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Old 5th October 2004, 07:21 AM   #131
Mark25 is offline Mark25  United Kingdom
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Quote:
Originally posted by JensRasmussen
Hello,......................
Changing the Q to achieve a “bump” in a HP filter in a SUB is one way of making the subwoofer player lower.
.............................
\Jens
Thanks for the reply Jens.

I had never considered the above! I'm guessing that the higher ranges of Q (ie above 1) will be of limited use to those staying away from a drivers natural roll-offs, like me.

Interestingly, i currently use an LR aligned active Xover, so there should be some interesting comparisions when i get the MOX built.

Thanks again.
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Old 20th November 2004, 09:07 AM   #132
Cobra2 is offline Cobra2  Norway
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Default ...many hours later...

- and still some to go...

Arne K
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Old 20th November 2004, 12:08 PM   #133
Bricolo is offline Bricolo  France
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Hi MOX users!

I'm not using the MOX, but I'm working on an active flter too.
I wanted to know: how's your grounding shceme in your active XO? Since there's multiple boards, each one having an output+gnd, input+gnd and power supply+gnd connector, can't this create some ground loops?

Thanks
Alex
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Old 20th November 2004, 03:18 PM   #134
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Hello Arne,

Nice one, how about some close ups?

\Jens
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Old 20th November 2004, 08:37 PM   #135
Cobra2 is offline Cobra2  Norway
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Default By request...

Quote:
Originally posted by JensRasmussen
Hello Arne,

Nice one, how about some close ups?

\Jens

Arne K
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Old 3rd December 2004, 07:31 AM   #136
MBK is offline MBK  Singapore
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Default HF rolloff with 100x settings?

Hi all,

I finally got to test my 8 boards. All filter sections are completed, inputs and buffers still to customize.

The results are very good and consistent for the 1x and 10x frequencies.

But at the 100x settings (the 1 nF cap) I have a consistent HF rolloff for the HP section, ca. -1 dB at 20k. The rolloff starts well below 20k, so that the crossover frequency shifts significantly and the HP and LP lines don't cross at -3 dB anymore.

Any ideas as to the cause? I eliminated the following:

- op amps: tested OPA 2134, TL072, NE5532
- input: buffered or unbuffered
- all 8 boards react the same
- soundcard has reference (its other channel)
- input buffer alone, or any HP settings with the 100 nF and 10 nF caps (1x and 10x) are perfect at 0 dB at 20 kHz - so it couldn't be in/out problems anyway.

Whay should the smallest cap cause a HF rolloff in a HP filter?? I can only think of lead inductance or parasitic capacitace to ground.

Pics attached, self explaining I hope. Note especially where the 2000 Hz HP, using 200 Hz x10, has 0 dB at 20 kHz, while the 2500 Hz HP, using 25 Hz at 100x, has -1 dB at 20 kHz. The 25 Hz at x1 or at x10 is perfect at 0 dB at 20 kHz.

MBK
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Old 3rd December 2004, 07:33 AM   #137
MBK is offline MBK  Singapore
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Default next pic

series of HP frequencies

MBK
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Old 3rd December 2004, 01:14 PM   #138
tiroth is offline tiroth  United States
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Well, now, this is interesting. Ages ago I posted a similar pic showing the same odd behavior and was really scratching my head over it. To me, it did not look like rolloff--Fc was dead on, and the slope looked to be zero outside the crossover. It just looked like it was -1dB down.

Looking at your post, I also do not think Fc has been shifted when the level is corrected for.

http://www.diyaudio.com/forums/showt...881#post383881

But Jens was unable to duplicate my results! What could be different about his setup and ours?
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Old 3rd December 2004, 01:26 PM   #139
MBK is offline MBK  Singapore
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Tiroth,

You're right, FC shifts only insofar as now the curves intersect only at higher F - in fact it is due to a loss of gain in the HP. It doesn't shift w/ frequency after it reaches its maximum, so it is likely not stray inductance/capacitance as I speculated. Could be that the formulas for Sallen key make some hidden assumptions which become invalid for certain ratios of caps and resistors... I didn't know/remember that you had the same results...

In retrospect it would have been better (for me) to cut all R values by a factor of, say, 5 (better also for low noise and for op amp balance regarding R on negative vs positive inputs...) and to multiply the caps by 5 (I don't need the 0.1 x setting anyway which would then require a very large cap). Hmm, or I can use an intermediate cap at the 0.1x position. But that would make for odd frequencies.

I guess now the only quick hack is to raise the Q for the HP to compensate. Or to live with it

MBK
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Old 3rd December 2004, 01:29 PM   #140
tiroth is offline tiroth  United States
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Well, if it is only a difference in level, just increase the gain by 1dB...unless you have drivers with identical sensitivity the interchannel gain won't be the same regardless.
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