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Old 5th March 2013, 12:30 PM   #21
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About MF: I still have a pair of the old Philips MF-3-ways somewhere, and before one channel gave up the gost, I must say they have good bass. The reason I feel MF has never really picked up is because the correction you want it to apply is very predictable; it is the usual 12 dB/octave downward slope you see below the -3dB point. The approach taken by Linkwitz to correct for this is much more sensible in my opinion.
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Old 5th March 2013, 12:51 PM   #22
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You can of course correct for more than just frequency response deviations with "real" motional feedback: You can also iron out nonlinear distortion.

The Sony sub under discussion has a current feedback loop added. With this trick, depending on whether one uses positive or negative feedback, it is possible to lower or rise Qts. If you make the current feedback frequency dependant you can even alter other TSPs as well. This was commercially used in the so called "ACE bass" by the Swedish company Sonab.

What Sony did we can't tell from the little piece of schematic that was posted. It is also difficult to say whether the design is well thought-out. It could in fact be anything form bad but loud to good and maybe not so loud.

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Old 5th March 2013, 01:33 PM   #23
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Great opportunity.

If you can "listen" to the system with a mic and analysis software, so much the better. It is amazing what you can do with a laptop and an old Radio Shack sound-level meter these days. I'm sure curious about how the output across the tuned range sounds in a small BR box with MF!!!

Short of opening the box and shorting the .22-ohm resistor, I don't offhand think you can do much of an A-B test of the effect of the MF. Wish you could.

Try pushing the cone. MF does this weird thing: the cone feels stiff when you give it a sharp push (because the MF fights back against your hand... depending on the frequency capabilities of the feedback loop, of course). A BR cone (without MF) is floppy.

Ben
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Old 5th March 2013, 01:33 PM   #24
tb46 is offline tb46  United States
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Hi Y'all,

Yes, I think ACE Bass or K.E. Stahl (Pat No. 4118600) would apply here, and bjorno has done a lot of work on this subject.

As to the schematic, the service manual is very detailed, and is available here:
Sony SA-W2500 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
(or, PM me).

Regards,
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Old 6th March 2013, 04:29 AM   #25
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Quote:
Originally Posted by audiomagnate View Post
Velodyne used to use accelerometers. I have an ancient ULD15 that works that way, but I don't see anything in the literature for this sub that says anything about motion feedback.
Since then Velodyne still use accelerometers in their servo subwoofers. It's still the most accurate way of implementing a motional feedback system
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Old 6th March 2013, 04:57 AM   #26
jamikl is offline jamikl  Australia
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Awhile ago I was given two Sony SA ED1A speakers which are described as Active MFB and contain one 4"speaker in each box. PDF manuals for this model are available free online and have the circuit diagram.
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Old 13th March 2013, 09:54 PM   #27
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Originally Posted by bentoronto View Post
But a ported box with motional feedback is an oxymoron.
I wonder if motional feedback has as little enthusiasm on this forum as I sense it does, because it demands skills both in mechanical and electronic fields.

Ben
Hey Ben,

Thought you might be interested in this:

http://www.powersoft-audio.com/en/co...tml?Itemid=111

The differential controlled IPAL system can make even a ported box track the input signal quite closely.
Ain't going to be available at Wall Mart though ...
Expensive toys!

Art
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Last edited by weltersys; 13th March 2013 at 09:57 PM.
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Old 13th March 2013, 10:47 PM   #28
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Thanks for link. Interesting pressure tranducer... something like the technology for air mass measurement in automobile fuel-injection throttle bodies.

Anybody who looks at those fabulous tone-burst traces (pages 12-23 from the link) has to shout, "I gotta have a motional feedback woofer." There is no other way to control a woofer cone like that.

Ben
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Old 14th March 2013, 08:05 AM   #29
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Quote:
Originally Posted by weltersys View Post
Hey Ben,

Thought you might be interested in this:

http://www.powersoft-audio.com/en/co...tml?Itemid=111

The differential controlled IPAL system can make even a ported box track the input signal quite closely.
Ain't going to be available at Wall Mart though ...
Expensive toys!

Art
Great stuff! I want one.
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Old 14th March 2013, 09:08 PM   #30
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OK... let me comment on the article link from Weltersys.

There are two components to the article (not written in English clear enough for my full grasp). The first has to do DSP modeling of something or other - looks interesting but over my head. This relates, I think, to shaping the input signal to the driver.

The second has to do with MF using a transducer that compares pressure (1) inside the cabinet with that (2) near the front of the driver, as I understand it. They claim their system can be used with any driver; they use an 18 inch driver in a BR cabinet for testing in the article.

Mic results with a four-cycle tone burst at 5 frequencies associated with the BR tuning are shown. The astonishing results are similar to my own tests years ago. As I said before, one look at the improvement due to MF and a serious fan of woofing will want to rush to DIY. I'd like to see anybody seriously deny those results look immensely impressive.

I was wrong to say MF in a BR environment is an oxymoron - it really is only half-peculiar. As the traces show, there are monumental improvements to a tone burst response as far as listening-by-eyeball can reveal. No doubt this improvement would be a great leap forward in any acoustic system including OB (freq corrected in the usual ways) and BR (which is ordinarily beyond easy correction).

The problem for me in attempting BR using voice-coil based MF is that the sound output as you pass through the broad resonant frequency region is that there is an orderly but convoluted relation between speaker sound output and cone motion. That is because at some frequencies, the port adds to sound (upper bump) or subtracts from sound (lower bump) or just holds the cone steady so it doesn't go booooom too much (system resonance). Go figure.

What is especially peculiar to me, pending learning more from this forum, is how you can take the dramatic changes in pressure inside a BR as you pass through the hills-and-valleys of resonance of a BR (as described above) and make the speaker output uniform? I think it is fair to say they authors are "shy" about the freq curve.

While we all know about the output of a BR box (as described above), I have little knowledge of what the pressures inside the box are doing.... let alone how the comparison of front versus interior pressures can lead to a flat response.

On the other hand, it is easy enough to understand how any kind of MF gizmo can result in the fabulous tone-burst results shown in the article, even in a BR cabinet.

Footnote: the MF literature has a hidden agenda that makes the article a little weird. That "agenda" has to do with claims to support patenting. Each author wants to show his/her system is novel, different, solves new problems, etc. and otherwise distinct from earlier efforts. Including this article.

Ben
PS - I am curious about that Walmart $99 speaker... but I am not thinking about replacing my Klipschorn and giant OB with the little Sony box, even if it has MF.
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Last edited by bentoronto; 14th March 2013 at 09:21 PM.
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