Commercial motional feedback woofer available sort of

Sony SA-W2500, saw it at Walmart for $100 but Sony says it is discontinued. This is a small box, presumably sealed, with a 10 inch "mica reinforced" cone and "motion feedback" with a 100-watt amp. Nobody seems to bother to mention basic specs like input, cross-over, controls.... and basic necessary information like that.

It seems to have the kind of voice-coil bobbin sensor coil (tactically massaged) that I believe is best way to get feedback signal. Seems to fulfill one of the oldest target goals of motional feedback R&D (going back to JAES 1954): making a smallish box do remarkable woofing.

I haven't yet found a meaningful review. But the Amazon review crowd is quite ga-ga as to value.

Anybody know about this woofer or Sony's efforts along those lines?

Ben
 
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Svendingo

Member
2006-11-22 1:37 am
So am I understanding correctly that this is some form of a servo "corrected" woofer?

What's to stop someone from redesigning the cabinet if it is a one note wonder; to achieve a more reasonable f/r?
That is a good deal for a not so simple circuit to implement well as DIY.
 
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.

I haven't asked Walmart to open the box to let me check it out. But a ported box with motional feedback is an oxymoron. Of course, theAnonymous1's post does seem to have an authoritative and definitive ring to it, so who am I to doubt him or her.

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

About Sony, they have sometimes produced HiFi gear of the first order (a famous turntable, for example). But maybe not any mass-produced speakers... for $79 on Amazon.

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
 
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So am I understanding correctly that this is some form of a servo "corrected" woofer?

What's to stop someone from redesigning the cabinet if it is a one note wonder; to achieve a more reasonable f/r?
That is a good deal for a not so simple circuit to implement well as DIY.
Right. A prime function of motional feedback is to "eat" the resonance.

Ordinarily, there are few frequency-correcting tricks in MF to worry about changing the box. In theory they are not needed. So putting the little driver into a big box ought to achieve a lot.

However, a crucial element of MF is the substantial frequency correction for radiation resistance (which is miraculously achieved by other drivers kind of automatically if accidentally, on a good day). MF deprives a driver of this automatic accidental correction so it has to supply it back using electronic correction.

But a 10 inch driver is under the thumb of decreasing radiation resistance at ALL frequencies of interest. So any correction in the Sony amp is already appropriate for it in any box.

Ben
 
I haven't asked Walmart to open the box to let me check it out. But a ported box with motional feedback is an oxymoron. Of course, theAnonymous1's post does seem to have an authoritative and definitive ring to it, so who am I to doubt him or her.

My apologies, what I though was a port must just be a Canadian cup holder.

[IMGDEAD]http://i50.tinypic.com/2zfkcc1.jpg[/IMGDEAD]
 
bentoronto,
What you may be missing is that in a real motional feedback speaker system is that the speaker itself is part of the electrical circuit usually. It can have a second voicecoil that is sensed or it can be reading the back emf output to derive the correction circuit. I doubt that if you opened this speaker up you would find any of that. It is more than likely just a marketing catch phrase for the dollar amount you are talking about. And yes Sony has been known for some very good sounding gear, but it is as I know under the Sony "ES" branded label only. Check those out if you like that brand, the normal Sony consumer product is nothing special that I have ever seen.
 

tb46

Member
2006-01-09 7:04 pm
Texas
Hi bentoronto,

Post #1: "...It seems to have the kind of voice-coil bobbin sensor coil (tactically massaged) that I believe is best way to get feedback signal...."

It looks like the "feedback signal" is a voltage from a small value resistor in the ground line of the speaker drive (also, it seems to be a vented design). I found the schematic here: http://www.diyaudio.com/forums/solid-state/229841-sa-w2500-sub-blowing-fuse.html . I'll attach a detailed view:

Regards,
 

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"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....
 
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.
 
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
 
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
 
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
 
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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