Cross over mod for B&W 802 nautilus - diyAudio
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Old 27th June 2011, 03:08 PM   #1
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Default Cross over mod for B&W 802 nautilus

Hi,

i am running 802 nautilus with krell fpb600, with krell krc-hr preamp and accustic arts dac2 mk4 dac. now the sound it very good and i want to add diy subwoofer at around 80hz but do not want to alter the above chain . so i need help regarding how to add lets say a good capacitor in series with the bass drivers of nautilus 802 to restrict output above 80 hz.

regards
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Old 27th June 2011, 07:53 PM   #2
DrDyna is offline DrDyna  United States
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The nautilus is tuned to 21hz already!

To answer the question, it appears that it is about 4 ohms, so to knock it off around 80hz you'd need about 500 mfd of series capacitance on it.

How much are you looking to spend on capacitors?
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Old 27th June 2011, 08:07 PM   #3
sreten is offline sreten  United Kingdom
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Hi,

You want to add a diy sub and restrict them to 80Hz ? Are you serious ?
No way IMHO would this ever improve anything, they don't need subs.

But if you really want monstrous low bass levels, decrease the input
coupling caps of the power amps for some bass roll off and run the
extra subs in parallel, both amps driven from the preamp.

Cannot see it sounding any better though, personally if bass is not
up to scratch I'd build some bass boost into the amplification chain.

rgds, sreten.

Series large electrolytics for vented boxes are a very bad idea.
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Last edited by sreten; 27th June 2011 at 08:09 PM.
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Old 27th June 2011, 08:48 PM   #4
DrDyna is offline DrDyna  United States
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I'm with Sreten, the Nautilus should already have solid response down to 21hz or so, according to it's tuning, and should be very easily able to do so driven with a Krell FPB 600. It's possible that having the Nautilus out in the room (out of the corners) causes a slight roll off. You might be able to leave them connected directly without caps and simply add subwoofers rolled off at whatever frequency the placement of the Nautilus is causing it to lose efficiency. You might have to experament, but you'll probably find that a couple of big subs playing from 20-ish up to about 40 hz or so might add just enough low end "ease" to be nice sounding.

I'd build a couple of good subs which you should be able to power with the same Krell without issue, slap a 16mh coil on them in series and give it a shot.

Maybe I'm crazy!

As far as output coupling caps, I'm not sure if I'd want to mess with a Krell! That's just me though.
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Old 8th July 2011, 08:58 PM   #5
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ok . lets rewind it. thnaks for the input though but 802 lacks the heart thumping bass below 40 and relieving the 802 below 80 would make mid bass clear.

Now the fpb is directly connected....NO signal coupling capaitor from input th output....and i dont want any active filter to distorld low level signal as well.....as far as just adding ablow 40 hz subwoofer would colour the bass from tow different drivers...imho
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Old 8th July 2011, 09:32 PM   #6
DrDyna is offline DrDyna  United States
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Quote:
Originally Posted by muddasirwaheedmalik2 View Post
802 lacks the heart thumping bass below 40 and relieving the 802 below 80 would make mid bass clear.
Well, if you have the means to try it, there's nothing wrong with building a pair of subs and using a simple crossover to give it a try. I would probably go a little lower than 80, though. It's up to you, a passive crossover is easy enough to design for subwoofers.
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Old 9th July 2011, 02:07 AM   #7
AllenB is offline AllenB  Australia
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As you move your speaker, the bass will change. I would add a sub without crossing the 802. The trick will be finding the right location for the sub. Level, phase, and location.
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Old 9th July 2011, 09:00 AM   #8
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Quote:
i dont want any active filter to distorld low level signal as well
Are you assuming that a passive crossover is transparent, while any kind of active crossover is going to distort and mess up the signal? Have you actually tested this idea?

It appears to me that your choices here mean that you give up the chance to improve things dramatically in order to avoid something at the limits of audibility.

Here's my suggestion. Put an active high pass on the mains at a point determined by where you stop getting "value" out of using the 802 bass. It probably will help smooth in-room response. Probably around 40 - 60 Hz. Then bring in the sub and again base the low pass point on where the response of the sub is useful. It can also smooth things out. I find that the end result of a good integration is far more significant than any imagined loss of fidelity with an active crossover.

A digital active also lets you shape the bass response, while a sub or a number of them also can give the firepower behind it.
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Old 9th July 2011, 09:14 AM   #9
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Quote:
Originally Posted by muddasirwaheedmalik2 View Post
ok . lets rewind it. thnaks for the input though but 802 lacks the heart thumping bass below 40 and relieving the 802 below 80 would make mid bass clear.
That "heart thumping bass" you refer to will actually be around 70-120Hz.
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Old 9th July 2011, 11:05 AM   #10
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I'd add a vote for active crossover or modification to the amp for active filtering.

Its difficult to get a smooth passive crossover for a vented box. The double impedance hump will give you messy response at the rolloff frequency. This won't help the blend to your subwoofer.

Sealed box highpass filters can be better and there is a class of 3rd order system that uses the series cap to flatten the response.

Go active.
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