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Old 13th August 2011, 11:35 PM   #1
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Default Vifa DQ25SC

Does anybody know if the plastic phase shield on the DQ25SC tweeter is actually useful or not? Is it just a physical protection device? I'd like to know if I can remove the entire face plate without affecting functionality.

I popped off the heatsink on the back and I plan to turn down the faceplate on a lathe to reduce the dipole offset when mounted back-to-back.

It looks like this tweeter, used in back-to-back pairing, could be a valid (or superior) alternative to the Neo3 for a dipole tweeter.

Click the image to open in full size.

If you want to try this yourself (these tweeters are not that expensive), do this:

1) Unscrew the Philips screw located in the center of the heatsink
2) Make sure the Philips screw is fully loose - note that it is not likely to fall out on its own, nor does it need to
3) Using needle-nose locking pliers (Vice-Grips), grab a fin and twist it like a clock hand, keeping the face plate stationary. The fin will bend but the heatsink should snap off.

For those concerned about heat dissipation because the heatsink has been removed - a thin piece of aluminum bar can be inserted between the motors of the tweeters to pull heat away much more effectively than the heatsinks that come stock.

Last edited by 454Casull; 13th August 2011 at 11:43 PM.
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Old 14th August 2011, 02:09 AM   #2
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Why did you call it a "phase shield"?

If you take it off you'll have to punch a small central hole in the diaphragm to compensate. Remember how some early dome tweeters had a hole?
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Old 14th August 2011, 03:31 AM   #3
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I think I read in some literature that it was called a phase shield. These tweeters were introduced a long time ago, though.

No, I don't remember about the early dome tweeters.
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Old 14th August 2011, 05:37 AM   #4
winslow is offline winslow  United States
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Someone was selling DQ25s on ebay a few months back minus the phase plug/shield.

I wad expect to see some droop in the top end if you removed it.
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Old 16th August 2011, 04:22 AM   #5
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Some progress here:

https://picasaweb.google.com/1044317...eat=directlink

The overall depth of the tweeter is ~16 mm. The outer diameter of the tweeter is 43 mm. The distance from the center of the tweeter to the face edge is 21.5 mm. The distance from the OD of the modified tweeter to the voice coil (outside of the dome) is 8 mm. The dipole spacing should then be 16+21.5 = 37.5 mm not including half the thickness of a aluminum bar for mounting and heat sinking (say, another 2 mm). This leads to a dipole peak of approximately 4400 Hz.

Compare this to the Neo3, which is 61 mm x 89 mm (applicable raw dimensions), averaged to 75 mm, gives a dipole peak of approximately 2300 Hz. So, we're looking at true dipole behaviour for almost an octave higher, with better power handling, higher maximum SPL (probably), and higher sensitivity, for half as much for the pair. Also, you can mount this unit slightly closer to your mid, to reduce vertical lobing, than you can the Neo3. Winner?

I am concerned with diffraction off the proximal circular edge, which is why I added the relatively generous 45 degree chamfer, but most likely the best way to go is to wrap the unit with some open-cell foam or felt.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Honestly, I don't think you can find a smaller good tweeter on the market right now... this tweeter, for $13.25 a pop at Madisound, is a serious beast.

Last edited by 454Casull; 16th August 2011 at 04:47 AM.
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Old 16th August 2011, 04:55 AM   #6
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Quote:
Originally Posted by 454Casull View Post
Does anybody know if the plastic phase shield on the DQ25SC tweeter is actually useful or not? Is it just a physical protection device? I'd like to know if I can remove the entire face plate without affecting functionality.
With rigid dome tweeter, cancellation at about 15khz-20khz would occur due to the phase induced distance between center and edge (metal cones are pistonic). The phase shield would delay the center radiation much like phase plug in waveguide.

At the cost of stored energy, of course.

Some investigation proved that WITHOUT phase shield is better

Vifa DQ25 Phase Shield removal surgery ...!
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Old 16th August 2011, 05:16 AM   #7
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Interesting. I don't know where you draw the conclusion that the off-axis response is superior without the shield than with; the responses look very similar... unless you are referring to the elimination of the peak in the top octave in the off-axis response.

How would you recommend removing the shield? I couldn't figure out how to remove the face plate without cutting into the glue, and that I did not want to do.

Last edited by 454Casull; 16th August 2011 at 05:20 AM.
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Old 16th August 2011, 06:40 AM   #8
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Quote:
Originally Posted by 454Casull View Post
Interesting. I don't know where you draw the conclusion that the off-axis response is superior without the shield than with; the responses look very similar... unless you are referring to the elimination of the peak in the top octave in the off-axis response.
Yes, the nasty peak.

Quote:
How would you recommend removing the shield? I couldn't figure out how to remove the face plate without cutting into the glue, and that I did not want to do.
With great care . I managed to dent one of the dome. They are fragile. The plastic is very hard so perhaps a hot knife is better proposition than what I used (pliers).
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Old 16th August 2011, 06:41 AM   #9
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So, keeping in line with the low cost of the tweeters, I think these would make a good match from 1800-2000Hz and below:

Aura NS6-255-8A 6" Paper Cone Neodymium Magnet Woofer

High sensitivity and a relatively small rear footprint. Unfortunately I cannot find any data on the distortion figures but the NRT motor is fairly reputable. Also, the other drawbacks, like the stamped frame, can easily be fixed by 1) magnet mounting, and 2) damping.

If only the paper gasket on the flange weren't so damn ugly...

Last edited by 454Casull; 16th August 2011 at 06:44 AM.
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Old 16th August 2011, 06:42 AM   #10
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Originally Posted by gainphile View Post
Yes, the nasty peak.



With great care . I managed to dent one of the dome. They are fragile. The plastic is very hard so perhaps a hot knife is better proposition than what I used (pliers).
Huh, pliers.

I was thinking of using a Dremel (rotary cutter) to do mine.
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