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Extreme Open Baffle Active Speaker inspired by Linkwitz LX521

Recently i visited Siegfried Linkwitz and heard his LX521.
I was positively shocked about the sound quality and the progress he made since my last visit when i heard the final version of the Orion.
I like the Orion, no question. I even build a pair for my friend.
The Orion is neutral balanced and throws a nice soundfield.
What it lacked for my ear is what i call drahma. It is more a polite speaker, never agressive and very clean. The aural attention is more in a wide cut midrange. At the edges of the spectrum, high up and deep down i would wish for more extention and definition. Transients come a bit slow so there could be more resolution and snap. This is more a speaker that does well what it can do well. It is all about avoiding glaring mistakes at the price of ommitting some of the details. Nuff said, the Orion has it´s time and place.
Nothing prepared me for what Siegfried had in stock.
The LX521 fullfills all my wishes and some...
The sound is now totally free from the speakers. I could not hear sound comming from the baffles, even in close miked pans. That is really unusual and welcome. They totally " disappear ". OK; this has being said many times in reviews, for example in Michael Fremers Review in the 90th of my original Virgos and it is true. I can only tell you that they even " disappear " more then i have heard before in a dynamic speaker. Another feature is also at work here that gives the illusion of the speakers not present : there is absolutely no preasure on the ears.
There is much more to say and i will later.
At the end of the session Siegfried encouraged me to build a pair myself.
I thought long and deep about it. I came to the conclusion that i should design my own version. Not in an attemped to better the LX521 but to have some fun. This thread is about this undertaking.
Some more notes about the sound i heard :
The LX gave the impression on being very wideband, subjectively the tweeter is more extended, crisp and fast. Quite an achievement with two inexpensive, small SEAS neodym
drivers. The bass is tighter, deeper, cleaner and has more slam and power.
Transition is seamless.
We could play very loud and dynamic. That gave quite a spectacular presentation.
It was emotional and accurate. Like a camelion the LX could turn from intimate and elegant ( King Singers ) to brutal and snapping ( live solo electric bass ).
We did not find time enough to listen to much classical ( i had too many questions and i can digest only so much from such an opulent meal ) but Mr. Linkwitz is convinced that strings are the LX friend. I have no doubt about it.
With extreme material the woofers and the lower mids moved quite a bit but i could not hear any distortion. Siegfried has a very big room so i can not imagine that even a rock and roll addict wants much more. This speaker simply has much more scope and range then the earlier Orion.
Of cause Siegfried told me : " This will be my last speaker ".
Sure, he said that when he made the Orion.
Some more notes that really challange the way we think in High End :
The speaker cables where 6m long Radio Shack 1.5qmm standart stranded with PVC isolation. Several non precious, magnetic metal connections in the chain.
I usually expect foggy and " dirty " treble that way but the treble was superbly clean, crisp, snappy and shimmering.
The speaker is active. That can not sound homogenius. The amplifiers are not as good as my 20.000,- $ tube, etc. blah, blah, blah.
He used a simple and good OPPO player. No footers, no roomtuning, no nothing...
Well, yes, and i knew it : the speakers are the weakest link in the chain !!!
The room too but with dipols and intelligent placement there is no need for extensive room tuning.
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So what did i do so far ?
I build the mid-treble baffles. That is really fast and easy.
I copied more or less the shape.
Siegfried told me that the shape is critical to maintain dipole action over a wide range.
He arived at the shape by experiment.
I understand the midrange shape but i had questions about the rather clumsy shape in the treble.
" That is to avoid that the radiation from the back tweeter comes around to the front ".
OK, i dig that.
The shape i use is diffent in that it uses Bezier curves instead of straight lines.
I do not think that is has a lot of consequences to the sound but it is a visual alternative.
In the treble my version has the biggest diffence to the LX521.
I use a dipole AMT donated by Mundorf.
Thanks Raimund.
I decoupled it by springs.
The midrange is the Scan Speak 10cm wideband.
I do not know if it is superiour to the Curve Cone SEAS.
I simply had it in stock.
I wounder if part of the " crispiness " of the LX comes from that Curve material.
When i scratched it with my fingernail it sure gave a crispy sound.
That is very diffent from soft material like Bextrene or unfilled Polypropylen.
Such membranes are known for loss of mircio detail.
The Curve cones obviously do not have that problem.
In the deep mid i use a driver designed by SEAS for me. It has a stiff paper cone, a huge magnet and long throw.


