Hello All. Thank you for this fascinating and educational thread on MFB. Several posts therein talked about KENWOOD's Sigma Drive and its ability to boost the magnitude of its amps' damping factor [DF]. Another option to boost an amp's DF is via positive feedback [PF]; maybe also via positive current feedback [PCF]. Thus PCF is metered in the target amp in addition to its standard negative feedback loop [NF]. By example the FirstWatt F7 by Nelson Pass.
Is there an application for PCF in systems using MFB? A possible way to add [PCF] is at the power output amp. By example, this will be enabled at LM3886 [U4] in the schematic shown by Armand in post #16.
Best
Anton
Is there an application for PCF in systems using MFB? A possible way to add [PCF] is at the power output amp. By example, this will be enabled at LM3886 [U4] in the schematic shown by Armand in post #16.
Best
Anton
Why do you want to increase the amplifier damping factor? The woofer is servo controlled so it does not improve the frequency response. For distortion reduction it is better to make the damping factor as low as possible, because that means it uses current drive (as opposed to voltage drive) which decreases inductance related woofer distortion.
An example of current drive combined with MFB is the Grimm LS1s-dmf. LS1s-dmf | GrimmAudio
An example of current drive combined with MFB is the Grimm LS1s-dmf. LS1s-dmf | GrimmAudio
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Thank you TBTL for the knowledge in your note, and for its attached link. The white papers therein are dense with valuable info.
The enhancement of the damping factor of a voltage source amp [VSA] is a beneficial effect of positive current feedback [PCF]. Still, [PCF] may have additional and unforeseen effects on a woofer; with and without MFB! Because; the mechanism of increasing an amp's DF via Sigma Drive which uses only negative feeback [NF] is different from that due to [PCF].
My curiosity is the driver to suggest experimenting with PCF in systems using MFB.
Best
Anton
This is over my head, but voltage and current feedback and their role when positive and negative in shaping the amp output is in "amp domain". Somebody should post a 4-way table clarifying their influence in output impedance because that explains a lot in a simple way.
But in "speaker domain", isn't all feedback negative? When looking at motional feedback systems, some feedback signal can arise from variations in current (or related cone motion) and some from voltage (or related to cone motion).
B.
But in "speaker domain", isn't all feedback negative? When looking at motional feedback systems, some feedback signal can arise from variations in current (or related cone motion) and some from voltage (or related to cone motion).
B.
I'm 5 years late to this thread but if anyone still cares here are a few things a learned about servo feedback subwoofers while working at Velodyne in the 1990's. Stiffness of the cone and connection to the VC are very important. The woofers I worked on had heavy, stiff paper cones. There was a flat aluminum ring sitting above the voice coil that created a very stiff connection between the VC and the cone. The accelerometer was mounted on top of the aluminum ring. The accelerometer itself was made in house in an air-tight housing. Loop gain was around 20db (i think...memory failing) Overall distortion at 20Hz was below 1% at full output. The hard part was designing a limiter that prevented the amp from ever clipping (I didn't do that) because without a limiter clipping sounds horrible due to the feedback. I still think these were some of the finest subs ever made and I still use my ULD-15.
Hello All. I have this question. Will a subwoofer using MFB and placed in the corner of a listening room suppress/attenuate standing waves [SWs]?
Please consult US 4,899,387 by inventor Nelson S. Pass which is entitled Active Low Frequency Acoustic Resonance Suppressor. He articulated very well the sonic effect of SWs in the Background Art section [page 1 lines 10-64].
Best
Anton
Please consult US 4,899,387 by inventor Nelson S. Pass which is entitled Active Low Frequency Acoustic Resonance Suppressor. He articulated very well the sonic effect of SWs in the Background Art section [page 1 lines 10-64].
Best
Anton
Wonderful to hear from an insider. Hope you can provide more insights to all of us.
You, me, and nearly everyone who has tried MF speak extremely favourable to its benefits, even if one distinguished person of my acquaintance didn't measure big THD improvements. Strange to read posters making fabulous mental and carpenter efforts with 6th-order boxes to gain some tiny advantage from a sub when MF is so promising.
Question: why did they use in-house built accelerometers? I see plans to DIY accelerometers using little bits of ceramics. But for maybe $80, an amateur can buy the famous ACH-01 which has audiophile-level low distortion and linearity.
B.
