why do you choose to mess around with bogus passive crossovers when you could go active and leave all that behind?
Regards,
Wayne A. Pflughaupt
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07-21-2004, 09:45 PM
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#2 of 26
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Member
Join Date: Jan 2002
Local Time: 03:06 PM
Local Date: 11-18-2008
Posts: 321
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You know my position on active; I'm of the same mind as you about this subject. I believe that there are so many advantages to active, especially for DIY. The dampening increase alone is worth the extra trouble and cost IMO.
Passive components cause so many anomalies in the high power signal path that often compensation networks are required to combat cretin driver’s Q’s. With active you hardly ever (if ever) need these compensation networks, so a strait electrical slope and conveniently adjustable amplitude is all you have to deal with in most DIY active applications. The fact that you can so easily change X-point and amplitude for each individual driver is a godsend if you want to quickly compare different alignments and fine tune to the room.
What Wayne said about
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Another thing: Years earlier I came across a white paper from Harrison Labs (the company that makes those “F-Mod” in-line filters) that said if you took a passive three-way speaker system that was being powered at about 200 watts and switched it to active, you could get the same output levels with a trio of 25-watt amps! Yup – a total of 75 watts active would perform like 200 watts passive!
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is what I can verify, when I changed my old passive DIY's to active I could play them to far greater SPL's with the same amps and not clip them, where before the distortion and lack of control of the drivers (decoupling from passive components) caused clipping prematurely. I had no idea that this little 2-way 6.5 woofer and 1" tweeter could have so much clean clear sound, they would play at concert levels now!
Nice post Wayne, and I couldn’t agree more with the notion of Active being the best DIY tool ever, and with Digital finally becoming more popular it's finally price worthy to consider an all digital Active network, save a bunch of preset curves and switch (A/B) quickly for a true comparison, this would be great to dial in a 2/3/4 way system. |
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07-21-2004, 11:21 PM
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#3 of 26
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Member
Location: 5280'
Join Date: Nov 1999
Local Time: 02:06 PM
Local Date: 11-18-2008
Posts: 3,355
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I feel it's a disservice not to share this 'epiphony' with a larger cross section of the DIY loudspeaker community.
Therefore I urge Wayne to share his insights with the DIY-loudspeakers mailing list. Tom Bennett, David Harris, Earl Geddes, Dan Wiggins, David Hyre, Thomas Danley, etc. certainly need to know this information.
In addition I'm sure that the unwashed masses such as Tom Nousaine, John Atkinson, Richard D. Pierce, Bascom King, et al, that post at rec.audio.high-end would benefit from these insights as well.
And lest we forget the likes of Siegfried Linkwitz, Charles Hansen, John Whittaker, and others at the dipole-list, and of course DIY Audio loudspeaker forum.
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07-21-2004, 11:23 PM
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#4 of 26
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Join Date: Sep 2002
Local Time: 08:06 PM
Local Date: 11-18-2008
Posts: 818
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I was just flipping through the PE catalog i got in the mail and that Behringer ultradrive pro sure looks nice. But what about the things you get with a nicely designed OX, the ability to correct dispersion, baffle diffractions, or are all those things "chasing tale" with problems going with a passive network? I haven't really looked into it, but i imagine that it would be rater nice to have a nice active setup, but i imagine that most are XLR and so that would add another component into the mix.
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07-22-2004, 04:57 PM
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#6 of 26
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Member
Location: Katy, TX
Join Date: Aug 1999
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Local Date: 11-18-2008
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I was just flipping through the PE catalog i got in the mail and that Behringer ultradrive pro sure looks nice.
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Personally I’m not fond of the extra AD/DA conversions this would require. Perhaps someday, when receivers and pre-pros have digital outputs, so that the signal can remain in the digital domain all the way to the amps. Not to mention Behringer isn’t exactly known for their pristine sonics or quality AD/DA converters.
Plus, I briefly used a mid-line pro crossover (analog) in my system several years ago, and it audibly colored the sound. Sibilants were “raspy” and it added some prominence to the upper bass. Since then I’ve been cautious about using budget pro gear in my system.
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F-Mods are passive line level, fixed frequency, fixed slope attenuators primarily intended for car audio.
The attenuation point is determined by a combination of the F-Mod device and the input impedance of the power amp. Use a amp with a different input impedence, and instantly get a different attenuation point and slope whether you want one or not.
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Yeah, I don’t see F-mods as the way to go; as you’ve noted, passive devices will be as problematic on the front end as they are on the back end.
