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Need help designing "tapped spiral TL" for Fane 12-250TC
Need help designing "tapped spiral TL" for Fane 12-250TC
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Old 17th May 2018, 08:50 AM   #1
bob4 is offline bob4  Germany
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Lightbulb Need help designing "tapped spiral TL" for Fane 12-250TC

Hi,

I have been thinking about a largeish build with a fane 12-250tc to get proper full range response with a cutoff as low as possible. A kind of back loaded horn/tqwt/mltl thing.


Is anyone up for helping me a bit in the process? I've been looking a bit into martin kings quarter wave material, and reading some threads on here about tqwt and mltl....

My original idea was to build something like this back loaded horn design:

Cyburgs-Viech von Berndt Burghard

One basic design premise of this horn was that it has no backchamber because of the high qts of the transducer. I suppose this would be beneficial with the 12-250tc as well, since it has qts=0.7.

On the other hand I recently read this paper

http://www.coolcat.dk/bjoern/QWforDummies.pdf

I'm leaning towards a straight line. Cab size 100 cm high, 70ish deep, 35-40 cm wide. (maybe try the 15" big brother in it later?)

I have a funny novel folding layout in mind, a sort of "tapped spiral TL"

Here is a very rough sketch to illustrate my idea. Please dont laugh..... Front baffle on the left, driver in the upper left, line terminus on the bottom to the same direction as the driver.

Opinions?
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Last edited by bob4; 17th May 2018 at 08:56 AM.
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Old 17th May 2018, 11:17 PM   #2
planet10 is offline planet10  Canada
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Need help designing "tapped spiral TL" for Fane 12-250TC
My initial take is, is this box big enuff?

And the folding and driver placement asks the question of whether the Zd is optimal.

With a Q of 0.7 one is likely going to need to damp the line until aperiodic so as to not have a big bump at the bottom.

dave
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Old 18th May 2018, 12:22 AM   #3
GM is offline GM  United States
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Yeah, ran a quick sim and a TL is ~450 L net tuned to 29 Hz once stuffed Vs a ~223 L, 38 Hz MLTL I did awhile back and it was the overall winner with the TL only having considerably less group delay down low enough not to be an issue for all but the relative few that are only happy with low Qtc sealed alignments.

GM
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Old 18th May 2018, 12:36 AM   #4
nigelwright7557 is offline nigelwright7557  United Kingdom
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By the time the back signal gets to the front of the speaker the CD will be on the next track at the front of the speaker !

I have put a Fane 15-300TC in a W bin and it sounds good.
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Old 18th May 2018, 03:23 AM   #5
bob4 is offline bob4  Germany
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I'm off to work soon, thank you for the replies. And kudos to the moderators for fixing my little picture issue! This place is amazing
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Old 19th May 2018, 08:13 AM   #6
bob4 is offline bob4  Germany
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Unhappy refreshing my memory on MJK.....

so I did

1) play around in horn response a bit. very basic inputs, BLH 1 PAR segment ~2,4 m long with 2000 sq cm mouth without back chamber. Results were spiky and ugly with low bass levels. Need to invest much more time and effort to get into HR, model some existing designs..... tedious. May do to that later

2)went to MJK's site and read this paper:

http://www.quarter-wave.com/Horns/BL...gn_Article.pdf

two interesting conclusions:
A) MJK's design approach starts from enclosure size. I like that, very practical.

B) mouth size of typical BLHs too small for proper horn loading at bass/sub freqs. My assumed mouth size of 2000 sq cm would provide horn loading only down to ~180-200 Hz. Even assuming half space loading, which doubles the effective mouth size it would be ~90-100 Hz which is not low enough.
-> TL theory applies

I signed up for the yahoo group. I want to get hold of those worksheets. Everybody on here seems to use them

All in all I'm beginning to have some doubts regarding the effort and box size, the general feasibility of my approach. I'm looking at a 350 L box to MAYBE eek out a bit of bass from this driver. Altough I have scrap wood I can waste on this, I want to have some confidence in what I'm building. Hopefully I can arrive at a worthwile design utilizing the MJK worksheets.
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Old 19th May 2018, 02:37 PM   #7
phivates is online now phivates  United States
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VC winding length 10.9 gap height 7.8 so the 3.5 xmax spec is p-p. Not a woofer.
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Old 19th May 2018, 03:49 PM   #8
Scottmoose is offline Scottmoose  United Kingdom
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No, but in fairness nobody claimed it was.

The 12-250TC is a 12in wideband driver with a Qt at the top end of what could reasonably be described as 'middling', a moderate Vas & Fs. By current enclosure size expectations, it wants quite a large box. For example, a Keele max-flat aligned vented cabinet is about 355 litres, with an Fb of roughly 27Hz. My friend Steve is happy with his in 80 litres sealed; depending on what amplifier he's running at the time system Q will be about 1 - 1.2. Suits his room & requirements, but like every other speaker ever built, won't work for everybody. The smallest I'd go would be the MLTL GM indicated above, or this Keele converted into MLTL form factor which should give some extra alignment damping.

As for horns -available HL BW is 18Hz - 134Hz (nearly). You can tune it low, but like any other vented alignment, you need bulk if you want gain.On top of that, if you heavily compromise the horn expansion you'll need to know what you're doing in terms of blending QW action into what BW is impedance matched, or you'll have a fairly rough response. That may or may not be terrible, depending on specifics; the room dominates < c. 300Hz & combined with our falling hearing acuity in the LF & our tendency to 'key off the peaks' we can put up with a rockier LF response than many believe. Worth it? Technically yes, in certain senses (e.g. dynamic range &c). From a practical POV, I doubt many would be willing to accept the large size of the enclosures (pity).

I suspect the smallest practical vented option that would blend gain & some output down to, say, the midbass would probably by a Karlson BP variation. Not to my taste but YMMV.
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Old 19th May 2018, 03:53 PM   #9
GM is offline GM  United States
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Hadn't noticed this! My MLTL sim is now limited to 100 dB/m/30 Hz/4 W/2 pi or ~103 dB/m in stereo.

GM
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Old 19th May 2018, 03:59 PM   #10
GM is offline GM  United States
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Quote:
Originally Posted by GM View Post
.......a TL is ~450 L net tuned to 29 Hz once stuffed......
Just noticed a typo ........should be ~19 Hz.........

GM
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