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Old 30th December 2009, 02:16 PM   #1
DIFORCE is offline DIFORCE  Australia
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Default Mark K's er18dxt

Here Is the design , a basic 2-way monitor using the Seas ER18RNX paper woofer ( same as In zaph's SR71 ) and the Seas 27 DXT tweeter .

The Seas ER18DXT ported two way

It's a rather new design but whats special about it is the Xover network using a second order on the tweeter , makes him play really low ... but it works .

Thing is I was going to build zaph's SR71 until I came upon this , on a meeting of fellow DIY'ers mark brought these things and they were best in show , You can see the link there .

even on zaph's own blog he considers this to be an upgrade from the SR71 , L18 builds ...
costs slightly more then the SR71 ... but probably outperforms it in any way.

your thoughts ? im seriously wondering about building these now
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Old 30th December 2009, 03:59 PM   #2
Jonasz is offline Jonasz  Sweden
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I would go for the DXT design. To me it looks like a much more modern design and I think we will see a lot more waveguides/DXT-lenses in the future, they seem to make so much sense.
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Old 30th December 2009, 04:57 PM   #3
DIFORCE is offline DIFORCE  Australia
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That's exactly what I thought Ordered the parts ( exept the xover parts... ) from madisound. Ordered overkill 28mm wood ... I will keep updating on this one.

Im planning on mundorf xover parts but we shall see
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Old 30th December 2009, 05:11 PM   #4
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While i appreciate the performance of the DXT, i can't help wondering if the Zaph mounted 27TDFC performs even better? F3 is definetly lower for the TDFC. Considering costs, they're about the same when adding the MCM waveguide. Thoughts?
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Old 30th December 2009, 06:47 PM   #5
Jonasz is offline Jonasz  Sweden
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Well, the powerrespons of Marks design is just awesome and that just isn't possible with a regular tweeter. Besides, both Zaph and JonMarsh are recommending Marks design so for me the choice is simple.
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Old 30th December 2009, 09:04 PM   #6
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Quote:
Originally Posted by Jonasz View Post
Well, the powerrespons of Marks design is just awesome and that just isn't possible with a regular tweeter. Besides, both Zaph and JonMarsh are recommending Marks design so for me the choice is simple.
I was comparing the Waveguide mounted TDFC to the DXT. Here's the HD plots with a single cap and WG mounted. Naybe you thought i was comparing the SR71?

Click the image to open in full size.

Besides recommendations, the graphs pretty much tell the whole story. This configuration could cross well to a lot of 8" woofers as well.
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Old 31st December 2009, 12:56 AM   #7
Jonasz is offline Jonasz  Sweden
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Distortion isn't everything, look at the powerrespons instead...
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Old 31st December 2009, 04:07 AM   #8
ojg is offline ojg
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This looks like a very good combination and interesting choice in crossover.

Mark states that this is a second order acoustic crossover, but with the capacitor on the tweeter the tweeter slope would ultimately be third order, no? (The tweeter by itself has second order rolloff)
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Old 1st January 2010, 12:13 AM   #9
DIFORCE is offline DIFORCE  Australia
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Im abit worried about marks choosen damping. He only suggests stuffing it with acusta poly fill without some foam sheets on the walls .. Naked mdf interior , Wont that make the enclosure less dead and encourge resonance ?
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Old 1st January 2010, 10:43 PM   #10
ucla88 is offline ucla88  Tahiti
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Hi guys,

A couple of comments.

The distortion profile is very good. I suspect that the DXT motor is very similar/identical to the TDFC motor. No doubt a larger waveguide will have less distortion. However, the distortion of the system, ER18+DXT is excellent and below a certain level, arbitrary decreases in distortion are probably not audible. The idea was to keep distortion well within a certain standard of excellence while optimizing the power response as best as possible.

I'd love it if Seas put out a larger DXT-the diy MCM is an excellent choice, but a pain in the rear to cut, mount, etc. This is a great way to go if you're up to the challenge and want a larger box. Jon over at HTguide is working on exactly this. John's MCM TMM is also an option, but big!

As far as dampening-if you want to pad the walls, this is fine. I've never found this that effective, but it will not hurt. I think that stuffing the interior aggressively is more effective lowering internal resonances. You might have to skip the batting and just use the polyfill if the wall treatment is thick.

As far as the order-it's second order acoustic 2 octaves on either side of the xover point, though, as noted the tweeter probably does have a 3rd order asymptote.

Again, the main benefit of this two way, as opposed to any other well executed two way, is that it does have much a much smoother power response. This requires a waveguide and a lower order xover.

Mark K
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