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Old 18th January 2010, 06:48 PM   #1
Tyson is offline Tyson  United States
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Default Drivers rolling off too soon - why?

Question for everyone - I am using a DCX 2496 crossover with a 3 way speaker. I find that when measuring the drivers independently, they start to roll off before the actual settings I have on my crossover.

For example, I want my tweeter (an SB Acoustics SB29) and woofer (a Peerlesss Nomex 830875) to have 2khz as their crossover point. If I set the woofers low pass to 2khz, it actually dies completely at 1.5khz. It's only if I set the low pass to 3khz that I get the proper roll-off slope at 2khz (gentle slope, about 6 db down at 2khz).

Same thing happens with my tweeter - If I set the high pass for it to 2khz, then it just dies at 2.5khz. If I set the high pass to 1.7khz, then it's a gentle rolloff just before 2khz (down 6db at 2khz).

This is consistent with my full-range measurements as well. If I set the woofer and tweeter to cross over at 2khz, then there is a deep notch in response between 1.5 and 2.5khz. Inverting the tweeter has no affect on the notch. However, if I move the woofer to 3khz, and the tweeter to 1.7khz, then I get a smooth response through the 2khz region.

Anyone know why this is happening, and if it's normal?
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Old 18th January 2010, 07:53 PM   #2
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Maybe you have a defective DCX ?

(that they are defective is quite normal... sadly)
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Old 18th January 2010, 08:04 PM   #3
Tyson is offline Tyson  United States
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No, I'm certain it's not defective, I had it modded by a very good modder and electrical technician. I also took it in for a checkup about 6 months ago and it's functioning perfectly.
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Old 18th January 2010, 08:09 PM   #4
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You say that its not working like it should, yet it is not defective? Well - try to compare it to a new/stock unit to see if the XO is working like it should.

Which filter type and slope are you using?

Try to measure the direct output from the DCX to see if that looks right.
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Old 18th January 2010, 08:13 PM   #5
Tyson is offline Tyson  United States
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I use either 4th order LR or 8th order LR.

Don't have access to a stock unit, unfortunately ....

My thinking was that the drivers already have an acoustic rolloff, and that the electronic crossover was combining with this acoustic rolloff to give a different shape and point for the rolloff I measure. I know this is an issue with passive crossovers, but I'm not sure how much that is also true for electronic crossovers....
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Old 18th January 2010, 08:17 PM   #6
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Ok, you should have -6 dB at the XO point. Measure the direct output from the DCX and you'll se if its right!

Yes - acoustic rolloff from the drivers will add up, so you might need to overlap the XO frequencies.
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Old 18th January 2010, 08:19 PM   #7
Tyson is offline Tyson  United States
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Ah, your answer is too obvious! Thanks, I'm not sure why I didn't think of that
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Old 18th January 2010, 08:20 PM   #8
tinitus is offline tinitus  Europe
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Its basic
Its why some passive xo "believers" have disbelief in active xo
Its due to drivers own natural rolloff
Its taken care of in a passive xo, and usually not in active xo, or at least not in the theoretical ones
Some do yes, but rarely

Now, your digital xo is very different
It should make it possible to shape the electrical xo function to suit the desired correct acoustical slope
So, theres nothing wrong
You are on to it

The evil dark side of this digital xo thing is that it needs measurements as much as a passive one
And you need to custom shape the slopes, factory fixed slopes wont work well

Whether it should be -3db or -6db at xo point depends on actual slope, whether its one or the other topology
LinkwitzRiley is -6db, and Butterworth is -3db, if I remember right
Which is which is determined also by shape of slopes
Its passive xo theory, but I think it still counts

I want a digital xo too, to paly with
But to be honest it scares me a bit
I mean, with passive component I know what to expect, well almost
But with a diggital I have no idea about whats going on
Funny ehh
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Old 18th January 2010, 08:24 PM   #9
Tyson is offline Tyson  United States
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That's what I'm learning. When I started, I just assumed I'd pick a xover point more or less at random, dial in a steep slope, and have everything work out fine. But the problem is that what is on the settings is not the same thing that comes out of the drivers. Finding this site (and the Holm Impulse software) has been a god-send and allowed me to make good measurements with my calibrated mic and Mobile Pre USB mic-amp. I should have done that in the first place, it would have saved me a lot of frustration....
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Old 18th January 2010, 08:25 PM   #10
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The reason I mentioned a possible problem with the DCX is that I've seen that the XO points and EQ's didnt work as the should, there were just plain wrong. That was a software error in the DCX.
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