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Old 31st January 2005, 06:46 PM   #1
bwbass is offline bwbass  United States
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Default Need help with PLLXO High Pass filter for full range's!

I've built a couple of 7l vented boxes for TangBand W4-1052sa's for use as full range computer/small HT speakers, and I'm powering them exclusively with a Sonic Impact T-amp.

I used a SBB4 alignment, which gives me good power handling and extension down to about 55hz, but below that the t-amp makes those litte 4" cones go nuts trying to play low bass. I simulated different filters in WinISD, and settled on a 2nd order high-pass filter at 65hz as the best balance of power handling and extension.

Problem is, the components for a high-level filter at that frequency (and at 4 ohms) would be:

A. Huge
B. Almost as expensive at the rest of the project
C. Detrimental to efficiency/distortion (?)

Then I read Dave (planet10)'s primer on PLLXO's: http://www.t-linespeakers.org/tech/f...ssiveHLxo.html

Seems like this would be the perfect solution and fit with the bargain-basemet hi-fi aesthetic of the whole thing. Problem is, I've never designed a filter before, and the PLLXO stuff is particularly confusing to me. I know I need to take the input impedance of the T-amp (10k ohms, AFAIK) into consideration, but I can't see exactly how this value fits into the equations for the 2nd order filter....

Also, will there be a problem since the two channels share a common ground at input on the 1/8" stereo jack of the T-amp?
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Old 31st January 2005, 07:19 PM   #2
bwbass is offline bwbass  United States
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... and is the choice of R1 purely arbitrary, or is there a reason for choosing 5k vs 10k, for example...?
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Old 31st January 2005, 09:17 PM   #3
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Quote:
Originally posted by bwbass
... and is the choice of R1 purely arbitrary, or is there a reason for choosing 5k vs 10k, for example...?

R1 wants to be about 1/10th as big as R2. The smaller R1 is the bigger the cap needs to be. You also don't want to load down the previous stage.

So if we want to maintain a 10k input impedance, than set R1 to 10k, and R2 to 100k. R2 will actually be in pararallel with the SI's input impedance so actual R2' needs to be about 111k.

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Old 31st January 2005, 09:21 PM   #4
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Default Re: Need help with PLLXO High Pass filter for full range's!

Quote:
Originally posted by bwbass
settled on a 2nd order high-pass filter at 65hz as the best balance of power handling and extension.
Keep in mind that a 2nd order PLLXO is going to be droopy (ie low Q) -- do you have the ability to adjust the filter Q in your modeling SW?

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Old 31st January 2005, 09:38 PM   #5
bwbass is offline bwbass  United States
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I think so... I'll have to check when I get home tonight. What Q value do you think would model this filter?

And just to clarify, by "low Q", do you mean that the corner will be less sharp and the transision to a true 12db/oct will take longer (happen at a lower frequency)?
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Old 31st January 2005, 09:41 PM   #6
bwbass is offline bwbass  United States
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Thanks for the clarification on the filter formulas, by the way!

Using 10k for R1 and 111k for R2, I get a C1 of 245 nf and a C2 of 24.5 nf. Does that sound right?
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Old 31st January 2005, 10:11 PM   #7
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Quote:
Originally posted by bwbass
Using 10k for R1 and 111k for R2, I get a C1 of 245 nf and a C2 of 24.5 nf. Does that sound right?
doing this off the top of my head (i really should dig out a ref to make sure i don't miss something)

10x10^3 x 24.5 x10^-9 = 2450 x 10^-6 = 2.45 x 10^-3 = 2.45 mS

1/2.45x10^-3 = 408 divide that by 2pi = 65 Hz

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Old 31st January 2005, 10:13 PM   #8
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Quote:
Originally posted by bwbass
I think so... I'll have to check when I get home tonight. What Q value do you think would model this filter?
~0.5, depends on the ratios of r1/r2

Quote:
And just to clarify, by "low Q", do you mean that the corner will be less sharp and the transision to a true 12db/oct will take longer (happen at a lower frequency)?
sort off.... it will also start attenuating earlier.

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Old 31st January 2005, 10:31 PM   #9
bwbass is offline bwbass  United States
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So the f3 will stay at 65hz in this case, it's just that the slope of the filter curve at this point would vary somewhat...

lower Q = Lower slope at f3 = higher start of the rolloff
higher Q = Higher slope at f3 = lower start of the rolloff

...right?

If this is the case, it should work out fine for my design... I'll model a .5 Q filter later tonight and see how it looks.
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Old 31st January 2005, 11:23 PM   #10
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not a perfect illustration, but blue is lower Q... ultimately thet should have the same roll-off

dave
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