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An Unbiased Comparison of Linear BL Technologies

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I would like to thank Neil for this write up. In no way am i here to make him wrong, but im here to merely add some points from the inside perspective at least about our lms coils.

in general we use 4 layer round wire. Thats the most basic subwoofer coil. We that in our TC9's and TC2's for example.

An liner coil version does not have extra winding at the bottom or top, but rather a liner reduction made in the middle area to reduce BL product where the driver is most efficient. As a result, mass goes down and inductance does too, but the BL product (in particular the L component only) drops on average by a factor of 30 to 35%. The flux (telsa) density in the gap is identical to a non-linear version.

A linear voice coil is a horrible trade off if the driver is under motored, be it LMS or anything else really does need to use a larger motor to gain that back which makes it more expensive. This is in fact why our 4HP motor works so well. The motor is too powerful for any reasonable low frequency driver and it was a prefect candidate for a big LMS. In fact we had a few tight gap versions that pushed past a BL^2/Re of 550-600 and a Qts of ~0.08. Ya... you're not going to make any bass out of that, but at 60Hz tuning SPL burp, hold on tight! The 4" vc allows more steel inside the the t-yoke which prevents saturation to allow us to take advantage of a 10" magnet to get more B. The LMS 18" pushed close to 89dB at 1 watt and we already have plans to reduce nearly 100 more grams and increase the L component of the motor to push it past 90 with ease by next year.

Does it mean LMS is better? In fact, any of the above designs can be used with great success to linearize BL, but its really the implementation that makes the difference. I will say one great advantage of XBL is the sublet space in the motor where a shorting ring can fit. I can argue that inductance has very little affect on the response of an LMS in action below 100Hz, but non the less the shorting ring will improve linearity and make the speaker a better one then if it did not have it. That is something that can not be implemented in the other designs.

As far a tolerances. LMS is actually one of the most accurate. The BL curve is nearly flat and only deviats when the voice coils are offset (too low/high) as a result of a manufacfuring defect. In the case of the 8200, this was the case and its important for me to say that we did not manufacture any of the production units! We only made a few which were prototpyes and we outsouced the rest. There were defects and a major lawsuit ontop of that, that i can not get into. Lets just say we were not happy. In fact, even in a non-linear voice coil, an offset to the coil such as the early versions of the 8200 will a non-linear behavior as such. It was not specific to the linear coil at all. Not to point the finger elsewhere, but I have seen major deviations in XBL designs too where the BL curve was a double hump, rather than flat. Its important that any of the above, including non-linear high sensitivity designs be manufactured correctly.

Edited by 7

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I would like to thank Neil for this write up. In no way am i here to make him wrong, but im here to merely add some points from the inside perspective at least about our lms coils.

And Kyle, thanks for posting. You clearly have the upper hand when it comes to the LMS approach and as you and I have discussed, the LMS-5400 has more than proven it's value in objective testing. I have many reasons to update this and I appreciate your insight. There are just a few things in your post I want to point out.

In fact, any of the above designs can be used with great success to linearize BL, but its really the implementation that makes the difference.

I worry that this thread doesn't state this enough. Just like everything in the audio realm, it is a manner of implementation for a given design goal.

I will say one great advantage of XBL is the sublet space in the motor where a shorting ring can fit.

It seems to me that this is taken advantage of quite frequently by Dan and I think it might actually be a downside. It seems to me that this might be a poor location to put some copper in terms of inductance variation, no?

As far a tolerances. LMS is actually one of the most accurate. The BL curve is nearly flat and only deviats when the voice coils are offset (too low) as a result of a manufacfuring defect. In the case of the 8200, this was the case and its important for me to say that we did not manufacture any of the production units! We only made a few which were prototpyes and we outsouced the rest. There were defects and a major lawsuit ontop of that, that i can not get into. Lets just say we were not happy. In fact, even in a non-linear voice coil, an offset to the coil such as the early versions of the 8200 will a non-linear behavior as such. It was not specific to the linear coil at all. Not to point the finger elsewhere, but I have seen major deviations in XBL designs too where the BL curve was a double hump, rather than flat. Its important that any of the above, including non-linear high sensitivity designs be manufactured correctly.

The ridging is a definite issue in the XBL^2 design that requires due diligence be paid to the motor geometry, specifically the coil length, top plate height, as well as the height and depth of the rebate. Very good insight on the 8200 also and it would be a shame if this was one of the straws that broke the camel's back regarding your relationship with Fujitsu-Ten.

