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Old 29th June 2006, 01:30 PM   #1
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Default Filters...which one to use?

I need a HP filter that cuts off everything below 45 Hz with a 24 dB roll-off.

If you designed PCB's, which would you choose?

1. Two 2nd order Sallen-Key Butterworths cascaded

2. A 4th order Sallen-Key Butterworth

3. Passive RC HP filter

4. Some other filter I didn't mention - please explain
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Old 29th June 2006, 02:04 PM   #2
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Try designing with the least number of parts and see where that takes you.
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Old 29th June 2006, 02:51 PM   #3
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1 and 2 are the same in terms of layout. 3 is impossible to realise in practical terms.

I would go for 1 because I prefer the Linkwitz-Riley response to Butterworth, but it really depends what your goals and application are.
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Old 30th June 2006, 02:41 AM   #4
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Circuit I'm going to use:
Click the image to open in full size.
It's a (24 dB/octave) 4th order Linkwits-Riley filter with input and output buffers at a low cut-off of 45 Hz.

Do you have a suggestion for an op-amp. I'm most familiar with the LM741, however they usually require a split supply (+12 0 -12) to operate in this fasion. This is for my car so I don't have that option.

I do know it is possible to voltage divide my battery using resisters, but that would only give me (+6.5 0 -6.5) .

I just don't want my subs to go into excursion, If I get a signal below 45 Hz, that's what'll happen, at least that's what WinISD has calculated.

I've looked everywhere for an adjustable subsonic filter, but they only seem to come with certain amplifiers. Where can I get just the filter?
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Old 30th June 2006, 09:11 AM   #5
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I'm sorry but a 45Hz filter is going to sound quite bad. You are not building a sub you're building a 'not very good at bass' speaker. I would take a revisit of your speaker design.

TL07x series op-amps would work fine in such an application, you could make an active supply splitter, or easier still would be just to AC couple the whole thing.
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Old 30th June 2006, 01:49 PM   #6
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Perhaps you can get a better scope of what I'm trying to do. At the bottom of this link, http://spkrbox1.spaces.msn.com/ , is my 4th order band-pass bass box. My actual design has changed a bit and I haven't had a chance to update it. My new design calls for a 9.5" X 4" port. I used WinISD to aid in the design and it calculates that anything below 45 Hz (@ 200 WRMS), my sub will go into excursion.

Any suggestions would be most appreciated.
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Old 30th June 2006, 05:41 PM   #7
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I skimmed through the page in about 30 seconds. All I can say is that you need to change your box design, maybe trade some efficiency for bandwidth, or maybe use a different driver. I haven't looked up the parameters of the one your'e using but it might not be suited to bandpass or vented boxes.

That box will be a one note trick, and when you add a 45Hz filter it will make it even worse. Also there will be no real deep bass.
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Old 30th June 2006, 05:45 PM   #8
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id reccomend a different sub, save the hassle and gain low end response
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Old 30th June 2006, 06:42 PM   #9
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I was afraid of that...back to the drawing board.
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Old 30th June 2006, 08:42 PM   #10
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Post up the T/S specs of the driver and lets see if we can come up with a better design.
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