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Old 2nd November 2010, 02:18 AM   #1
mfaughn is offline mfaughn  United States
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Default Translating Active XO to Passive XO? How to?

Hi,

I have built some open baffle two-ways using Fostex FE103E and reasonably nic CTS 15" alnico woofers from a Magnavox console. I am currently bi-amping them and using an Ashley SC-22 active crossover. Can anyone point me to a resource that would help me figure out how to turn my settings on the active XO into an equivalent passive XO. I'd like to be able to run these off a single amp.

As it is now I find crossing over at right about 1K sounds the best to me and that the FE103e must be attenuated relative to the woofers in order for it to sound balanced.

I also have some Fostex FT-17H (i.e. Realistic 40-1310A) units that help out as well. I currently have them paralleled with the FE103e and tune them with L-pads.

Any help would be most welcome.

Thanks,
Michael
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Old 2nd November 2010, 04:17 AM   #2
Pano is online now Pano  United States
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Well, it can be quite a task. The SC-22 is 12dB slope, IIRC, so that's not too hard to imitate.

But what you really need are impedance measurements of your drivers on your baffle. Feed that into Jeff Bagby's Passive Crossover Designer spreadsheet and you can tweak away to your virtual heart's content.

With some value tweaking of the caps, coils and resistors, you can find a response curve that closely matches your Ashely. You can see the effects of an active crossover in the spreadsheet and let that be your target. Or just follow the built in 12dB target curves.

I've done this a few times with pretty good success. Having accurate impedance measurements is the key. You don't even need frequency response measurements, just impedance. Good luck!
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Old 2nd November 2010, 10:38 PM   #3
theresa is offline theresa  United States
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You really cannot design an accurate passive or active crossover without knowing frequency response. Design tools that don't use this information are not going to result in correct response. Its a daunting process. I use active crossovers and find them easier to find a correct crossover level/phase/distortion/time alignment. Of course with a lot of luck you might get it just right. I am looking forward to using REW now that the measurements can be directly loaded into the miniDSPs.
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Old 2nd November 2010, 11:16 PM   #4
mfaughn is offline mfaughn  United States
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Well, I was thinking of getting one of the Dayton woofer testers anyway...This adds another tic in the "for" column. Gotta, gotta unload some other stuff to justify buying anything else though...

Thanks for the education.
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Old 3rd November 2010, 07:07 AM   #5
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Hi MichaŽl,

It is a very hard to go to a passive crossover for a beginner !
You need measurements tools to get Impedance and Response
Some simulations tools like Basta / MJK worksheets can do the job. But I have found Basta, not very precise in the treble, if you want to align phase response. I don't have any idea on MJK worsheets.

Do you have the T/S parameter of the woofer ? MJK have done a similar project. If the woofer QTS is higher than 0.6, I think you can simply copy his project. Less job to do. If the Qts is less than 0.5, the woofer is not suitable for open baffle.

I wrote a tutorial, how to easily measure T/S parameter (in French sorry)

Have Fun

NB
There is also John K. software.

Last edited by jerome69; 3rd November 2010 at 07:27 AM.
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Old 3rd November 2010, 12:42 PM   #6
mfaughn is offline mfaughn  United States
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I'm fairly sure the woofer has a high Qts. It was pulled from a Magnavox console where it was being used in what amounts to a Uframe (open back cabinet). My understanding is that they didn't have T/S parameters back when they made that console but that they did go to a lot of effort selecting drivers that worked well in the applications they put them to. My project looks a lot like the MJK open baffle, a bit taller and wider and more distance between the drivers. The woofers sound pretty darn good in there
now. I might mod them to into an H or U-frame on the bottom but I'm not sure they really need it.

Francais? pas problem. i started skimming through and I got most of it. I'm the only person living in my house that isn't French. I can read better than I can speak or write and I can get help if I need it.
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Old 3rd November 2010, 01:25 PM   #7
Pano is online now Pano  United States
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Quote:
Originally Posted by theresa View Post
You really cannot design an accurate passive or active crossover without knowing frequency response.
Quote:
Originally Posted by jerome69 View Post
It is a very hard to go to a passive crossover for a beginner !

OK - true enough if starting from scratch, but that's not the case here. Mr. mfaughn already has an active setup he likes, a passive equivalent is all that is desired. That can be calculated once the actual impedance curves of the drivers are known. The WT2 or WT3 will do that very well.

If the slopes and frequencies of the active filter are known, then they can serve as targets in the simulation. Again, I suggest the very good and free Passive Crossover Designer. It may take some time to learn the program, but with help from the forum, it won't be that bad. Shootz! I can probably do it for you.

Measured frequency response is not needed here, in fact it's a hindrance. Looking at the acoustical response will only confuse this job. The electrical function is what you to imitate. Knowing the impedance curve of the drivers and the DCR of the inductors will allow you to do this.

For example:
  • The active crossover is set at 1Khz, 12dB Bessel. You like the way this sounds.
  • Measure the impedance curves of the drivers. (with L-pad if used)
  • Plug those curves into PCD.
  • Choose 2nd order Bessel 1Khz as your target.
  • Tweak the component values of the passive filter until its electrical response (filter function) matches the target.
Voila! C'est fini!
I've done it, it works.

Will it be exact? No, not likely. But it can be close enough and may even sound better. If the amplifier output impedance, the DCR of the inductors and the L-pads are taken into account then it can be very close indeed.

Important fact to remember: We are not starting from scratch. That IS difficult. All we are doing is imitating a 2 way active filter that is already good to go.
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Old 3rd November 2010, 01:37 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Whilst the OP has a active setup that appears to work, MJK's article
shows that some baffle effects can be manipulated to advantage
by using offset electrical c/o frequencies to create a symmetrical
acoustic c/o point. There is scope for overall improvements.

rgds, sreten.
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Old 3rd November 2010, 01:50 PM   #9
Pano is online now Pano  United States
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Oh yes, absolutely! Open baffle crossovers usually don't follow box crossovers - at least for the woofer. There is certainly room for improvement. But I was not going to open that can of worms.
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Old 3rd November 2010, 07:20 PM   #10
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I am very pleased to see some American people are able to read French

No problem Pano ! Your idea is working and it is the solution .
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