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My New Line Array--It's a Modified CBT24
My New Line Array--It's a Modified CBT24
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Old 11th October 2017, 07:15 PM   #1
Jim Griffin is online now Jim Griffin  United States
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Default My New Line Array--It's a Modified CBT24

I have started a new project. It is a new line array derived from Don Keele’s seminal Constant Beamwidth Transducer (CBT) activities. At this point the cabinet has been designed and built but surface finishing (painting) and detailed assembly need to be completed. Likely, it will take 4-6 weeks of work to complete this effort and hear music.

This project evolved from a fortunate snap decision I made late in summer of 2016. Madisound ran a sale on SBA 65 (SB Acoustic SB65WBAC25-4) drivers which reduced their cost by nearly 50% so I purchased more than 50 of these jewels. The SBA65 drivers have a molded plastic frame with a neo magnet. Usually, I over think my DIY speaker design before buying drivers but in this case I did have a specific idea in mind. My goal was to eventually build a full range line array with these drivers. During the past year I considered various configurations—straight arrays in either monopolar or a dipolar arrangement but had not settled on the best way forward.

While I have read Don Keele’s CBT work over the years, I really did not seriously consider a CBT design. But at the 2017 MWAF in Dayton I had a chance to talk with Keele about some of his latest work and to see/hear his CBT24 that he developed for Parts Express. Don sent me pre-release copies of his October 2017 AES Convention presentations. One paper, coauthored with Canadian researchers, was on a dipole CBT design. After reading that paper I was convinced that a dipole CBT was not what I really wanted to pursue. Don also shared several details about the CBT24 (called the Epique by P-E).

Keele’s sales points for the advantages of the CBT24 vs. conventional systems including: Even coverage with no sweet spots, elimination of floor reflections, reduction of ceiling and floor reflections, broadband constant directivity, flat power radiation, ability to play extremely loud with low distortion, and the system is time aligned and phase linear. He says that it can reproduce square waves at all locations in front of the speaker when properly equalized.

After studying the CBT24/Epique design I decided to proceed toward using my SBA65 drivers in an improved version of the CBT24. Thus my new line array project is a modified version of Don Keele's CBT24/Epique arrays.

What didn’t change in my version versus the Epique was the number of drivers and the arc. The new array is still 24 drivers and 36 degrees arc. Beyond those basics my new DIY array deviates from Keele’s work. I'm using a different (and better) 2.5" driver than the units used in the Epique units. The SBA65 is a 4 ohms driver vs. the Epique's 16 ohms. Several users on DIYAudio and other forums have commented upon or directly compared the SBA65 to other 2.5” to 3” diameter drivers. Most notable in comparison the SBA65 has a relatively flat frequency response with a rise in the 10k-20kHz range.

The drivers used in the Epique have 16 ohms nominal impedance while the SBA65’s 4 ohms impedance which necessitates that the shading network used in the Epique need changes for my modified CBT24.

For my arrays the cabinet--while looking similar to the Epique's--has several key changes. The new baffle is customized for the SBA65 drivers as there are significant mounting changes vs. other 2.5” drivers. A single unequalized SBA65 in a sealed box will get down to an F3 of 140 Hz for a half to one liter volume. For the modified CBT24 I changed the width of the tower to 4.5" vs. 3.5" for the Epique. Internal bracing is added which strengthens the enclosure. The SBA65 drivers are spaced 2.625” center-to-center apart.

One focus for me was on improvement of the base/tower interface. My needs were for stronger attachment of the two elements plus addition of easier separation for moving and transport of the arrays. The solution was to add an internal U-shaped metal plate with welded nuts inside the enclosure. Screws attach the base to the bottom of the tower and thread into the captured nuts inside the tower.

Just like the Epique my modified CBT24 is intended to be used with subwoofers and equalization to cover the entire frequency range. As mentioned earlier the lower F3 of the basic array is 130 to 140 Hz without equalization. For realistic bass levels subwoofers would be needed. I expect that equalization will be necessary to tweak the frequency response to perfection.

