The Triton MTM Grows Legs--A MLTL Design - diyAudio
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Old 21st December 2012, 12:31 PM   #1
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Default The Triton MTM Grows Legs--A MLTL Design

At the November 2012 Central Kentucky DIY Speaker event I heard Bryan Keane’s Triton MTM bookshelf speakers. Bryan used Jeff Bagby’s prototype Triton design for Creative Sound Solutions (CSS) as the model for his speakers. I was impressed with the imaging and clarity of Bryan’s speakers and they were easily the best bookshelf speakers at the Central Kentucky event. Jeff’s design is superb and well executed as explained in the white paper on this design at:

http://meniscusaudio.com/images/Trit...Write%20up.pdf

Bryan Keane’s Triton MTM build thread is:

Jeff B's CSS Triton High Resolution Monitor

Robert Woods has a similar build thread at:

"The CSS 'Next Generation' High Resolution 'Triton' Kit" designed by Jeff Bagby

The drivers are interesting in their size (small) yet low distortion realization. Both have patented XBL2 technology which was developed by Dr. Dan Wiggins during his tenure leading Adire Audio. Dan invented a driver design that extends the linear travel of the moving parts of the speaker so that measured distortion levels are reduced compared to standard technology.

Under the sponsorship of Creative Sound Solutions (a Canadian DIY loudspeaker parts distributor), the new XBL drivers are the CSS VWR126X mid-woofer and the LD25X tweeter. The very wide range VWR126X is a 4.5" diameter unit. Along with the XBL motor system, this driver features a woven Polypropylene cone, rubber edge, cast aluminum frame and copper dust cap. The likely driver application of the VWR126X is as a premium mid-range in multi-way systems. The LD25X tweeter features a small flange and the XBL motor system—the first for a tweeter—and is perfect for MTM and other limited baffle space designs. The distortion levels are some of the lowest measured. Both of these drivers were introduced to the market for DIYers early in 2012 and you can check several threads on DIYAudio and Parts Express Tech Talk forums threads on the Triton MTM bookshelf design.

My MTM design with the CSS drivers is realized as a mass loaded transmission line (MLTL). The MLTL configuration extracts optimum bass performance from these drivers per this size enclosure. I call MLTLs speakers with legs--the stand or base for a bookshelf speaker aren’t necessary as the MLTL yields a tall floor standing cabinet. MLTL design efforts are aided by Martin J. King’s MathCAD based worksheets.

The basic assumptions for my efforts were to retain several key elements of the Bagby Triton bookshelf design including the cabinet wide (7.5 inches), driver spacing, and the tweeter height at 35 inches as Jeff recommended. The ultimate goal was to be able to use the original Triton crossover design and its inherent baffle step correction. My contribution was to lower the bass 3 dB down frequency of the bookshelf speaker which was measured at 52 Hz by Jeff.

After working through the MJK worksheets, I have a MLTL MTM design that satisfies the design goals. It is a straight MLTL with no fold or taper so it is simple to construct. The 3 dB down frequency response is 37 Hz while the port terminus dips at 43 Hz. The outside dimensions are 7.5” wide by 8” deep and 42.5” high. Assuming 0.75” thick walls, the internal cross section dimensions are 6” wide by 6.5” deep, and 41” high. The tweeter is centered at 7.75” from the inside the top while the mid-woofers are centered at 3.5” and 12” from the top respectively. A one inch baseplate would raise the tweeter to a 35" height.

Similar to most vented box designs, the length of the port tube determines the 3 dB down frequency for the bass response of the MLTL. For the 3” diameter port tube the tube length is recommended to be 6”. During my simulations I studied various other port lengths ranging from 4.5” to 9“. For 4.5, 6, 7.5, or 9 inches lengths, the 3 dB down frequency response is 40, 37, 35, and 33 Hz, respectively. The 7.5” and 9” lengths would necessitate either a curved tube inside the enclosure or extension outside the box.

