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PostPosted: 17 Dec 2014, 22:31 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Hi Folks,

I just recently got the shells of these great looking open baffle speakers built by my friend Keith which he had build for himself a number of years ago. Originally he had the H-fram open baffle speakers setup with the 8" Fostex FE206E and the 15" Eminence Alpha 15A. The open baffle speakers shells are what I'll call a "modular" design that will allow me to use them with a Eminence Alpha 15A bass driver and a number of different 3 to 8" fullrange drivers . I am planning on using a 2-way active crossover (Ashly XR1001) to start off with. It is a fairly inexpensive variable crossover unit and I figure with a microphone+RTA system that I have in hand, I should be able to find good crossover points. I'll post more as I make progress.
Attachment:
Open-Baffle-Shells.jpg

Cheers


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PostPosted: 09 Jan 2015, 16:44 
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Unfortunately I've been sick since before the new year so progress has been very slow. Until now I've only really had time to clean up the speaker baffles and mount the 15-inch Eminence Alpha 15A bass driver.
Attachment:
Mounted-Eminence-Alpha-15A.jpg

Previously I had mentioned the speakers are "modular". By that I meant that they can be taken apart into three pieces. The lower H-Frame, the vertical stem and the upper round open baffle. This allows the speakers to be somewhat adjustable. By rotaing the stem 180 degrees along its vertical access you can change the position of the high frequency driver allowing better time alignment with the low frequency driver. See the photo below.
Attachment:
Modular-Open-Baffle-Speakers.jpg

Cheers


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PostPosted: 17 Jan 2015, 21:07 
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Location: Clifton Park, NY USA
The H frame looks like a great build. Maximum strength against internal pressure, minimum cross-section, really nice. The OB is in my opinion not such a great build. By centering the driver in the circular baffle you have set up the worst case baffle defraction profile, the ripple will be at a maximum since the OB edges are equidistant all around the driver. It would have been better to offset the driver or use a irregular shaped baffle shape.

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PostPosted: 19 Jan 2015, 11:29 
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Hi Martin, thank you for the comments, they are very much appreciated.

That is good news about the H-frame because there is no ability to adjust it. I knew that the centered HF driver was not ideal, but I hadn't taken the time yet to look into the technical details of the issue. The good news is that there is the possibility to make adjustments.

With regards to driver centering, the back cutout is for an 8" driver, so since I will be using 3 and 4" drivers I can change the centering. The upper baffle really contains 3 pieces. The Rear baffle with the 8" cutout and mounting hardware, the nice looking two tone Front baffle and in the case where a small driver is used there is a Driver baffle sandwiched in-between the front and the rear where the driver is mounted.

That said, I do have the ability to place the driver off center somewhat. How much of an off-centering do I need? With the circular front baffle that may not look so great, but with the modular design I can always create a new front baffle. What would be considered an irregular baffle?

Another option I have is that I can very easily rotate the stem say 5 or 10 degrees (by just drilling a couple of holes) so that it is off axis with the H-Frame. Is there any merit to that?

I do have your upgraded MathCAD worksheets so I will have to fool around with them an try build a model to see what adjustments I can make to improve the speakers.

Cheers
Attachment:
Open-Baffle-Shells-notes.jpg


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PostPosted: 21 Jan 2015, 19:36 
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Location: Clifton Park, NY USA
Offsetting the driver in the circular baffle will help even out the response. Having a square baffle with the driver offset an uneven amount in two directions would also be an option. You might try simulating the baffle shapes and driver posiitons in the EDGE program to get a handle on the dB of ripple and try to optimize the geometry.

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PostPosted: 21 Apr 2015, 12:47 
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I finally got the cabling together to setup the biamped open baffle system.

Attachment:
H-Frame-OB-Fostex-FF85WK.jpg


The fullrange driver is a Fostex FF85WK which I find works real well with this sort of setup. For the crossover I am using a a 2-way Ashly XR1001active crossover. The preamp is a Cambridge Audio DAC Magic Plus. The Oddwatt KT88 monoblock amps power the top end of the system while a DIY FirstWatt F5 Amplifier is used to drive the bottom end (15" Eminence Alpha 15A). Yes, serious overkill on the amplification end. For summer I'll probably use a chipamp for the bottom end.

So far I have been playing around with crossover points between 200 and 400Hz. I've been setting the level by ear so it will be interesting to see how far off I am when I RTA the speakers and set the crossover and level properly.

I find this an improvement over the previous open baffles which used the same drivers but with a passive crossover. The H-frame does extend deeper relative to my previous flat OB setup.

Cheers


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