Advice on crossover? - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 02-21-11, 05:10 PM
Mark Techer

Join Date: Jan 2008
Location: Brisbane, Australia
Posts: 613

Quote:
Theresa wrote: View Post
I've never known someone who is experienced with speaker design to say that "its not that hard" to design a crossover.
Speaker design is more of an art from than an exact science. I take no offense to the term "black magic" if that works for you. I think the reason many look at as too hard is they think too deep into it. I've made (and sold) seven 7.1 systems in the last 11 years, and all of them worked (or I doubt the buyers would have handed over the cash if they did not sound good). So either I understand the WHY or I got really lucky and I somehow I don't luck applied each and every time.

In its most simple terms, a typical LR crossover is a parallel rejection electrical circuit - ie it rejects frequencies above the set value of the LP and below the set value of the HP. A Cap has no "resistance" at or above the Xc and will present an ever increasing resistance to frequencies below the Xc. Coils have no "resistance" at or below the Xc and will present an ever increasing resistance to frequencies above the Xc. Because electricity is lazy, it always chooses the path of least resistance. The values chosen to represent the point Xc are based on frequency and impedance load.

So whilst a crossover is a combination of Caps and Coils (and some resisters), it wants to see the speaker drivers as fixed resistances even though speaker drivers behave nothing like fixed value resisters. In a 2 way passive design, you have a LP and HP and I would suggest that no less than 12dB/oct is used simply because you have the ability to correct phase issues by inverting the tweeter's polarity. Each time you add a Cap or Coil, you get a +45° (for Coils) phase shift and a -45° (for Caps) phase shift. So for a 2 way LR at 12dB/oct, you have 2 Caps and 2 Coils resulting is a 180° phase shift between woofer and tweeter. Invert the polarity of the tweeter and you now have both drivers in phase.

Something important to consider is that many components have a tolerances of at least +/-5% where even expensive caps like those from Sollen and Bennic are not 100% precise. 10% is quite a margin.

The first step in a custom design is finding out the Fs of the Tweeter? Given the OP has to replace the tweeters anyway, he can choose to use the same ones or something else. The Fs is the resonant frequency of the driver and why they got fried when playing un-flitered frequencies below that. You want to set Xc at least 1 octave above Fs and Fs will be listed in the spec sheet of the driver. The Fs of the VIFA XT tweeter I use is just 550Hz, so in theory, an XC of 1100Hz could be used. In my passive systems, I used 1550Hz. Most tweeters however will require Xc to be at or above 2K.
Mark Techer is offline

Old 02-21-11, 05:23 PM
Senior Shackster
dd

Join Date: Dec 2006
Location: Upstate NY
Posts: 118

^^^Good art comes from experience. You seem have it, and the tools. My point is that a shot gun approach (op's) to save a pair of fairly decent speakers would be the wrong approach.
digital desire is offline
Old 02-22-11, 06:17 PM
Mark Techer

Join Date: Jan 2008
Location: Brisbane, Australia
Posts: 613

Quote:
digital desire wrote: View Post
^^^Good art comes from experience. You seem have it, and the tools. My point is that a shot gun approach (op's) to save a pair of fairly decent speakers would be the wrong approach.
I agree.
Mark Techer is offline
Old 02-23-11, 11:45 AM
Senior Shackster

fbov

Join Date: Aug 2008
Location: Pittsford, NY
Posts: 302

Quote:
caliberconst. wrote: View Post
...I posted this in the DIY section because the boxes of these 9's are trashed, so I am going to re-use all of the drivers,ports and build new cabinets.
Well, what's the plan now that the tweeters have joined the boxes and all you have are mids and woofs?

Looking at the spec sheet, this was a 4-driver, 2.5-way design. You will not find a similar crossover on the shelf. Presumably the pair of woofers was the 0.5 part with the mid running open to the tweeter cross. You can learn to characterize drivers (electrical and FR) and use crossover design tools to mate these drivers with a new tweeter, which will be chosed for sensitivity unless you want to pad the rest of it down. Try asking on RE Tech Talk or HTGuide forums, but I'll bet you get a lot of "are you sure you want to jump in the deep end?"

Have fun,
Frank
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