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Old 10th March 2005, 09:26 AM   #1
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I currently have my sub crossed at 100Hz, and I have a linkwitz transform. I feel that it is time to create proper preamp with high pass filters. Basically, is my current circuit ok, or should I add some unity gain buffers anywhere? I calculated the values for the filters using the SLOA093 document from Texas Instroments. I have built each filter seperately on breadboards, and they work ok. I just dont know if I can stick them together like I have.
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Old 10th March 2005, 10:01 AM   #2
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You can have a look here and compare that with the TI papers.
The sheets includes everything like subsonic filter, input buffer, linkwitz transfrom and variable gain.
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Old 10th March 2005, 10:59 PM   #3
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Thanks for the link, but ive already had a play with the Linkwitz Tranzform Program, and other spreadsheets.
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Old 11th March 2005, 05:59 AM   #4
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all filters show are 2nd order, likely making them 12dB/oct unless they are signifigantly overdamped which i'm betting against given the closeness of values.

note that the source impedance is not shown. you may acheive better results adding a buffer on the inputs.

have not checked the values but it is likely the rumble filter and linkwitz transform filter will be near the same frequencies.
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Old 11th March 2005, 09:02 PM   #5
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Yes, all your filters are indeed 2nd order, 12dB/octave.

You need a buffer at the very end to prevent the pot from getting loaded down.
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Old 13th March 2005, 07:03 PM   #6
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So adding buffers at each end is the only change I need to make?

Are the values of the components ok?

[edit]: Sorry if ive confused anyone with the orders of my filters. I thought 2nd order was 6db/oct.
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Old 13th March 2005, 07:20 PM   #7
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Yeah, a buffer at the front end would be a good idea because there are a few stages to drive and you can never be sure what the source impedance is. A buffer on the low out mono is also needed as mentioned for the pot.

I would also add a resistor of 22-100 ohms at the output of each op-amp that will drive a cable to swamp against capable capacitance possibly sending the op-amp into instability.

You might also like to simply parallel two 0.1uF for your sub low-pass as well instead of using a 0.22uF, but this is being a little picky and I guess you are short on space. I'd also look into cascading two of those filters to cut off more sharply.
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Old 13th March 2005, 09:45 PM   #8
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Quote:
Originally posted by Evan Shultz
You need a buffer at the very end to prevent the pot from getting loaded down.
Why not place the pot before the rumble filter instead?
No additional buffer needed.
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Old 14th March 2005, 07:05 AM   #9
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I may disagree. Depending on the value of the pot, the filter will be affected.
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Old 15th March 2005, 09:09 AM   #10
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Quote:
Originally posted by richie00boy
I would also add a resistor of 22-100 ohms at the output of each op-amp that will drive a cable to swamp against capable capacitance possibly sending the op-amp into instability.
The buffers, or all the opamps?

Quote:
You might also like to simply parallel two 0.1uF for your sub low-pass as well instead of using a 0.22uF, but this is being a little picky and I guess you are short on space. I'd also look into cascading two of those filters to cut off more sharply.
Which filters? The lowpass?
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