An Extremis 6.8 Offset Reducing Taper TL Design

[valveitude]

Howdy ,

I recently purchased a set of Extremis 6.8's (thanx herm), and set out to find a transmission line design for them..no luck. I was kind of surprised about this, as the spec's seemed to indicate they would be a match made in heaven. Not to be deterred, I got access to MJK's Mathcad vault, and fired up the Mathcad worksheet, and AutoCad, and between dealing with a pair of sick grand babies, went to work . After a few emails to Martin asking questions on how to enter the data manually (Martin provides fantastic support for his worksheets), I got things rolling and came up with this design...



I plan to run a dipole ribbon off the side with the driver, so the final baffle shape hasn't been worked out. Here is the “sections” outline that I used to model it in MJK's “Offset-TL” worksheet...



Here is the projected freq. Response...



And the impedance curve...



I put together a PDF file that I copied all the Mathcad worksheet fields into (excluding baffle step..my dinosaur of a PC can't handle that section) and posted it here ( it's 300k, so it's to big for an attachment). It has been a l-o-n-g time since I built a pair of speakers, I have no experience with TL's (except falling in love listening to them), and I was hoping that the more learned of the members here could give it a once over. I would really like to know if I have overlooked anything, and your general impression of the design before I draft up the cut sheet for them.

Edit: I forgot to mention the spl graph was with a 10W input, it is a 86dB/1W driver after all .

Thanx,
Casey

[bjorno]

Casey,

I plan to run a dipole ribbon off the side with the driver

Exactly what I would do if using a dipole ribbon, if your front faced drawing shows the free standing left frontal speaker.
Otherwise in my opinion, it’s a good-looking Tl plan and well done simulations I find no reasons to remark upon.

It has been a l-o-n-g time since I built a pair of speakers

Are you kidding?

b

[valveitude]

Hi bjorno-

...if your front faced drawing shows the free standing left frontal speaker.

It is.

Otherwise in my opinion, it’s a good-looking Tl plan and well done simulations I find no reasons to remark upon.

Good news , thank you for taking the time to review the simulation.

Are you kidding?

Afraid not..it's been about 25 years .

-Casey

[bjorno]

Afraid not..it's been about 25 years

That’s about the years ago I tested this concept for the first time.

It’s still my favourite design though I’m nowadays using a mid driver between the tweeter and the bas driver.

What about your choice of ribbon driver? I’m also curious of the proposed crossover.

b

[valveitude]

What about your choice of ribbon driver? I’m also curious of the proposed crossover.

The ribbon will be a DIY design I've been playing with (on paper anyway). As for the crossover, this will be an active setup with a 200W
tube buffered BPA200 driving the Extremis, and a tube SE driving the ribbon. The crossover point will ultimately be determined by the ribbon's performance, but I am shooting for 300hz. One of the reasons I chose the Extremis is the low distortion in the midrange, this gives me a little flexibility if I need to crossover higher. The slope will also be picked based on the ribbon. I would idealy like to use a dual slope with an initial rolloff of 6dB/octave. I believe this would give me the most seemless transition between the 2 drivers.

As a "stopgap" until I can build the ribbon and required amps, I am considering converting my Paradigm Titans to dipole and using them in place of the ribbons.

-Casey

[valveitude]

An intersesting observation, and a couple questions:

Changing the Mathcad model of the line to ignore the last 1" flare, the response below 100 hz gets slightly less flat (a small hump). It's very slight, but noticible.

I'm planning on using 1" mdf, is this thick enough to eliminate any bracing of the 12" wide center Baffle ? Any other material recommendations ?

-Casey

[Cal Weldon]

Casey, I'd be very interested in how this turns out. I have a pair of Extremis gathering dust. They need a good home.

I have a pair of Titans in the garage along with a small powered woofer. They are fine for casual listening but when I do a side by side with others, I find them quite light in the top end.

I look forward to the results of your ribbons. How long does it take to diy them?

[valveitude]

Hi Cal-

Casey, I'd be very interested in how this turns out. I have a pair of Extremis gathering dust. They need a good home.

It will be a while yet before I know just how good it all works out..I have to finish my TT project, and build the amps for them (these will need considerably more than the Fisher 800 I'm using can muster) before I make these. In the mean time I'm trying to get the "plan" in place so that I know my goal. As I find (make) the time, I will be drafting up a set of detailed plans, and will be posting them here. Perhaps you can build them before me, and let me know how they sound . I'm only half kidding here, unless someone points out a flaw in my thinking (always a possibility), and the model is way off (not very likely based on results others have gotten from MJK's worksheets), provided you have the power behind them, these should rattle the rafters with bass..not to mention the very clean mids.

I have a pair of Titans in the garage along with a small powered woofer. They are fine for casual listening but when I do a side by side with others, I find them quite light in the top end.

Yep..modded with better caps and sound deadening, they sound way better than they should at the price, but that last octave just-aint-there. I think I can get my hands on a pair of the old (80's) Radio Shack (Panasonic) ribbons from my brother to take over the 10k, and up, duties.

I look forward to the results of your ribbons. How long does it take to diy them?

Good question. The last pair I made (15 years ago) took a couple of weekends. If you haven't already, I would highly recommend reading the ribbon thread started by dhenryp linked here, it's long, but covers just about all you need to know about building ribbons with inexpensive rare earth magnets.

-Casey

[valveitude]

Doh!!

Finalizing the drafts/cutsheet,I realized my 48 1/2" front/back/side pieces won't work out so well cutting from a 48" wide sheet.. like I said it's been awhile

Ran a quick sim this morning to see if an inch shorter would make any difference (1/2"x2)..it didn't seem to. I'll redraft and sim a "construction friendly" version soon.

-Casey

[Ed LaFontaine]

A sheet of mdf is an option.... it comes 49" wide x 97" long.