What fun!
Do you plan on wrapping a feedback loop around each woofer? or apply MFB on one woofer and use the MFB derived signal to drive the other woofers.
I missed this comment last night...rather intriguing. As you suggest, if stable and tuneable this would allow use of much larger loopgains than typically possible with MFB systems.
This is one of the big questions a want to investigate. It will save me a lot of work and $$ if one MFB signal proves adequate.
I have ripped the dust cap off on one of my cheap drivers and molded a new one in glass fibre. But the weight landed on 23 grams which I find a bit high since moving mass of the woofer cone is only 65 grams. So I have ordered a lot of carbon fibre sheets and will try making a platform the same way as chriscam showed in post #90. http://www.diyaudio.com/forums/subwoofers/239941-analog-servo-sub-9.html
I plan on making the platform by sandwiching two 1mm carbon-fibre sheets with 10mm high 1mm wide carbon support beams between them for a total weight of about 13 grams. I have no experience with carbon fibre material but I think this will give a very stiff platform with a very high resonance frequency
chriscam: Do you know the resonance frequency of your platform?
I think I had mentioned before that I had good results using MFB from one driver providing a corrected drive signal for two woofers used in a force canceling type arrangement similar to the Genesis G-928 subwoofer. Distortion was essentially identical for both woofers when measured with microphone near each woofer. I seem to recall that Genesis has used one sensor to correct all woofers in an array, but I can't seem to locate the webpage where I read this. I have never tried to use MFB for dipoles though, so will be interested to see your results.
I'm not sure if your local hobby shop would carry this, but I used carbon fiber balsa core laminate for mounting disks. It was very stiff and light, disk for a 2" VC weighed about 6 grams. I don't recall what brand it was, but I used the thinnest sheet they sold.
Here is a link to the type of material I am talking about.
http://dragonplate.com/docs/DPSpecBalsaCore.pdf
If using the output from one sensor to drive multiple woofers, one would of course want to at least make sure all moving masses were equal and loaded in the same way.
Still, would be better to use drivers from a reputable manufacturer known for manufacturing consistency rather than just the cheapest drivers around.
Still, would be better to use drivers from a reputable manufacturer known for manufacturing consistency rather than just the cheapest drivers around.
Yes, all woofers need to be equal and be equally loaded for this to work.
I agree that using better drivers is the best option. No doubt.
But I believe it is better with multiple cheap drivers that one or even two expensive ones. Less x-max = less distortion and more distributed drivers = less room modes.
The ultimate solution to room modes is DBA which is my long term goal.
Double Bass Array ? Wikipedia
Thanks for the link bolserts. Using that material gives me about the same weight as making my own sandwich and saves me the trouble of making it. Expensive though..
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Got to say that the idea of using feedback on one system to control other systems seems kind of conceptually suspect and just doesn't feel right. But I've been trying to think of examples where this is done. In a car engine, you have exhaust status controlling many cylinders although that is usually through a single throttle body; in a modern V engine, you likely have two sensors.
But then multiple drivers, which can be a very effective configuration for woofers or tweeters, is a hybrid idea too.
Ben
But then multiple drivers, which can be a very effective configuration for woofers or tweeters, is a hybrid idea too.
Ben
What I said was not that you should be using expensive drivers, but that at least you should use drivers that are manufactured with good QC so that the effectiveness of a single controller is maximized. Drivers with wildly varying T-S or Klippel specs will not work well if you control them all the same way.
Can you describe what it is that doesn’t feel right to you?
The additional woofers don’t affect the feedback loop.
Do you have any issues with using equalization circuitry upstream of a power amplifier to flatten the response of a non-MFB woofer? If not, you could think of the signal derived from a MFB woofer as the measured equalization required to flatten response(and signal correction to lower distortion). Applying this equalization/correction to additional woofers is conceptually no different than utilizing a Linkwitz transform or other equalization circuits upstream of the power amplifier driving the woofers.
I’ve often wondered if the equalization/correction could be analyzed and defined(via DSP?) as a function of signal frequency and magnitude. If so, you could stick an accelerometer on the woofer dust cap, perform some frequency sweeps at various signal levels and let the DSP number cruncher go to work. Then remove the accelerometer and use the derived function to equalize and lower distortion for any number of woofers(same type, same box size) and avoid stability issues as well.
Hmmmmm....interesting. I can't think of any examples off hand either. Huh.
Well, perhaps using one thermostat in one room to control several HVAC units servicing multiple rooms in a large section of a building. Not the best example, but it is done. It only works well if the thermal/ventilation related properties of each room are similar and the capacity of each HVAC unit is similar.
JOKE (or no joke?)
Gosh, we have all been wasting our time trying to encircle woofer drivers with motional feedback schemes to sharpen transient behaviour, curb over- and under-shoot, and reduce distortion. Here is much simpler proposal. In a thread about loudspeaker state of sophistication on the Multi-Way forum, somebody posted the following information reflecting their grasp of negative feedback theory in their critique of motional feedback.