No. MFB controls cone motion. The coupling between sound and cone motion is weak so standing waves do not significantly affect cone motion.
If you use an externally mounted microphone as a sensor instead of the more common piezo accelerometer, it might suppress standing waves a bit. There are however better feedback techniques if you want to build an active bass trap.
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Hello Baldin, TBTL, and bentoronto. Thank you for your replies. The patent US 4,899,387 [Nelson Pass inventor] teaches an example of an active bass trap.
I've been interested in this subject for a long time. As an example, I have a recent post in the thread "DEF Amp" in the Pass Labs Forum. I discuss in it and others therein systems under study. Please peruse it and this other thread in the Pass Labs Forum which is entitled "DIY the device of US 4,899,387". It also tackles a similar subject matter.
imho, positive current feedbackl [PCF] and MBF have a potential to work well together.
Best
Anton
I've been interested in this subject for a long time. As an example, I have a recent post in the thread "DEF Amp" in the Pass Labs Forum. I discuss in it and others therein systems under study. Please peruse it and this other thread in the Pass Labs Forum which is entitled "DIY the device of US 4,899,387". It also tackles a similar subject matter.
imho, positive current feedbackl [PCF] and MBF have a potential to work well together.
Best
Anton
Interesting. I don't know for sure but I think sound pressures created by the standing wave would be too small to affect the servo sub but...It could be tested by using a sine wave at the standing wave frequency and seeing if there is a difference in the accelerometer signal when the sub is placed in the corner as opposed to the center of the room. Might try it sometime.
Back in the 1990's small accelerometers were not very good and expensive. Now days they seem to be in everything and are cheap.
Hello Speaker Dude. Thanks for your post and suggested valuable experiment. I looked at the schematic of ULD-15 from Hifi Engine. It has a highly detailed and robust power amp which drives the loudspeaker in bridge mode.
It is possible to inject any power amp with positive current feedback [PCF]. But what sonic value will PCF add to it and to the current line of Velodyne's great performers?
Best
Anton
It is possible to inject any power amp with positive current feedback [PCF]. But what sonic value will PCF add to it and to the current line of Velodyne's great performers?
Best
Anton
I have not studied active bass traps, so I cannot suggest other methods. I know it's a bit bold to make such a statement that x is not good without suggesting better y. So here is the explanation why x is not optimal. If you hugely simplify a room to a one dimensional thing, forcing zero sound pressure on one side of the room makes that side of the room act like a pressure release surface, so it will reflect (though with a 180 degree phase shift) sound instead of absorbing it.
That's it. That's amazing (and maybe similar to Pass's). $2000/each.
Odd they should say it makes no sound: doesn't it vibrate a driver to counter the pressure, even if you can't hear it? There are all kinds of noise fighters (long ago, power plants installed them to cut down noise for neighbours). So does this make a negative wave or what?
One of the remarkable things about MFB (that we love to show our friends... and is a sort of demo that the MFB concept really works) is the cone is rock hard. That is, when you push on the cone*, the cone and amp push right back at you. IT'S ALIVE. Here, I guess what we are seeing is the opposite: when the pressure hits the corner, the cone "crumbles" into softness. May not be as apparent with this absorber because "pushing" is very low frequency and might be below the range of the AVAA device.
Most absorbers work on eating velocity and so a corner is bad. But this works on eating pressure in a corner.
They use a mic but that is pretty unfeasible for MFB. But here, for reasons I am to dim-witted to produce right now, a mic must make good sense.
Is there anything about this system that couldn't have been implemented 50 years ago?
And thanks, Speaker Dude, for your posts. More? As a product, MFB has always been hard to commercialize because of instability, crazy owners, house pets, over-driving, and other out-of-bounds circumstances that destroy commercial systems. Any insights on that topic?
B.
*you pushing on the cone is an erroneous signal, eh, so it pushes back
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Hi,
upping this thread again ;-)
Analog Devices offers new MEMS accelerometers which look rather promising at first glance as their noise is by orders lower and their bandwidth considerably higher than the former generation of MEMS accelerometers.
Is there any reason why these shouldn´t be useable for subwoofer applications?
jauu
Calvin
upping this thread again ;-)
Analog Devices offers new MEMS accelerometers which look rather promising at first glance as their noise is by orders lower and their bandwidth considerably higher than the former generation of MEMS accelerometers.
Is there any reason why these shouldn´t be useable for subwoofer applications?
jauu
Calvin
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