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Old 3rd August 2005, 12:58 PM   #1
Geez is offline Geez  Hungary
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Default Is it possible: passive subsonic filter?

Hi!

I was wondering if someone has already built a subsonic filter to prevent excessive cone motion.

Using BassBox 6 at the External tab in the Driver Properties window I could experience with capacitor and inductance values but it looks like I would need 200-1000 microFarad and 20-60 mH (3-8 Ohm DCR) values which are very expensive.

As long as I read on other forums that they sell Behringer 2×3 way active crossovers for $200 I'm not shure the passive is the right choice.

Daniel
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Old 3rd August 2005, 02:31 PM   #2
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You would not want to make a passive subsonic filter for the reasons you have already pointed out. The components required would be too large and expensive.

I have always used a steep active subsonic filter.
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Old 3rd August 2005, 02:38 PM   #3
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Not to mention that it's much easier to build a steeper active filter and you do not suffer from wavering frequency response nor lack of damping at the cutoff frequency.

You can even build a nice neat active subsonic filter complete for under $50 all in, and that's being generous.
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Old 3rd August 2005, 04:35 PM   #4
simon5 is offline simon5  Canada
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For the coil, you could use a big iron core, or wiring your inductor on a big transformer core.

The DCR will be near 0.

As others said, probably better to go active hehe!
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Old 4th August 2005, 11:20 AM   #5
Geez is offline Geez  Hungary
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Thanks for the reply's.

In Thiele's calssic paper he writes about the passive filter assisted bassreflex alignments. If a passive low frequency high pass filter is so expensive then these aligments: B5, B6, C6 are not good choices I can conclude.

Daniel
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Old 4th August 2005, 11:35 AM   #6
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I see no reason why one shouldn't attempt a passive subsonic filter !
Just don't put it at the amp's output !!!!!

Regards

Charles
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Old 4th August 2005, 11:47 AM   #7
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Thiele's paper was written in the 60's before op-amps were readily available. Electro-Voice and others came out with electronic "bass boost" circuits (~B6 alignments) in the 70's. Some of these circuits may be interesting for you.
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Old 9th August 2005, 05:50 AM   #8
rcw is offline rcw  Australia
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Default re passive filters

If you are making your own power amplifier you can get a second order high pass filter by choosing the value of the input coupling capacitor, and the capacitor that shunts the feedback leg to ground. This has a fixed Qt=.5, and is suitable as a part of the SC6,C6, QB5 class III alignments.
If only a single capacitor can be used then this can be part of a B5 alignment, in practice a back to back pair of electrolytics that has a nominal total capacity of around twice the one called for in theory will give a result that only varies by a few db. from the ideal if put in series with the driver, this can also be used as a second order filter if you choose an input coupling capacitor that is equal to the theoretical value.
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