Common with vented boxes, the MLTL mid-bass drivers will have increasing cone extension at the low end of their frequency range as input power increases. For this MLTL the linear cone extension reaches maximum cone extension at an input of 30 watts at 37 Hz. For the VWR126X drivers the linear extension is specified to be 5.8 mm. The SPL produced at this limit is 105 dB. While these MLTLs aren’t subwoofers, this is an impressive SPL level for small mid-bass drivers.

Prototype speakers have been built to demonstrate the performance of the MLTL design. Pictured are the Triton MLTL MTM speakers. The cabinets are made with cherry hardwood side and top panels (no veneer on these speakers!) and laminate covered MDF front and rear panels. The cherry hardwood and the copper color dust covers provide a stunning appearance to these speakers. The port tube and terminal plate are located on the rear of the boxes. My units use a Corian baseplate with floor pads.

Inside these cabinets are braces which strengthen the enclosure. While the cross-sectional area of this MLTL is small, my experience is that the highest pressure inside the box is at its top. Four braces are used inside the box. The first brace is under the top as the brace encircles the perimeter of the top. Just beneath the lower mid-bass a ledge brace encircles the box and a similar brace is placed at the bottom of the box. Finally, a bridge brace adds structure to the area of the tweeter. All braces are implemented with 0.75” square hardwood dowels.

For damping and frequency response smoothing the volume from below the drivers to the top of the box is filled with teased Acousta Stuf damping material. A wire screen rests on the ledge brace and retains the stuffing within this volume. The simulated stuffing density was 1 pound per cubic foot which equates to approximately 0.33 pound for each enclosure. Another damping technique is to line the internal surfaces of the panels with Sonic Barrier VE-1 vinyl sheets. Essentially, this material mass loads the panels so that any box vibration would be attenuated--these cabinets pass the knock test!

The crossover network derived by Jeff Bagby was divided into separate tweeter and mid-woofer boards. The tweeter board was mounted inside the box behind the top woofer (on the rear panel) while the mid-woofer network was placed behind the terminal plate on the front panel.

I’m just into the break-in of these speakers at this point but they have the same magic that I heard with Bryan’s speakers. Jeff’s design images like crazy and now you have additional bass extension. Hence, this design can make you grin. They reproduce music with excellent results. Only if you need bass within the 20 to 40 Hz octave, would you have to add a subwoofer.

In future postings I will more include additional details on this MLTL design.

Click the image to open in full size.

Last edited by Jim Griffin; 15th September 2013 at 01:56 PM. Reason: correction
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Old 21st December 2012, 12:57 PM   #2
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I would like to see the impedance response in mjk's sheets to see if it resembles traditional BR saddle.
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Old 21st December 2012, 01:24 PM   #3
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I presume Jim will eventually share more information on this design, possibly the modeled impedance graph. I can assure you that there will be a BR-type saddle in that graph because all TLs are inherently 4th-order systems just like a BR or vented box. Some people will stuff the life out of their TLs in order to flatten the lower-frequency peak in the saddle, which has two effects, decreasing the slope of the roll-off below the knee in the system response while also virtually killing all of the output from the mass-loading port. Thus, F3 is raised substantially, negating any bass benefits.

From what Jim has posted, this appears to be an outstanding design again that provides Jeff's excellent design with additional bass, just like Maynard Goff's ML-TL versions of Jeff's Continuum.
Paul

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I would like to see the impedance response in mjk's sheets to see if it resembles traditional BR saddle.
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Old 21st December 2012, 03:45 PM   #4
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buzzforb,

I'm using MJK's worksheets with MathCAD 8 so I have limited latitude to change the scale of the graphs. Hence, my impedance graph has a 120 ohms full range scale. The double saddle curve is there although surpressed. The peaks occur at approximately 40 Hz with 6-7 ohms and 100 Hz with a 11-12 ohms peak. Otherwise the plot follows along the 4 ohms level as it should.

