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Old 20th September 2014, 02:17 PM   #1
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Default 3" full range in MTM config

I have recently build some MLTL enclosures to play around with different drivers in various configurations, biamped and crossed over using a minidsp.
The first effort comprised two 6.5" peerless nomex and a Vifa XT25 in mtm format. After trying various slopes and crossover points in the vicinity of 2k,
I decided to swap the tweeter and replace them with Tang Band W3-1364sa.
Wow!
Crossed at 500hz they sound very sweet with bass extension very acceptable.
But, I have a question.
Should I enclose the back of the TB? I thought about a 4" dia stuffed tube similar to that used in the old KEF Concerto midrange driver.
In the meantime, I just mounted the little bamboo cone driver in the same baffle cutout where the XT25 was located without providing any separate enclosure for it(yet)and thought that I would probably hear significant distortion as a result, but I don't.
There is 1 pound of stuffing In the top 20" of the cabinet - would this help in reducing any distortion? Or maybe my aging ears can't detect it?
Peter
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Old 20th September 2014, 02:34 PM   #2
xrk971 is online now xrk971  United States
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You have discovered the magic of FAST ( Fullrange assisted subwoofer technology, or a two-way crossed over relatively low in 200 go 300Hz range.) Are you using a passive crossover or active biamping? Generally, you want to isolate the fullrange from the sound of the woofers directly hitting the back of the cones. It can cause distortion, not to mention excessive excursion as a passive rad for the woofers. Sealing it with a stuffed tube should work. Other drivers to try are Vifa TC9FD, Fostex FF85Wk, and Faital Pro 3FE25 (or 3FE22). Those drivers can all behave like tweeters when liberated of having to produce anything below 200Hz. Can you share design of the MtM MLTL? Photos?
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Old 20th September 2014, 05:55 PM   #3
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Thanks for the reply. I will install a tube at the back of the TBand at some point. It will be interesting to hear the difference, if any.
The MLTL cabinet is based on dimensions provided by another diyaudio member when I browsed for enclosure for peerless 830875 tl.
The internal dimensions are 7.5"x 14" x 40" with 3"dia x 2.5" port at the bottom. 1lb poly stuffing in top 20" or so. My Radio Shack meter shows good extension to 40hz and drops off by 32hz.
The sides of the two cabinets are made from one sheet of 3/4" mdf and the removeable baffle, top and bottom, from 3/4" cherry.
I plan, eventually to try the full range TB with Vifa 8" woofer in a TM format which should provide even more low end extension, though I'll have to lengthen the port.
Also have a pair of 10" dayton sub drivers that would require cutting into one side, so I'll put that effort on the back burner.
All these alternates are biamped with a pair of Symasym amps, a B4 pre and minidsp.
I'll try and upload a picture when my laptop is back in business. I have'nt figured out how to upload images from my iPad to this website.
Peter
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Old 20th September 2014, 07:53 PM   #4
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The mid-tweeter needs to be in its own box, otherwise it will be modulated by the bass driver. We have had great success using aperiodic midTL out the back of the box. You could do that by running the cardboard tube out the back of the box.

Using a FR as mid-tweeter allows one to get the woofers close enuff that they can be less than a 1/4 wavelength at the XO, the XO can be pushed below the bafflestep frequency so that BS can be easily dealt with with a simple level adjustment. This usually falls in a place where the drivers have lots of overlap allowing for a simple XO. If the XO is 1st order, and since the drivers are effectively co-incedent, one can get VERY good time performance.

We have done 2 MTMs so far: an ML-TL and a standmont. (follow the links for more pics & info)

Click the image to open in full size.

Click the image to open in full size.

Next up, we are going torevisit the ML-TL but with the new Mark Audio Alpair 7p and A12pw.

dave
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Old 20th September 2014, 08:34 PM   #5
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Very interesting - thank you.
My woofers are 10.75" o.c. Equating to approx 1/4 wavelength of 310hz, I think. I'm not sure how to use this information to good effect.
The baffles are 9" wide.
If I enclose the back of the tweeter with an open ended tube 4" dia, is stuffing the tube just a matter of trial and error to achieve the best performance?
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Old 20th September 2014, 09:28 PM   #6
mikje is offline mikje  United States
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This post is timed perfectly. I'm working on a TMM with a Tang band W3-881Si abd two CSS WR125s.
Dave, you mentioned crossing so the woofer is less than 1/4 wavelength at xover. Is that using the FS of the woofer? If not, what frequency related to the woofer should one use?
Thanks,
Mike
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Old 20th September 2014, 10:06 PM   #7
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Quote:
Originally Posted by pjblakey View Post
Very interesting - thank you.
My woofers are 10.75" o.c. Equating to approx 1/4 wavelength of 310hz, I think. I'm not sure how to use this information to good effect.
Close enuff, i get 315 Hz.

Quote:
The baffles are 9" wide.
That gives a 1st approximation of BS -3dB of 507 Hz, so as generality one will want to XO between ~360 & 500 Hz.

Where you cross depends on driver response and XO slope, usually lower slope needs to XO lower. This is a place where measuring across the XO point to get the flattest response can be helpful, but keep in mind that this is getting into the range where one is getting into room effects if measuring in a room.

One could push the 1/4 wland BS numbers closer by adding some extra material to the cabinet sides, but really they are close enuff.

Quote:
If I enclose the back of the tweeter with an open ended tube 4" dia, is stuffing the tube just a matter of trial and error to achieve the best performance?
To start you use an increasing density of stuffing as you approach the terminus. You can use GMs clik test for a 1st approximation as you tune, or measure the mid-tweeters impedance, looking for flattest response.

A thread for Tysen and variations on FAST

dave
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Old 20th September 2014, 10:07 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

F <= 1/4 wavelength of the driver spacing.

At 340Hz 1/4 wavelength = 25cm.

However box width determines the best x/o point
more for simple FASTs than such considerations.

Series 1st order can work better than parallel.

rgds, sreten.

There is generally no need to worry about optimum stuffing of a mid tube compared
to a bass box too much. For a TMM you can triangulate a panel between the top
and front below the driver, ands just well stuff the space behind the driver.
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Last edited by sreten; 20th September 2014 at 10:19 PM.
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Old 21st September 2014, 10:44 AM   #9
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Default pics

here are a couple of pics of the tang band mtm
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Old 11th October 2014, 05:17 PM   #10
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Default Little Fountek jewel

I replaced the tang bands with Fountek FR58ex and a further "improvement".
It's the first time I have used and aluminum cone anything and am very pleased with the overall sound and detail. Of course, the output is down quite a bit, but in my medium size room, just fine. Very easily compensated with the minidsp. Currently crossed at 500Hz LR4.
The tang bands, in comparison were a little shouty when cranked up and a little ragged on the high end.
For $18 a piece, they are little jewels.
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