Translating Active XO to Passive XO? How to?

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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
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
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!
 

theresa

Member
2009-08-07 12:06 pm
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.
 

jerome69

Member
2008-02-18 11:33 am
Lyon
Hi Michaël,

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.
 
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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.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
You really cannot design an accurate passive or active crossover without knowing frequency response.
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.
 
Possibly going backwards. It is as much an exercise in exploration, learning, etc. as anything else. As it stands the speakers don't quite do enough for me, not enough that I'd commit to leaving them in use long term. On the upside the are very pleasant sounding and they could be listened to all day without fatigue. They lack in the detail and imaging departments. Knowing they could work well with just a single amp would mean they might live longer. I've also had about enough of big, room dominating speakers -- might help if I got the Altec 1236, Snell Aiii, and DQ-10's out of the same room...the cargo adds up. I recently completed some Fonken's and with the addition of a sub they are a nice flavor of the month.
 

jerome69

Member
2008-02-18 11:33 am
Lyon
Detail and imaging are the highlight of open baffle.
A lack of microdynamics too ? The loudspeaker phase might not be enough coherent ... You should time align or physical align the 15" and the 4". Perhaps lower the actual crossover point to 400/500Hz.
Believe me it is a fantastic game territory to explore ;)
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
One problem with most active crossovers is that you can't split the HP/LP frequencies, they are locked together. Spreading those frequencies apart is one of the basic "tricks" to getting good results. And using different slopes on different sections.

FWIW, crossing an OB woofer much lower than the desired crossover point and then boosting its overall level often works wonders to bring it back into balance.
 
One problem with most active crossovers is that you can't split the HP/LP frequencies, they are locked together. Spreading those frequencies apart is one of the basic "tricks" to getting good results. And using different slopes on different sections.

FWIW, crossing an OB woofer much lower than the desired crossover point and then boosting its overall level often works wonders to bring it back into balance.

That is true for the cheaper analogue ones using pots on the front to adjust xover frequency but not for the better analogue ones using plug-in cards.
With those any combination of slopes and frequencies is possible.
 
I use BSS FDS360 but they are not being made anymore. They are usually around £100s/h, new they were around £700 I think. They also allowed for 360deg phase adjustment in the xover region and parametric and shelving eq cards were also available. Some pro audio dealers will still supply these.
They were used for P.A. systems and studio monitoring so most def. pro applications.
On the whole the good ones have been replaced by digital ones these days.
 

uvellani

Member
2004-10-15 9:06 pm
That is true for the cheaper analogue ones using pots on the front to adjust xover frequency but not for the better analogue ones using plug-in cards.
With those any combination of slopes and frequencies is possible.

If you have a 3 way like the behringer 3400 you can use different freq cutoffs for the woofer and tweeter. Set it to 3 way mode and use the high and low bands and skip the mid.
 
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