Openminded--A Mark Audio Alpair 10.2 Open Baffle Project - diyAudio
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Old 11th July 2011, 08:14 PM   #1
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Default Openminded--A Mark Audio Alpair 10.2 Open Baffle Project

I’ve wanted to try an open baffle (dipolar) speaker design for a while. But I wouldn’t settle for less than a full range speaker system with excellent dynamics and sound presentation--no compromises. For simplicity I wanted to start with a two way system that marries a top notch wide range driver with a musical woofer.
I had used both the Mark Audio Alpair 10 and 10.2 (second generation) drivers in a MLTL enclosure as reported previously on DIYAudio. From hearing the new A10.2s in the MLTL boxes I heard improvements vs. the original A10s. Thus I thought that the A10.2 would be a good candidate for the mid/tweeter driver in the new project.
As most of you realize, full range open baffle coverage with a single driver—even one as remarkable as the A10.2—really limits the amount of bass you can achieve from the speaker design. Fortunately, the work by Martin J. King and followers show excellent results from the use of a large woofer—say 15 or 18 inches diameter—with a wide range driver to cover the mid and tweeter ranges of the frequency band. Martin analyzes various bass configurations on his website and I chose the H-frame design which yields good results with reasonable size for the overall speaker.
Normally, a high Q woofer is selected to provide essential EQ to boost the woofer output for bass frequencies if a passive crossover is used between the H-frame woofer and the broadband driver. For my project I planned to use an active crossover, so I looked for woofers that had good extension, high 90’s dBspl, and adequate response to below 40 Hz and reasonable overall Q. After a search I chose the Eminence Kappalite 3015LF Neo driver (Qts of 0.39, 99.8 dBspl, 9.6 mm extension, etc.). A low Q woofer like this one needs equalization to help assist its response so the crossover box so I anticipated EQ’ing across the frequency band to aid both the woofer and A10.2. For my crossover network I’m using the dBx DriveRack PA+ which features equalization over 28 bands. You can read other specs on the DriveRack and Kappalite 15 on the manufacturer websites.
Then I got to building. The H-frame for the Kappalite 15” driver is 18 x 18 inches high/wide and has a 16” depth (outside dimensions). The frame is built from 0.75” thick cherrywood (sides and top) and MDF. The A10.2 open baffle was sized to yield open baffle performance down to the low 100’s Hz. The A10.2 baffle is 18” wide and 19” high and has 3” thick hardwood side rims. With these dimensions the H-frame has a dipolar dip near 300 Hz so my goal was to crossover the woofer to the A10.2 so that their response would blend around 200 Hz.
I used a small Parasound preamplifier before the DriveRack and four channels of my Rotel 5-channel (125 watts) HT amplifier to power the speakers. RCA to XLR adaptors are used on the inputs and outputs of the crossover box. I have not heard any gain compression within the signal path for this gear so gain distribution seems in order.
When I finished assembly of the speakers the tweaking process with the crossover started. The tweaking with the DriveRack doesn’t take long as you can flip switches and twist controls to your desires. Initially, I tried to work toward flat response but the sound was a little off for my taste. I measured a slight rise in the frequency response that I suspected would need correction. With the DriveRack RTA microphone I tried the Auto EQ function with the mic across the room about 15 feet from the speakers. The Auto EQ process plays pink noise in the room and you get to choose between a set of contours. The flat response EQ wasn’t my cup of tea. Next I chose the live performance profile Auto EQ setting and bingo! I’m hearing the sound that I sought.
Now I’ve been playing my favorite CDs with these speakers and I’m pleased. The music is clean and tuneful without any excuses. Others have concurred about the natural sound and how it sounds like music. The Alpair 10.2 produces exceptional mids and the highs are spot-on without the need for a separate tweeter for my listening. Furthermore, I’ve become a devotee of dipolar bass. If you normally listen to closed or vented box speakers, you owe yourself a chance to hear open box bass. No muddy bass, no boxy sound, good integration with the room, etc. are just some of the benefits.
Last week I entered the speakers in the open unlimited category of the Parts Express Midwest Audiofest in Dayton, OH. My speakers won first place in this class so I was very pleased with the results and to know that others like what they heard.
Finally, it was fortunate that I purchased my Eminence drivers before the insane escalation in Neo driver prices this year. Other lower cost woofers are available that can do the job in this configuration so I encourage others to build it and tell us how your ears benefit.

https://picasaweb.google.com/1026835...80330646289442
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Old 11th July 2011, 08:28 PM   #2
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I was just wondering when someone would attempt an OB with the larger MAs!

Can you speak to the dynamics of the MA 10.2 in this configuration?
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Last edited by chazdrumzalot; 11th July 2011 at 08:31 PM.
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Old 11th July 2011, 08:45 PM   #3
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The crossover relieves the Alpair 10.2 from most of any bass handling issues. I can touch the surround and I don't feel significant energy nor feel/hear over driving of the 10.2.

Frankly, you will want to turn-up the volume as the sound is clean and clear. Fortunately, my drivers have several hundred hours on them so they are well broken-in.
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Old 11th July 2011, 10:16 PM   #4
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Here is a link to additional photos of my speakers.

https://picasaweb.google.com/1083207...KFxZnUlNrfrQE#
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Old 12th July 2011, 03:04 AM   #5
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WRT diffraction, wouldn't you have better results offsetting the alpair? Or is it not really an issue with this design?

And by the way, really beautiful and elegant design as always.
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Last edited by chazdrumzalot; 12th July 2011 at 03:15 AM.
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Old 12th July 2011, 03:00 PM   #6
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Offset or not is a good question. I did not complete a study for this design so there is no offset.

However, I would be in good company if I did not use an offset. Dr. Linkwitz points out that while an offset may help diffraction on the main axis, it causes some problems off axis, especially for an open baffle driver.
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Old 12th July 2011, 03:05 PM   #7
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Yes, I read your comments on the matter on the PE forum. That's an interesting point, and furthermore, at least one can say that centering the speaker is good aesthetics.

I wonder what would be required to produce your results using just passive components. It would first require a higher-q woofer for the H-frame. I wonder how the beta-15 would model in an H-frame? I think its sensitivity would be a good match, so hopefully no need to pad either driver...
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Old 12th July 2011, 07:40 PM   #8
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If you are interested in other Eminence or Goldwood woofers, go to Martin J. King's website as he has several white paper/projects on various woofers and specific configurations. He has a passive crossovered Eminence 15" Alpha or Goldwood 18" and I think specific examples of Jordan JX92S or Fostex full range drivers on his site as well. He talks about various Eminence woofers in another white paper.

If you have his spread sheets, you can plug in your choice of woofer and full range driver and see how your variation functions. For open backed woofers, the dipolar configuration eats up sensitivity so it would be best to work the details to see if you choice of woofer and wide range drive can be passively crossed. Generally, a higher Q woofer needs less low end EQ which makes passive crossovers do-able. But an active crossover can solve a lot of aliments for this kind of configuration plus permits other choices of drivers.
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Old 12th July 2011, 09:19 PM   #9
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Neat. I think H-frames are great and sitting something on top of them is an easy way to get full rich bass.
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Old 13th July 2011, 01:17 AM   #10
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Godzilla,

What big *** mid driver is that?
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