Jim
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Old 21st December 2012, 04:55 PM   #5
pkitt is offline pkitt  United States
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That's strange, Jim. I also use MathCad 8 and I have essentially unlimited abilities for setting scale factors on the graphs, not that it matters all that much as far as this specific issue is concerned. My understanding is that it's Martin's worksheets that allow this ability, not MathCad. What version of the ML-TQWT worksheet are you using?
Paul

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Originally Posted by Jim Griffin View Post
buzzforb,

I'm using MJK's worksheets with MathCAD 8 so I have limited latitude to change the scale of the graphs. Hence, my impedance graph has a 120 ohms full range scale. The double saddle curve is there although surpressed. The peaks occur at approximately 40 Hz with 6-7 ohms and 100 Hz with a 11-12 ohms peak. Otherwise the plot follows along the 4 ohms level as it should.

Jim
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Old 21st December 2012, 06:59 PM   #6
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Thanks Jim for posting your impressions and thanks for your efforts in creating a new Triton.

Give them a few more weeks and you will be even more impressed. I can't wait to start the conversion of my MTMs right after Christmas, I hope.
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Old 21st December 2012, 07:17 PM   #7
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Paul.

I have the worksheets on my old XP computer and the sheet is dated 9/10/06 so it is a 2006 version. I can change the look of the graph, formating, such as the number of grid lines but no entry that I see will change the scale of the data. Autoscaling gives me 120 ohms Y axis and that seems to be unchangeable at this time. Perhaps you can suggest something off line to straighten me out on plot set-up.

Thanks,

Jim

Last edited by Jim Griffin; 21st December 2012 at 07:18 PM. Reason: correction
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Old 21st December 2012, 08:13 PM   #8
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Jim,

What do you mean by a bridge brace?

What is the Zp of the vent?

Damping exists only in the ~top third of the line?

dave
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Old 21st December 2012, 09:13 PM   #9
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Jim,
Very cool idea to "extend" on my design. I really like the execution too. I would expect these to be very impressive for the driver complement. The Tritons have some impressive bass for what they are, but it is pretty much all mid and upper bass. Of course, no one really expects a stand mounted monitor that size to dig too deep. If your design takes it another half-octave deeper then that really will be something.

I struggled a bit with a peak in the upper bass in the Tritons due to the capacitive load of the upper impedance peak due to the box tuning and the woofers resonance. I didn't add additional conjugates to flatten it, and no one seems to comment on it, so it must be pretty benign. However, if your design reduces that impedance peak magnitude then this should really flatten and extend the bass. I think your speaker has the potential of taking my design to another level, and I'm all for it.

You are right, the Tritons image like gang-busters. This must be a feature of the low distortion drivers and the lower level information retrieval they are capable of in the midrange and highs that account for this. I didn't do anything in the crossover design that I wouldn't have done in other designs, yet these still image better than any of my other speakers that I could compare them with. If you want a speaker that images, then this is the one.

I bet yours are a real joy to listen to. I hope to hear them myself sometime; I would love to hear how the MLTL performs with these little drivers. Again, nice job, and contrats on a great execution of the design.

Jeff
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Old 21st December 2012, 10:47 PM   #10
pkitt is offline pkitt  United States
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I'm pretty sure your 2006 version does not allow changing scales on the graphs and you'd have to "purchase" Martin's current worksheets to get that feature, which may or may not be all that valuable to you. I really like it, though, as it allows seeing some graphs with a lot more resolution and allowing you to make subtle changes for improved performance.
Paul

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Originally Posted by Jim Griffin View Post
Paul.

I have the worksheets on my old XP computer and the sheet is dated 9/10/06 so it is a 2006 version. I can change the look of the graph, formating, such as the number of grid lines but no entry that I see will change the scale of the data. Autoscaling gives me 120 ohms Y axis and that seems to be unchangeable at this time. Perhaps you can suggest something off line to straighten me out on plot set-up.

Thanks,

Jim
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