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Old 17th October 2005, 07:04 PM   #1
DonoMan is offline DonoMan  United States
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Default Trying to design my first active XO

I'm doing a 3-way Extremis/WR125S/XT19, crossover frequencies currently at 506Hz(LR2)/4kHz(LR4). Baffle is 9" wide (which is why I chose 506Hz for crossover)

Can anyone see any problems or give me any tips regarding this circuit? Does it need phase correction?

Note: the labels on the output jumpers are where they connect to next, not what the output there is. Will be using an OPA549 for Extremis, LM3875 for WR125S, and probably a LM4700 for the XT19.

I plan to use OPA134s for the crossover, as I already have a bunch.
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Old 17th October 2005, 07:22 PM   #2
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I'd put the BSC circuit before the mid/woofer split. Your XO is very close to the "center frequency" of the BSC so it will need affect both drivers.
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Old 17th October 2005, 08:04 PM   #3
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Isn't the BSC frequency 300/ft, so 300/.75 = 506.6...Hz?
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Old 17th October 2005, 08:24 PM   #4
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Right, that's the center frequency, 3dB out of a total of 6dB. The total effect extends an octave or more above and below there. Here's the response of an 18" baffle with no compensation. Move the curve up an octave for your 9" baffle so the center is around 500 instead of 250.

Click the image to open in full size.
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Old 17th October 2005, 08:56 PM   #5
DonoMan is offline DonoMan  United States
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Changes:
-Moved BSC to include mid
-Made BSC 5dB not 3dB (Extremis starts rolling off around 400-500Hz too, and I like bass anyway)
-Subsonic @ 15.9Hz on woofer (should this just be first order instead of second?)

Better?
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Old 17th October 2005, 09:18 PM   #6
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Ah, I went back and looked at your component values and realized you were going for 3dB total BSC in the first one. My mistake. So, what you had would have given you 1.5dB at the center frequency of 506Hz and 3dB total. How much total BSC you need will depend on how close you put them to the walls. What you have now looks good to me but I'm at the limit of my active circuit expertise. Maybe Davey will notice this thread and look your circuit over. He has much more experience than I.
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Old 17th October 2005, 09:30 PM   #7
DonoMan is offline DonoMan  United States
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I already know 5dB is hot for my application when used as BSC only, but the Extremis is already rolling off from around the same frequency on its own, so I expect it to flatten it out slightly.
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Old 17th October 2005, 11:17 PM   #8
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Just for grins, try Carlos' advice on bypassing opamps. My Linkwitz based xover has a 33 uF electrolytic paralleled with a 0.1 uF film (took some hacking) on both rails for each opamp, and about 1000 uF bypassed with 10 uF mylar and 0.1 uF film on each rail at the power supply input. It sounds quite nice, thank you very much. OPA134s can also drive fairly low impedances, so I'd also try throwing a 2K resistor at the amp inputs just to load down the cable some.

If you're not running phono, you might not necessarily need 12 dB / octave subsonic, but depending on how low your woofers go you might want to use a 6 dB / octave switchable rolloff below about 50 Hz to compensate for room gain.


Cheers,
Francois.
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Old 17th October 2005, 11:38 PM   #9
DonoMan is offline DonoMan  United States
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Well, an Extremis is still a pretty small driver, considering that I'm running these full range... I thought a subsonic would be a good idea, especially since these will be used as my all purpose speakers, including for movies, and I currently don't have a sub, so they'll be doing all of the bass work for now...
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Old 17th October 2005, 11:46 PM   #10
DonoMan is offline DonoMan  United States
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More common component values...
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