I’ve toyed with the idea of modding my Genesis speakers to active. I’ve already got my eye on a couple of amplifiers that would be ideal for the project. For one, Yamaha makes a dandy little low-powered 4-channel amp that sells pretty cheap on eBay. Adcom also used to make a low-powered 4-channel amp that would also work well. I could sell my hunkin’ 250-watt Adcom amps and have money left over. 
The guy at HTT is braver and more patient than I am, making his own crossovers from scratch. I’d just use off-the-shelf hardware. I’ve toyed with a few ideas, like:- Using AudioControl Phase Coupled Activators. They’re a 24dB/octave design with Linkwitz-Riley alignment, which would probably allow me to safely push the Genesis tweeter down to 1000Hz (!). The PCA is designed to be a sub crossover, but they use plug-in chips for the crossover frequency, so functionally you can choose any frequency you want. I’m currently using one in my system between the subs and mains and I’ve been very happy with the sonics. The downside is that it would take a second PCA to separate out the lows for the subs. Fortunately the PCA’s specs are so-o-o excellent that even passing the upper frequencies through two of them shouldn’t have any affect on sound quality. However…
- I could probably just as well use an AudioControl three-way car audio crossover to get everything in a single unit.
- Another piece of hardware I’d like to try, just for the sake of comparison, would be an Ashly pro-audio crossover. They also have 24dB/octave slopes, but with a unique feature, a “Response” control that allows you to tune the response curve between the filters. Basically, the “Response” control allows you to utilize either Linkwitz-Riley alignment or Butterworth, or a combination of the two. As a practical matter, the control lets you compensate for the droop in response you usually get between the filters – very cool.
Regards,
Wayne A. Pflughaupt |
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07-23-2004, 02:47 PM
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#7 of 26
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Member
Join Date: Apr 2004
Local Time: 04:06 PM
Local Date: 11-18-2008
Posts: 84
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Wayne,
> These two things were enough to convince me that passive crossovers are basically so much crap: They suck out amplifier power and audibly change the sonic characteristics of a driver. <
You're preaching to the choir. My JBL 4430s have been biamped since I bought them more than ten years ago. When I switched from the passive crossover to an active (a Rane AC22) two weeks after I bought them, the improvement in clarity was immediately obvious. And these are high-end professional loudspeakers, presumably having as good a passive crossover as can be made.
--Ethan
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07-23-2004, 03:26 PM
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#8 of 26
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Member
Join Date: Jul 2001
Local Time: 04:06 PM
Local Date: 11-18-2008
Posts: 1,576
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Hi, I'm the guy with the active Kit281s. I'm not really ready to present the results of this project, but once I've tweaked the system to be as good as it can be, I will try to make PCBs available so it is easier for other owners to do the upgrade. You can read the HTT thread, but don't worry... I will post the whole deal here when I get farther along.
As far as I can tell there is just as much "voodoo" in the active design. Computer software has made it easy to predict the complex summed frequency responses of passive crossovers just as well as active ones. It's too bad that with regular speakers drivers it is often not a good idea to use symmetrical, consistent slopes (which most off the shelf crossovers have).
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07-23-2004, 04:39 PM
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#9 of 26
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Join Date: Jun 2001
Local Time: 04:06 PM
Local Date: 11-18-2008
Posts: 7,403
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Did you do any sort of response measurements post-active in order to compare then with what your final version will be?
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07-23-2004, 05:02 PM
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#10 of 26
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Member
Location: 5280'
Join Date: Nov 1999
Local Time: 02:06 PM
Local Date: 11-18-2008
Posts: 3,355
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And these are high-end professional loudspeakers, presumably having as good a passive crossover as can be made.
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Ethan,
You been in the business long enough not to be that naive.....
Michael P.
You're doing what must be done to properly create an active XO for any speaker. And you're finding out how much work that process is .... 
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These two things were enough to convince me that passive crossovers are basically so much crap:
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Really?
Wayne,
How many optimized high-end passive crossovers have you designed and built? And how many comparisons have you made between those same crossover and active ones custom designed and built for the same drivers?
Actually, for that matter how many crossovers of any kind, have you designed and built? Some how I get the feeling that all this just pure speculation on your part.
The use of generic active XO's, even those with 24dB/octave LR slopes pretty much ignores all known fundamentals for how loudspeaker should be designed. Advocating the use of any generic active XO that lacks the ability to compensate for things necessary for proper levels of loudspeaker performance (baffle step compensation, impedance compensation, etc, etc,) is folly.
As interesting aside to this entire discussion is that really high-end design Wilsons, Genesis, etc, primarily use passive XO's. Now the people spending $100,000+ for a pair of loudspeakers can certainy afford whatever it takes for optimal performance. And my guess is that designers such as David Wilson, et al, would use active electronics if there were any sonic benefit.
So one could conclude that not all passive XO's are crap.
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