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Old 23rd December 2008, 08:42 PM   #1
scottw is offline scottw  United States
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Default Visions of PLLXO's danced in his head....

Familiar story, open baffle with 8 inch wide range, passing the lows (< 200Hz but yet to be determined) off to a 15 inch prosound driver. Just became bi-amp capable and going to try a 2nd order PLLXO for low pass.

Have found the T-line site helpful:

http://www.t-linespeakers.org/tech/f...ssiveHLxo.html

but little else available on line or in forums. And I still have some questions:

Is a line level 2nd order (R's and C's, no L's) still going to cause a 180 degree phase change? I probably should know this.

2nd order PLLXO is a cascade of two 1st order while maintaining an impedance ratio (1:10) of the components (more or less with exceptions). What is the result if you reduce this ratio to 1:5 or 1:2?
Does it effect the slope of the xo or does it effect the frequency extremes-i.e., roll off the highs or lows?

Thanks,

Scott
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Old 23rd December 2008, 09:05 PM   #2
Ron E is offline Ron E  United States
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Default Re: Visions of PLLXO's danced in his head....

Quote:
Originally posted by scottw
Is a line level 2nd order (R's and C's, no L's) still going to cause a 180 degree phase change?

What is the result if you reduce this ratio to 1:5 or 1:2?
Yes.

Model it with LTSpice and see the answers to both questions.
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Old 23rd December 2008, 10:18 PM   #3
scottw is offline scottw  United States
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I'm afraid the only "spice" I use is in the kitchen.

But I guess it's getting to the point where I need to download it and learn to use it.


Scott
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Old 24th December 2008, 12:24 AM   #4
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If you reduce the ratio, the 2nd section loads down the 1st section, so the XO starts shifting around. If you diyed your amps or are willing to go into them, inserting 2 1st order filters will get you more consistency.

The passive filters create the same phase rotation as any analog filter would.

dave
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Old 24th December 2008, 01:38 AM   #5
Pano is offline Pano  United States
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Hey Scott. Ditto what was said above.
Plus as you change either source impedance or load impedance things will shift a bit. As they get closer to each other the overall attenuation will rise.

Do you know what your source impendance and load impedance are? Or can you give us an idea of what you are using?
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Old 24th December 2008, 07:57 AM   #6
scottw is offline scottw  United States
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Thanks for the replies.

Quote:
If you diyed your amps or are willing to go into them, inserting 2 1st order filters will get you more consistency.
Hey Dave, do you mean put one 1st order in front of the amp and another in between stages in the amp? Might be tough to do cause I plan to try a number of xo points till I find what works. And this may be quasi-temporary.


Quote:
Plus as you change either source impedance or load impedance things will shift a bit....Do you know what your source impendance and load impedance are? Or can you give us an idea of what you are using?
Hmmm...Source is a TVC (S&B Mkiii) and the output impedance does vary; but in the range I listen it will usually be from a few Ohms to 100 Ohms (measured at 1kHz).

The amp for the lows is an integrated sand amp with around 50K input impedance-actually DCR stays at 55k till the volume gets to 75% then it tapers to 40K at full volume. It drives the 15's now with a speaker level 2nd order with this amps volume control at about 12 o'clock. So I've got room for a little insertion loss.

My original thinking is if I could cheat on the 1:10 ratio and increase the impedance it may help prevent loading the TVC too much. But it may not matter that much, I may just try it as recommended and see how it does.


Scott
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Old 24th December 2008, 04:38 PM   #7
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Quote:
Originally posted by scottw
, do you mean put one 1st order in front of the amp and another in between stages in the amp?
exactly. You won't get the droop of 2 cascaded passive RC.


Quote:
Hmmm...Source is a TVC (S&B Mkiii) and the output impedance does vary; but in the range I listen it will usually be from a few Ohms to 100 Ohms (measured at 1kHz).
Give it a try. If the highs go away you don't have enuff grunt.

Quote:
The amp for the lows
It is only the HF amp that usually has a problem.

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Old 24th December 2008, 05:03 PM   #8
Pano is offline Pano  United States
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Yeah, that TVC gives you a nice low output impedance.

You shuld have no troubles with that setup. I thought you might be driving it out of a passive pot preamp or something.

Dave, what do you think is a good value for his R1 in the PLLXO?
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Old 24th December 2008, 11:42 PM   #9
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Quote:
Originally posted by panomaniac what do you think is a good value for his R1 in the PLLXO?
The filter would be happiest if he does 0.5 uF + 5k

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Old 25th December 2008, 06:55 AM   #10
scottw is offline scottw  United States
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Thanks guys.

I'm diggin' through my spare parts bin to see what caps I can match up for the job.


Scott
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