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Old 9th November 2008, 09:34 PM   #1
MikeN is offline MikeN  United States
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Default subwoofer with no crossover

Hi guys. First time poster. I understand the kirchoff's law, and can wield a soldering iron fearlessly, but I am naive when I get past fixing a toaster.

My intention is to buy a couple of m-Audio AV 40 powered computer speakers. I want to add to them a standard powered subwoofer. From my reading, this means that ideally I need a line level crossover network to separate the frequencies at something like 80 Hz. However, PLLXO boards like www.marchandelec.com/xm46.html are overkill for me.

It occurred to me that I could just use the rolloff of the AV40's as a de facto high pass filter. Check the frequency response graph:

http://www.digit-life.com/articles2/...udio-av40.html

I could feed the line-out of my computer (or portable cd player, or mp3 player) into the line-in of the subwoofer, and also into the line-in of the AV40's. Then I set the crossover frequency of the subwoofer to 90 Hz. I do not use the subwoofer outs. The AV40's are down about 3dB at that frequency. Thus, assuming the rolloff of the AV40s is roughly like that of the subwoofer, the sum of the two lines should be roughly flat, right?

Does this sound plausible? Or am I completely bolloxed? Advice is much appreciated.

Mike
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Old 9th November 2008, 09:46 PM   #2
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Default Re: subwoofer with no crossover

Quote:
Originally posted by MikeN
a standard powered subwoofer.
Hi Mike,
A standard powered woofer has a plate amp that takes care of all that including a variable XO frequency. Do you mean a woofer powered by a standard kind of amplifier/receiver?
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Old 9th November 2008, 11:30 PM   #3
MikeN is offline MikeN  United States
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> A standard powered woofer has a plate amp that takes care
> of all that including a variable XO frequency. Do you mean a
> woofer powered by a standard kind of amplifier/receiver?

Cal, the powered subwoofer outputs at speaker level, so I can't feed that into my powered speakers. Otherwise that would have been perfect.

I looked in vain for a subwoofer that would output both line level and speaker level. I found that Daytons have a line-out, but that is unfiltered, and intended for daisy chaining more subwoofers. If you know of one that has a high pass line-out that would be great.

Thanks -- Mike
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Old 10th November 2008, 02:45 AM   #4
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From memory the O-Audio BASH plate amp uses a filtered line out and the price on those isn't exhorbitant

http://www.oaudio.com/500W_SUBAMP.html
500 watts may be too much for the application how-ever

EDIT
Another way may be to use a line level splitter sending one line to the sub and use the output control on the sub ( IE the dial XO) until it sounds right, low roll-off is around a 100 and most plate amps seem to have controls that go from 40-.. 120, just play with it until it sound good enough.
Both get the full signal but only use the part needed
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Old 10th November 2008, 03:39 AM   #5
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Default Re: subwoofer with no crossover

Quote:
Originally posted by MikeN
PLLXO boards like www.marchandelec.com/xm46.html are overkill for me.
http://t-linespeakers.org/tech/filters/passiveHLxo.html

dave
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Old 11th November 2008, 12:04 AM   #6
MikeN is offline MikeN  United States
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> From memory the O-Audio BASH plate amp uses a filtered
> line out and the price on those isn't exhorbitant
>
> http://www.oaudio.com/500W_SUBAMP.html

I see what you mean. From the manuals it seems likely that the line out is filtered. However, building a subwoofer is more than I was intending for this modest effort.

> Another way may be to use a line level splitter sending one
> line to the sub and use the output control on the sub ( IE the
> dial XO) until it sounds right, low roll-off is around a 100 and
> most plate amps seem to have controls that go from 40-.. 120,
> just play with it until it sound good enough.
> Both get the full signal but only use the part needed

That was what I was thinking, though I probably didn't express it properly. I was worried that this method might have unacceptably large bumps or dips near the XO point. The end result should be good, but is not intended to be audiophile quality. You have confirmed that this should sound ok -- thanks Moondog!
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Old 11th November 2008, 12:04 AM   #7
MikeN is offline MikeN  United States
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> http://t-linespeakers.org/tech/filters/passiveHLxo.html

Planet, thanks for the link. I had found this one while doing my homework before asking you guys for advice. I am capable of making such a filter if I have the values for the resistors, etc., but I am out of my depth when I have to choose the values to match the impedance of my amps.

Do you think that these FMod filters, made for autos I think, would be decent quality, and do exactly the same thing? I can't tell if they are line or speaker level, but perhaps that doesn't matter.

http://store.hlabs.com/pk4/store.pl?section=9

Mike
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Old 11th November 2008, 12:19 AM   #8
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You can do as you described, but I'd put a 50Hz high pass on them as a rumble filter, too, something like the FMOD would be fine.
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Old 12th November 2008, 07:16 PM   #9
MikeN is offline MikeN  United States
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> You can do as you described, but I'd put a 50Hz high pass on

Thanks Badman.

Cal, after checking more separately-sold plate amps I see what you mean about how they often have a filtered line-out. Doesn't seem to be the case for the fully built powered subwoofers though.

Ok, now I know enough to move forward. Thanks for your help, guys -- it's much appreciated.

Mike
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Old 2nd December 2008, 07:17 PM   #10
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Quote:
Originally posted by MikeN
Do you think that these FMod filter
An FMod is an encapsulated version of the PLLXOs i linked and makes assumptions about your amps input impedance.

dave
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