"Measuring with a mic and correcting with eq or dsp IS a feedback loop. And it's generally adequate as long as you keep the system within it's linear boundaries, you design the system so that it doesn't have issues that can't be corrected with eq or dsp, and you are reasonably aware and proactive of the effects the room has on the system."
Funny thing, perhaps an even simpler approach to reducing harmonic distortion in woofers is by using the "feedback loop" from audio reviews in magazines.
Ben
rainy day in Toronto
Gosh, we have all been wasting our time trying to encircle woofer drivers with motional feedback schemes to sharpen transient behaviour, curb over- and under-shoot, and reduce distortion. Here is much simpler proposal. In a thread about loudspeaker state of sophistication on the Multi-Way forum, somebody posted the following information reflecting their grasp of negative feedback theory in their critique of motional feedback.
"Measuring with a mic and correcting with eq or dsp IS a feedback loop. And it's generally adequate as long as you keep the system within it's linear boundaries, you design the system so that it doesn't have issues that can't be corrected with eq or dsp, and you are reasonably aware and proactive of the effects the room has on the system."
Funny thing, perhaps an even simpler approach to reducing harmonic distortion in woofers is by using the "feedback loop" from audio reviews in magazines.
Ben
rainy day in Toronto
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The only trouble with that idea is that microphones only accurately measure amplitude, not phase. Phase is most definitely part of distortion. To make a mic have correct phase, the distance to the speaker driver would have to be zero.
Quite true. And clearly there's no feasible way with present technology to use a mic for driver acoustic feedback except maybe in some very limited range.
But more erratically, I think this person is saying you can do frequency sweeps with REW and from that sort of a-priori information, you can "model" your woofer and achieve perfection of cone motion.
Wouldn't it be a wonderful world if that is all you needed to do!
Ben
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Not sure what systems you mean?
The old Philips used a cheap/early accelerometer and "modest" drivers and, at the time, seemed like a frustrating effort to get a "quart of soup from a pint bottle." But I have since learned to keep my biased opinion of accelerometers to myself since economic quality accelerometers seem to be showing up and, with MUCH engineering care, looking good.
The Kenwood system, if you are referring to the circuit on the fabulous old Basic series of amps, is very little different than the bridge method of feedback (if you think of the speaker wires as part of the bridge) which I always like. But I didn't think they thought in those terms ever. Actually, almost any amp can be readily re-configured into a "sigma" concept.
Ben
"Not sure what systems you mean?"
I mean has the group resurrecting the Phillips MFB servo compared it to the more modern Sigma Servo system and done listening comparisons...
Both systems have their merits and I was wondering if either was deemed "better". The Sigma Servo is certainly easier but doesn't include driver distortion in it's error correction such as the Phillips.
I mean has the group resurrecting the Phillips MFB servo compared it to the more modern Sigma Servo system and done listening comparisons...
Both systems have their merits and I was wondering if either was deemed "better". The Sigma Servo is certainly easier but doesn't include driver distortion in it's error correction such as the Phillips.
Hi,
Sigma Servo is nothing but a classical global feedback for the amplifier.
Only with the possibilty to choose the point where feedback is sampled.
In other words ... Instead of sampling the output voltage only inside the amplifer, it allows to include the speaker cables into the FB-loop, thereby sampling at the speakers input also.
Onkyo featured similar with their Super Servo drive from Pre to Power Amp.
It might have been of some use with power amps that had to drive very low impedance speakers over long cables.
Most important difference to a MFB is that it does not include the speaker itself into the feedback loop.
jauu
Calvin
Sigma Servo is nothing but a classical global feedback for the amplifier.
Only with the possibilty to choose the point where feedback is sampled.
In other words ... Instead of sampling the output voltage only inside the amplifer, it allows to include the speaker cables into the FB-loop, thereby sampling at the speakers input also.
Onkyo featured similar with their Super Servo drive from Pre to Power Amp.
It might have been of some use with power amps that had to drive very low impedance speakers over long cables.
Most important difference to a MFB is that it does not include the speaker itself into the feedback loop.
jauu
Calvin
The design is meant to be something like the remote-sensing feedback system used on lab power supplies - if that gives some context to some readers. It takes the feedback from the speaker terminals, downstream from the amp.
I think you'll find it can introduce some current feedback and therefore that it is as I described: bridge MFB circuit if you consider the speaker wires (and any added little resistance you need to introduce to get enough feedback) as part of the bridge.
Ben
For those unfamiliar with the Sigma circuit the attached pic will give you and idea of the progression of Kenwood's efforts to wrap more of the amplifier/speaker interface inside the feedback loop to maximize damping factor...which was a big buzz word back in the day. This configuration does not connect the speaker as one leg in a balanced AC bridge for distortion reduction.
While the link remains available, you can download their 24 page brochure on it.
I nice reminder of the amount of technical content some audio companies used to provide.
http://www.cieri.net/Documenti/Kenwood/Sigma-Drive Technical Guide.pdf
While the link remains available, you can download their 24 page brochure on it.
I nice reminder of the amount of technical content some audio companies used to provide.
http://www.cieri.net/Documenti/Kenwood/Sigma-Drive Technical Guide.pdf
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