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Here are the responses of the drivers i measured free field on axis.
The crossover frequencies are supposed to be at 120Hz, 7 Khz, active 4th order and 1kHz
first order passive.
As far i i can tell this will be posible without problems.
I will use the Mini DSP anyway so i can linearize or shape the response more or less at will anyway.


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As you can see the 6dB electrical filter does not result in an acoustic 6 dB filter.
The drivers overlap quite wide with the correct polarity. When i use the wrong polarity you see a wide suckout that is aproximately centered on the desired 1kHz crossover point.
Correctly build the responses adds nicely with no succouts. That is easy to EQ in an active system. The combination is easy to drive. The impedance minimum is 7.7 Ohm but over a very wide range it is much higher.
I measured on the upper midrange axis.


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Your measurements and listening impressions of the dipole Mundorf AMT will be of interest to many dipolers. Do you think you will have the curiousity to build a mid-tweet baffle with dual domes like the LX521 for comparison? The mini-DSP should simplify the work.

If you shock mount your mid-treble baffle above an H-frame dipole woofer your speaker could use 12" dipole woofers in the same volume as Linkwitz's 10" with their support stand.


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No, i do not have a plan to build Linkwitz original but there are some around in Germany so we could arrange a comparison.
Actually i build this in my spare time for my contribution to Frickelfest.
Thanks for the drawing.
I have two woofers that i would like to try.
One is a set of downfiring subs i build for the Enviee baffle last year and one is a cardioide with LAT drivers.
If non of them does not work i will try something similar to LX521.
I have M-Dipols with 30cm woofers but that is a fixed installation.


2009-10-04 12:10 pm
Joachim, very interesting work. I much respect your second opinion about any speaker's performance. I believe many are looking to understand what makes SL's latest design so good; how much is it to his original baffle design and/or special XO objectives for the target radiation pattern, and how much he may have just lucked out on the choice of drivers.

I also hope to learn from your work if SL's bass section can be replaced fairly easily. Bass is room dependent anyways and I like the idea of distributed bass with a couple of woofers concealed around the room. (I also have $$ invested into 4 B&C 12inchers and this is holding me from building the original LX521s :headbash:).

>>only if I could make a small suggestion :): you make it hard for people on the forum to follow in your footsteps when you use proprietary drivers in your designs. do you think you could also try a derivative with something available to the rest of us? (even if your home version comes first to you).

I am also interested in your opinion: would you expect a visaton B200 to be able to make it in the lower mid if crossed to that 4inch SS (I assume we are talking 10F4424; the same you used in mini mpl?).

we are aiming to also try SL's design with slightly higher sensitivity drivers, right?

Danke Vielmals.
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You can always build the LX521 as is. It is one of the great speakers of our time.
You can get the drivers and other supplies easy. Everything is well documented and supported. I do not see a real problem when you add another woofer to the LX521 top.
Concerning my design i do not plan to trade with drivers and i will not disclose what drivers i use in the treble and in the lower midrange. I can only say that this drivers are made to my specification.
The upper mid is obvious a Scan-Speak wideband driver.
What i may offer if there is enough interest, is a complete kit.

Yes the drivers i use have higher sensitivity but that does not help much where it matters : in the lower mid, upper bass. Here excursion is necearry after the formular :
Area times excursion.. So when you use a B200 that is rather limited in throw it does not work.
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