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Old 14th May 2017, 02:55 PM   #1
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Join Date: Sep 2012
Default Great Balls of Prestige

LATEST UPDATE: My complex sphere build. Almost finished!

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So I have had a pair of Wilmslow Audio Prestige speakers for the last few years.
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I heavily modified the stock cabinets, but have always planned to make new ones as they were not to my design and I don't think they are that good, even after my modifications.

I have spent a long time thinking about their replacements.

Having built these:

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I realise the positives of spherical enclosures. The best diffraction results possible (no edges or corners), with virtually no wall vibrations (no corners or edges to allow modal behavior - just one single surface.

The only disadvantage is the internal major resonance at the diameter of the sphere. Not an issue for tweeters, or the VM752 volt mid-range dome as they both have sealed backs. But with some internal bracing this can easily be overcome anyway - just break up the internal volume as you would any normal box.

So spheres it is for me.

Then there is the issue of the pairing in the Prestige of the Volt RV3143 to the Volt VM752. The dome has to be crossed above 500hz - preferably about 600, and the RV3143 12" driver really doesn't like going that high. I never felt it 'kept up' with the 3" dome.
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Click the image to open in full size.

So what driver could hold it's own with the VM752 from about 80-600 hz? (I will be covering 90hz and down with 4x12" subs in stereo dual opposed configuration, as I think it is important to separate these frequencies with different enclosures that aren't physically connected).

After a lot of consideration and thought, I settled on the new SB acoustics 7.5" satori woofer - the MW19P-4. This will sit in a sealed sphere of 9 litres with a Q of 0.7 and nicely get me to about 80 hz, it is also very high performance with a SOTA motor, and very low Le/distortion etc. so should keep up with the VM752 much better than the RV3143.

Click the image to open in full size.
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So.....onto designs.

I decided to use Ikea bowls of different sizes as I have in previous builds - I was very impressed with the strength and internal damping of the bamboo block construction. But as the VM752 dome is rear mounted, I need to make the mid-range sphere's back detachable. So I went with the brushed steel bowls for the backs.
Click the image to open in full size.
Click the image to open in full size.

This decision was made after I tried damping the steel bowls with 4mm silent coat (a damping product for cars).
It is a constrained layer damping product that is specifically engineered for use with 1-2mm thick steel. I have used it before on my speakers - it works OK with MDF and plywood, but on the thin steel it is designed for, it is amazing. The steel bowls with this product layered up are deader than I have ever achieved with wood constructions. It demonstrates that constrained layer damping really is a finely engineered balance of physics.

Anyway, I digress. So, I actively choose the steel backs over bamboo backs after experimenting with getting the most inert spheres I could.

I then had the issue of rear mounting the VM752 into the dome shape. After trying a few different approaches (and scrapping a few bowls), I settled on using a very large hole saw, then a table sander to achieve a near perfect fit.
*CLICK ON IMAGES AT THE END FOR LARGER PICTURES*

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I routed a lip onto the bamboo bowls to countersink them into the steel bowl, which will allow me to bolt them on and therefore be removable if I need to access the VM752 mid-range dome.
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The MW19P-4 will just be screwed directly onto the bowl, so the steel back will be glued and screwed into position.
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The Scanspeak 7000 tweeter is then mounted into a small bamboo bowl to allow a decent round-over and attempt to minimise diffraction. It will be mounted as close as possible to the mid-range dome to minimise lobing.
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Click the image to open in full size.

I plan to use some bamboo chopping boards to create the stands - each ball will be held in position by the stand and isolated from the others by sorbothane mounts.

Anyway here are some progress photos to show you where I am at with this project. This is obviously a work in progress, so fit and finish isn't yet where I want it to be, but for the initially rough up, I think it will be an interesting enclosure design!

More to follow...
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Last edited by bushmeister; 7th June 2017 at 10:02 AM.
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Old 14th May 2017, 03:06 PM   #2
zman01 is offline zman01  Bangladesh
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Interesting path you've chosen, keep the updates coming.
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Old 14th May 2017, 03:26 PM   #3
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Join Date: Apr 2005
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Fun they remind me of the focal egg speaker that was in speaker builder some time in 85 they replaced a pair of audio pro satellites that I blew at a party
This picture I took from the web
I brought my parts from Winslow audio ,I love the eighties
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File Type: jpg IMG_0561.JPG (104.3 KB, 240 views)
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Old 14th May 2017, 08:04 PM   #4
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I am now thinking if I can make the subwoofers inert enough using push-push dual opposed configuration, I may integrate them into the base of the speakers.

I have ordered 4 of these to do the job:
Click the image to open in full size.

At 108 each, 20mm X-max, and excellent verified Klippel measurements, I couldn't resist.

I have also ordered 1200mm length of 10" diameter 10mm thick PVC schedule 40 pipe. This is almost the same diameter as the spheres- so the design should be nicely balanced if I have the sub tube end on at the base. I am hoping the dual opposed set up will cancel out all the mechanical vibrations particularly given they are on the end of an incredibly stiff pipe.....some nice vinyl wrap and it might work......

I will keep you posted.
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Old 14th May 2017, 09:16 PM   #5
zman01 is offline zman01  Bangladesh
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Those subs look like they mean business.
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Old 15th May 2017, 06:06 PM   #6
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Dual opposed works wonders. My 2x15" ppsl sub barely vibrates when pushing 110dB+. While my little 10" feels like it's going to take a walk at 90dB. Bonus points if you mount the 2 drivers using the same threaded rods, doubles as bracing since you "pre-stress" the cabinet.
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Old 15th May 2017, 06:26 PM   #7
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Quote:
Originally Posted by Zonneschimmel View Post
Bonus points if you mount the 2 drivers using the same threaded rods, doubles as bracing since you "pre-stress" the cabinet.
This is genius! As I am using a large pipe, threaded rods to mount them is a great idea!

Pipe and 45kg of subwoofers will be arriving this week so I will get some pics up! I have also picked up some brushed steel vinyl wrap so the PVC pipe will match the steel bowls.

Still not sure if it will be too much with the subwoofers integrated into the main speakers. I do like being able to move them about a bit to get best in room response.....
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Old 15th May 2017, 10:52 PM   #8
msh1999 is offline msh1999  Israel
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looks awesome, good luck with the project!

i wonder, why you didn't go for pmc TL cabinet design then?
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Old 16th May 2017, 05:44 AM   #9
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Quote:
Originally Posted by msh1999 View Post
looks awesome, good luck with the project!

i wonder, why you didn't go for pmc TL cabinet design then?
If you mean loading the rv3143 with a TL cabinet instead of the ported box....what difference would that make?
Check out the response graph of the rv3143 in my first post. Pushing that driver up to meet the volt vm752 at 600hz is not ideal in my book.

So it calls for making the 3-way into a 4-way...

If I'm turning it into a 4 way....May as well pick the best drivers for the 20-90hz and 90-600 ranges. So a 7.5" midwoofer, and 4 big crazy excursion subs....are much better than a single high le 12" woofer.

Hope this makes sense.
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Old 16th May 2017, 02:54 PM   #10
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Thought you might like to hear how effective 4mm constrained layer damping is on the 1mm steel bowl.

I presume because the resonant frequency is so high already (incredibly stiff steel hemisphere) and the wall is thin and relatively light, the damping product works so much better than it does on MDF and plywood. The treated dome is utterly dead.

Listen for yourself (be aware there is a slight echo in the room which you can hear off the walls, this isn't the treated dome!):

https://youtu.be/MpkUbizbBjI

Last edited by bushmeister; 16th May 2017 at 02:57 PM.
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