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Dallas II build - Phase shift?
Dallas II build - Phase shift?
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Old 12th July 2019, 11:15 AM   #1
WayneAudio is offline WayneAudio  United Kingdom
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Default Dallas II build - Phase shift?

Hi All - I'm a COMPLETE NOVICE - throwing myself in at the deep end by building the Dallas II's and will be using the 208ez (sigma) and T90a super tweeter.

For the T90a, I will be using an MDF 'tunnel' - purely for aesthetics (and to keep the wife happy!). This tunnel design for the tweeter is not open for debate / critique.

However, I would like to know from you guys if there is anything I should be genuinely worried about regarding phase alignment / shift? I found the attached image about phase-shift which was simple enough for even me to understand. In short - If you were me, would you look to set the super tweeter tunnel back a little so that it better aligns with the 208ez (albeit completely messing with mine and my Wife's OCD by not sitting flush with the baffle!)?

If having the T90a tunnel flush with 208ez is going to cause no-to-very little issue, then I'm keeping it as planned!!!

I've also attached some photos of pretty much where i'd got to with the Dallas II's so you can see what I'm aiming for here.

Many thanks!
Attached Images
File Type: gif TM_lobe_time_aligned.gif (7.2 KB, 171 views)
File Type: jpg -658174887_20181129_144547_2753928.jpg (771.2 KB, 170 views)
File Type: jpg -2082265988_20181129_144558_2550831.jpg (740.7 KB, 163 views)
File Type: jpg 142786561_20181129_144710_2487604.jpg (599.8 KB, 159 views)
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Old 12th July 2019, 12:56 PM   #2
Scottmoose is offline Scottmoose  United Kingdom
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Well, the acoustical centre of the voice coils (a somewhat nebulous concept) should be physically aligned if you don't want to do it electrically.

I'm not going to critique the design of your 'tunnel' for the T90a, other than say however that if you're going to stick it in that thing, in all honesty you'd be better off not bothering with it at all. We're not talking 'a bit of a compromise' here; we're talking affecting behaviour so badly that it will almost certainly sound worse than if you do without.
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Old 12th July 2019, 01:13 PM   #3
WayneAudio is offline WayneAudio  United Kingdom
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Quote:
Originally Posted by Scottmoose View Post
Well, the acoustical centre of the voice coils (a somewhat nebulous concept) should be physically aligned if you don't want to do it electrically.

I'm not going to critique the design of your 'tunnel' for the T90a, other than say however that if you're going to stick it in that thing, in all honesty you'd be better off not bothering with it at all. We're not talking 'a bit of a compromise' here; we're talking affecting behaviour so badly that it will almost certainly sound worse than if you do without.
Appreciate you going easy on me! Why would the tweeter sound so bad in it?I thought it was better than the 2 'dowels' you get in the box which it would just sit on.
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File Type: jpg 1722384495_20181124_210446_2275495.jpg (623.9 KB, 149 views)

Last edited by WayneAudio; 12th July 2019 at 01:17 PM.
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Old 12th July 2019, 01:32 PM   #4
Rev. Billy Bob is offline Rev. Billy Bob  United States
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That's a huge vertical center to center distance for a supertweeter.

You might be able to dial it in for one tiny sweet spot, but expect a lot of lobing if you move around at all.

If it were me, I'd think about rear firing the super tweeter or bouncing it off the ceiling.

If you really want to front fire the super tweeter, I'd think about mounting a tube in the unused space in front of and above the diagonal piece of the cabinet and putting the super tweeter in that.

You want the drivers as close as possible, and it becomes more critical the higher the crossover frequency.

You also might want to consider experimenting with a very high order crossover.
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Old 12th July 2019, 01:33 PM   #5
Scottmoose is offline Scottmoose  United Kingdom
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Ah, well: depends. I was assuming the dome was buried deep within a tunnel along which it would radiate. If it's in effect simply mounted at the front of a long cylinder which is being used to aesthetically conceal it, that's a different story. In the former case you'd have horrendous standing waves / reflections (just try talking along a tube) so this should be avoided like the plague for HF use. In the latter case, shouldn't be a big deal. Circles aren't great for diffraction, but the T90a is horn loaded & thus relatively directional, so shouldn't be a big deal.

As noted, there's quite a distance between the drivers which isn't ideal, but you should get away with it since it's mostly just providing some assistance with harmonics, even if it doesn't look ideal on a graph.
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Last edited by Scottmoose; 12th July 2019 at 01:36 PM.
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Old 12th July 2019, 01:34 PM   #6
WayneAudio is offline WayneAudio  United Kingdom
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Quote:
Originally Posted by Scottmoose View Post
Ah, well: depends. I was assuming the dome was buried deep within a tunnel along which it would radiate. If it's in effect simply mounted at the front of a long cylinder which is being used to aesthetically conceal it, that's a different story. In the former case you'd have horrendous standing waves / reflections (just try talking along a tube) so this should be avoided like the plague for HF use. In the latter case, shouldn't be a big deal. Circles aren't great for diffraction, but the T90a is horn loaded & thus relatively directional, so shouldn't be a big deal.
Yes, it's definitely the latter ! you're a star - thanks for your thoughts.
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Old 12th July 2019, 02:34 PM   #7
xrk971 is online now xrk971  United States
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Dallas II build - Phase shift?
What you want is a smooth waveguide expanding out while giving the tweeter the necessary setback. If you put any sharp hard edge like a (1mm even) lip, a sharp edged ridge, not to mention a tunnel (tube), it will adversely affect the sound. Carefully designed tweeter reflectors or diffraction devices (like ones used above some tweeters from SAW) can work but they are modeled with FEA to provide the required sound.
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Old 12th July 2019, 05:02 PM   #8
planet10 is offline planet10  Canada
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Dallas II build - Phase shift?
Quote:
That's a huge vertical center to center distance for a supertweeter.

You might be able to dial it in for one tiny sweet spot, but expect a lot of lobing if you move around at all.
The tweeter needs to be as pysically close to the FE208 as possible. Even then the seperation will be too great to get any phase alignment but for one spot. Fortuneatly up where these will be crossed it is not overly critical.

The void above the woofer is an ideal place to put up.

The mounting cylinder, if you don’t do the right thing and abandon it, needs to have a heavy chafer/bevel on the front edges.

dave
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Old 12th July 2019, 05:03 PM   #9
planet10 is offline planet10  Canada
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Dallas II build - Phase shift?
Quote:
What you want is a smooth waveguide expanding out while giving the tweeter the necessary setback.
Built into the tweeter already.

dave
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Old 19th July 2019, 09:35 AM   #10
WayneAudio is offline WayneAudio  United Kingdom
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Quote:
Originally Posted by planet10 View Post

The void above the woofer is an ideal place to put up.

The mounting cylinder, if you donít do the right thing and abandon it, needs to have a heavy chafer/bevel on the front edges.

dave
Hi - Thanks for this. Which of the 4 options attached for the T90a (super tweeter) placement, would you expect would produce the best results? I have also embedded a link to a YouTube video which I found really helpful - which supports your views.

I welcome mothers' thoughts too. Thanks all.

YouTube
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File Type: jpg Slide1.jpg (73.9 KB, 46 views)

Last edited by WayneAudio; 19th July 2019 at 09:44 AM.
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