Kevin Kendrick has started a build thread on the modified CBT design on the Parts Express Tech Talk forum so you can link there for construction details and in process photos.

Kevin's construction thread is at:

Jim's new CBT Arrays - Techtalk Speaker Building, Audio, Video Discussion Forum

Last edited by Jim Griffin; 11th October 2017 at 09:46 PM.
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Old 11th October 2017, 07:56 PM   #2
Kevin K is offline Kevin K
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Hope to get a chance to hear these one day Jim. I can only imagine how awesome they will be! Never asked in any of our emails, will the enclosures be painted Black or did you have another color in mind?
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Old 11th October 2017, 08:17 PM   #3
Dr1v3n is offline Dr1v3n  United States
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First, fantastic! I'll be a regular reader of your project! Don Keele was an inspiration for my build as well.
Do you mean these drivers (I think there might be a typo in your 2nd paragraph):SB65WBAC25-4
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Old 11th October 2017, 09:05 PM   #4
Zarathu is offline Zarathu  United States
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Ummmm..... Aren't they going to begin to develop comb filter distortion at around 5200hz, when the CTC at one wavelength reaches 2.7 inches? Perhaps the curve has some impact on it? Is there a surface mount ribbon that we are not yet seeing?
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Last edited by Zarathu; 11th October 2017 at 09:20 PM.
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Old 11th October 2017, 10:04 PM   #5
Jim Griffin is online now Jim Griffin  United States
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Kevin: The color that I like is called Nebula Gray Pearl which is a medium gray with some sparkle.

Dr1v3n: Thanks for the typo correction on the part number of my drivers. I made the change in my message.

Zarathu: I'm a convert from some of my earlier comb line statements. Comb lines can be heard if you move up and down 1-2 feet away from a straight array but move to the listening position (several feet away) and they will blend so that they are not noticeably audible.

Also a full range array with these small drivers will have tradeoffs in the quality of their treble coverage. I'm not expecting perfection so I'll have to listen and report how they sound once finished. Every line array--indeed every speaker--has compromises.

Some wide band ribbons are out there but very expensive (and too long for an array like these) and not amenable for a curved surface speaker like these. It is all about how much money you have to spend.
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Old 12th October 2017, 12:23 AM   #6
xrk971 is online now xrk971  United States
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My New Line Array--It's a Modified CBT24
SB65 is one of my favorite full range drivers. It's got a. Wet nice top end almost like an aluminum dome tweeter that can go down to 100Hz. Nice project - looking forward to reading more about it.
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Old 12th October 2017, 02:08 AM   #7
Jim Griffin is online now Jim Griffin  United States
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Mr. X,

Thanks for the comments on the SBA65. I followed your small driver comparison studies and noticed how well this driver compared vs. the various contenders.

Jim
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Old 12th October 2017, 05:10 PM   #8
HankF is offline HankF  United States
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I'm glad you've started Jim, and I have decided to not buy the PE kit, but follow your (and PE's) thread. Did you CAD the cabinet plan and then have the MDF (or BB?) CNC routed?
Edit: Okay, I just read the PE thread and saw they were CAD and CNC'd.

Last edited by HankF; 12th October 2017 at 05:35 PM.
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Old 12th October 2017, 06:22 PM   #9
Jim Griffin is online now Jim Griffin  United States
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Hank,

Kevin Kendrick lives/works close to/and in Fort Worth. I can provide contact info if you wish to pursue.

Kevin was able to take my Visio drawings plus some info from Don Keele and turn that into engineering drawings for CNC. We did not have drawings for the P-E's CBT24 (the Epique) but between my inputs and Kevin's knowledge he was able to proceed.

Kevin is an active DIY speaker builder plus he has work experience within his profession to create and build mechanical models. This enabled him to anticipate and solve issues without much direct involvement from me.

Last edited by Jim Griffin; 12th October 2017 at 06:25 PM.
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Old 12th October 2017, 06:28 PM   #10
HankF is offline HankF  United States
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Please do, Jim. My daughter and her family live in Fort Worth. Go Frogs!
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