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Old 2nd March 2013, 11:00 PM   #11
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Default "Feedback" or transformer?

The first subwoofer with "server" I remember working with was in the early '70s, the Infinity Server-Statik. It always looked like a transformer to me.

At one point, Phillips built a powered speaker with an accelerometer in the dust-cap of the woofer. That sounded pretty good, and was a concept I approved of. Using a coil wound with a voice-coil still sounds more like a transformer than a feedback sensor to me....
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Old 2nd March 2013, 11:07 PM   #12
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jplesset,
If I remember correctly some form of positional sensor or input differential circuits were used when this was first proposed but it was a patented application and never caught on. I have seen it where there was a second coil in a separate gap used for location and measuring displacement was used. An accelerometer would be used in a differential circuit also I would think looking at the difference of direction between the cone travel and the sine wave input. I haven't read those papers in so long, they were included in one of the AES compendiums on loudspeakers.
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Old 2nd March 2013, 11:25 PM   #13
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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.
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Old 2nd March 2013, 11:35 PM   #14
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I would think that because of where they are sourcing the electrical signal that they are measuring the back emf voltage given off by the voicecoil moving in the gap.
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Old 2nd March 2013, 11:43 PM   #15
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Quote:
Originally Posted by theAnonymous1 View Post
It's definitely ported. I've never been impressed by any Sony audio product and this thing just looks like it sounds bad. Boomy mass-market junk.
Until theAnonymous1 posts a picture we can read or writes that the picture he or she has posted is an SA-W2500, I'll stick with exactly what I replied before:

"Just doesn't make much sense from the theory. Of course, doesn't mean some really dumb company wouldn't do it."

Ben
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Old 3rd March 2013, 12:04 AM   #16
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Thanks to all for largely sensible discussion.

Before starting the thread here, I read a thread at an A/V forum that quoted a discussion that seemed to me to indicate a special coil on the VC. That is a sensible approach for Sony/consumer in the various ways that matter to a consumer business. Extra coil, current feedback, or Wheatstone Bridge, can be very cheap to implement in a powered speaker and the benefits of MF are very impressive. Sorry if wrong and I mis-posted. As I said before, one of the R&D goals of MF is to produce great bass from small boxes.

I can't read the schematic kindly supplied by tb46 and MF schematics can be deceiving (sometimes intentionally so). But using what is essentially current feedback to simulate motion feedback is an old and quite useful approach (although I used a Wheatstone Bridge in the MF amps I built). A mega-bit copy would be nice or using PMs to get them from you. BTW, some of the first Kenwood super-amps had trick connections that, in effect, used the speaker wires for current feedback... as did a number of amps in ancient times when narrow bandwidths made feedback a less perilous undertaking than today.

MF in a ported tuned box is totally screwball, but yet still within the territory of HiFi screwballland. Anyone with an understanding of the perverse relation between cone motion and sound output in a BR will understand why. Of course, maybe that wouldn't stop Sony. Even if MF in a BR is actually counter-productive in the bass region, it has benefits everywhere else in the compass. And it is possible, the tuning is nothing like the usual TS-BR theory and it does work beneficially in some unusual way with MF.

Ben
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Old 3rd March 2013, 04:43 PM   #17
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tb46 kindly sent me detailed information. Thank you.

No question, there is a tube running from the back, as in theAnonymous1's photo, to near the front. Ported box it is... but not sure if it is a traditional tuning or set low, kind of like a "small leak" design which is a very good sub design.

Yes, motional (current) feedback from across a .22-ohm resistor in the negative leg (as tb46 already said in his post) which is fed differentially through some frequency adjustment, to an op-amp just for the purpose. I'm not sure if this is as a bridge or not.

Glad to have all this great help, esp. tb46. My curiosity is sated, although I'm still puzzled over how you can make MF work with BR.... or why Sony thought so.

BTW, seems this series of units reviewed pretty well but the auto-on circuitry was so flawed owners were annoyed (and stayed away - my 2-cents).

Ben
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Old 3rd March 2013, 05:44 PM   #18
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They have them in stock at my local Walmart. I'm tempted to pick one up just to see what it sounds like.
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Old 3rd March 2013, 10:45 PM   #19
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Quote:
Originally Posted by audiomagnate View Post
They have them in stock at my local Walmart. I'm tempted to pick one up just to see what it sounds like.
I saw a few reviews at Amazon but could draw no solid conclusions. One person's "boom" is another person's "great bass."

A good DIY person can adjust inputs and outputs to get the auto-turn-on to work right and to balance loudness against the main speakers.

When I researched MF at Bell Labs long ago (world's largest anechoic chamber at the time), I often relied on brief pulses (not that an anechoic chamber is needed to test with pulses). Dramatic difference between a speaker's pulse output with and without MF in terms of both initial blast and in reduction of after-burbles. Exactly as you'd expect.

The difference was dramatic, both by ear and on the Polaroid pictures we took off oscilloscopes, the best image technology of the time. And audio sonograms.

That quality should be apparent in a resonant box even if it has a weird low-bass response, assuming the Sony has enough MF to matter.

With all the heat at this forum about off-beat stuff like tapped horns and finite baffles, you'd think a coherent logic like MF would be a keen interest.

Ben
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Last edited by bentoronto; 3rd March 2013 at 10:50 PM.
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Old 5th March 2013, 12:12 PM   #20
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I told a friend about this sub and he just texted me that he bought one. I'll try to give it a listen/help him place it in the next day or so. I sold him a pair of JBL L1s a few months ago, so if it's decent his system should sound pretty good.
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