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I am not near as clever as many out there and I have many years of education to go before I am prepared to fully engineer products from beginning to end. There is a possibility, however, that I will be working with a very well known transducer designer on some home audio prototypes (I personally think the car audio market is a really, really tough place to make a buck with any longevity).

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I am not near as clever as many out there and I have many years of education to go before I am prepared to fully engineer products from beginning to end. There is a possibility, however, that I will be working with a very well known transducer designer on some home audio prototypes (I personally think the car audio market is a really, really tough place to make a buck with any longevity).

You are so right on the car audio comment. Home audio is much more proffitable, and from the sound of it yoru more honesty than snake oil and that's rare in that scene.

I also wanted to thank you for this thread. I just learned more about design of a sub than I ever knew before. I love learning so much and audio, something I have a passion about, is one of the most diffocult to grasp. No one spends the time to make it easy, verry old boys club it feels at times, but this broke down allot of good info.

You made this verry easy to understand and even had pictures! we love pictures.

You deserve a standing ovation... or better yet, I'll have a nice lager in your name. I will promptly drink another after that so it's not lonely when I get home.

Thank you sir!

Edited by dem beats

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This technology isn't limited to subwoofers ;)

This is verry true, but for the past couple years I have obsessed with bass. I was trying to decide on how to do a DIY sub for the HT, now I got the truck.... and if I get caught up in thinking about mids and tweets.. Oh man...

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It seems to me that this is taken advantage of quite frequently by Dan and I think it might actually be a downside. It seems to me that this might be a poor location to put some copper in terms of inductance variation, no?

Well obviously i have never made one, its really a simple test. Make one with and make one without and measure the THD%, or even do a non-linear dynamic measurement. I will say copper can't hurt and you can never get enough of it in a motor. If its not taking up space in a critical flux gap, then by all means, I would add it. There are some rare cases where high inductance is actually somewhat good because of its peaky nature around 60Hz. A La car audio SPL competition comes to mind. In fact one of our big pro audio neo drivers has a big inductance peak around 60 and its rather nice for sound reinforcement. But in most case, linear is linear and thats what we shoot for.

The ridging is a definite issue in the XBL^2 design that requires due diligence be paid to the motor geometry, specifically the coil length, top plate height, as well as the height and depth of the rebate. Very good insight on the 8200 also and it would be a shame if this was one of the straws that broke the camel's back regarding your relationship with Fujitsu-Ten.

There were a few straws and that was really the last so and we decided to cease all Eclipse orders just last year. In the end they are going with less expensive products now and its really the wrong business for our products. Just the fact you can get an SW8200 for less than a Type-R mortifies me.

Edited by 7

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so im gonna dig this one back up...

awesome write up! fairly easy to understand from a layman's persepective.

out of curiousity, are there any good informative books on this subject matter?

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Thanks bro, gives me a bit to think about tonite :)

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Good read. There are some variations to those as well such as the split gap/dual motor setup of the Orion H2s.

Any idea what the treatment is on the W7 series?

I will be adding the Differential Drive (aka Dual Gap) technology when I get a chance. Though it does allow for some flexiblity in terms of keeping BL linear, it shows more benefits in power handling by increasing surface area and air flow over the coil substantially.

The W7 is a very simple method: it's a standard overhung design with some machining done in the pole piece. If the right amount is done in the right places, you can achieve surprisingly linear BL, but it becomes increasingly more challenging to achieve this at very high levels of excursion. Of course, I'm of the opinion that a speaker that takes 1kW with 30mm of Xmax is more than enough for my ears.

I think your due for a write up on dual gap motors! :lol:

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Good read. There are some variations to those as well such as the split gap/dual motor setup of the Orion H2s.

Any idea what the treatment is on the W7 series?

I will be adding the Differential Drive (aka Dual Gap) technology when I get a chance. Though it does allow for some flexiblity in terms of keeping BL linear, it shows more benefits in power handling by increasing surface area and air flow over the coil substantially.

The W7 is a very simple method: it's a standard overhung design with some machining done in the pole piece. If the right amount is done in the right places, you can achieve surprisingly linear BL, but it becomes increasingly more challenging to achieve this at very high levels of excursion. Of course, I'm of the opinion that a speaker that takes 1kW with 30mm of Xmax is more than enough for my ears.

I think your due for a write up on dual gap motors! :lol:

JBL does have a white paper on the technology;

http://www.jblpro.com/catalog/support/getfile.aspx?docid=1253&doctype=3

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