Old thread here: http://www.diyaudio.com/forums/multi-way/132777-flux-modulation.html
So we know there are a few ways to reduce flux modulation. Shorting ring around the base of the pole piece, copper right inside the coil gap, copper on the top and/or bottom of the pole piece extension, etc.
http://www.diy-audio.narod.ru/litr/FaradayRingsVoiceCoilImpedance.pdf
I recently (re-)discovered Eighteen Sound's brilliant AIC technology:
http://www.eighteensound.com/static...118th_AES_Barcelona_inductance_minimizing.pdf
There is a coil wired in parallel, wound in reverse (I assume), fixed on the pole piece inside the coil gap. The induced flux from the fixed coil counteracts the induced flux from the voice coil. It seems perfect... Is it?
One of the benefits of AIC is that sensitivity increases slightly from a higher BL as the forces from the coils work sympathetically. However, the further away the voice coil gets from rest position, the less help the fixed coil can give. The other issue is that flux modulation in Le(i) and BL(i) vary based on excursion due to poorer coupling.
How do we fix this? Split the fixed coil into two sections a la SplitGap/XBL^2, and even better, make the voice coil gap with that technology as well. See here: https://lh6.googleusercontent.com/-...I/AAAAAAAAAlk/qKfhENvhcIE/w740-h986-no/13+-+1
Anyway, this is all just conjecture without seeing real data from something like the 12NDA520. It does seem like the perfect way to eliminate flux modulation, though it wouldn't hurt to throw in a shorting ring around the base just in case.
By the way, someone tell me again why we don't use electrically conductive voice coil formers? Wouldn't that be just as good as copper on the pole piece?
So we know there are a few ways to reduce flux modulation. Shorting ring around the base of the pole piece, copper right inside the coil gap, copper on the top and/or bottom of the pole piece extension, etc.
http://www.diy-audio.narod.ru/litr/FaradayRingsVoiceCoilImpedance.pdf
I recently (re-)discovered Eighteen Sound's brilliant AIC technology:
http://www.eighteensound.com/static...118th_AES_Barcelona_inductance_minimizing.pdf
There is a coil wired in parallel, wound in reverse (I assume), fixed on the pole piece inside the coil gap. The induced flux from the fixed coil counteracts the induced flux from the voice coil. It seems perfect... Is it?
One of the benefits of AIC is that sensitivity increases slightly from a higher BL as the forces from the coils work sympathetically. However, the further away the voice coil gets from rest position, the less help the fixed coil can give. The other issue is that flux modulation in Le(i) and BL(i) vary based on excursion due to poorer coupling.
How do we fix this? Split the fixed coil into two sections a la SplitGap/XBL^2, and even better, make the voice coil gap with that technology as well. See here: https://lh6.googleusercontent.com/-...I/AAAAAAAAAlk/qKfhENvhcIE/w740-h986-no/13+-+1
Anyway, this is all just conjecture without seeing real data from something like the 12NDA520. It does seem like the perfect way to eliminate flux modulation, though it wouldn't hurt to throw in a shorting ring around the base just in case.
By the way, someone tell me again why we don't use electrically conductive voice coil formers? Wouldn't that be just as good as copper on the pole piece?
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Aluminium formers exist all over the place, from cheap cheap to expensive drivers. Due to the VC and the former moving in unison there is very little compensation and we wouldn't want it. Having the VC buck against it's former to which it is firmly secured to would be akin to welding the drive shaft to the chassis. < Extreme example for sure 😉 Also note that even when AL is used it is split eg it will not short the flux. Also Faraday rings et al do nothing for bass, they are only effective at much higher midrange frequencies.
They do use aluminum formers because they are very conducive thermally BUT normally they are split or have a small gap so that they are NOT a shorted turn on the moving system which is not normally desirable. A shorted turn in the magnetic gap is a good thing as its counter emf prevents the lines of flux from moving (as much) when “pushed against” by the voice coil current.
While the best place to put that shirted turn is IN the same gap as the VC, that is also the most costly place to put it if you have a fixed flux as a target, Cubic volume in the gap X B field in the gap X fudge factor = cost of magnet structure.
Best,
Tom
While the best place to put that shirted turn is IN the same gap as the VC, that is also the most costly place to put it if you have a fixed flux as a target, Cubic volume in the gap X B field in the gap X fudge factor = cost of magnet structure.
Best,
Tom
Hi Bill
I hope your doing well, I have an e-mail I never finished to you, maybe over Christmas holiday I can finish it and finally send it.
I think you and tinitus hit on some areas worth elaborating on since my first answer left many holes and “the magnet” is an interesting system which can be seen like an electric circuit.
In electricity we have Voltage and Current in a magnetic circuit we have the B or filed intensity or flux density and H the magnetizing force.
Air or Vacuum has a permeability or ability to conduct magnetic flux of 1, iron can have a permeability into the thousands meaning when presented with a given magnetizing force H, it has a flux density thousands of times higher than in air. One could say that higher permeability is something like lower resistance conductors are in an electrical circuit and is why we use iron to conduct the flux to the gap.
A coil with current passing through it in place of a magnet can accomplish the same thing as a magnet and if that coil has a magnetizing force proportional to the number of turns times the current with the magnetic path factored in. Thus a field coil with a current source is a purely constant H force (and often, the Rdc of the coil and Voltage source becomes in effect a constant current source).
The Voice coil is also a source of H, it has current and turns in the gap and so this H is on top of the constant force in the magnet. When it pushes against the magnetic flux the fly’s fall into the simple soup.
So, we have a magnetic gap, it has a thickness and surface area and like Voltage and Current, H and B, that area and thickness has what one can think of as magnetic impedance.
A magnet like ceramic or alnico has what you could think of as a magnetic impedance curve or BH product. The total load on the magnet is the leakage loss (lost flux not in the gap) and the load the gap presents. A magnet itself is ideally the right combination of magnet thickness AND area so that when connected to the gap (area and thickness) and leakage losses, the magnet is operating near or at the peak of it’s load line or “BH product”.
With magnets like ceramic and alnico, a large enough voice coil H force can shove the magnet’s operating point away from that maximum product and that modulates the field strength in the gap. Alnico magnets are rather fragile this way and so the way they made recharging them less of a problem was to use a large magnet volume than normal and deliberately increase the flux density at the pole tip to the point of saturation (where the irons permeability falls sharply and becomes more like air so far as magnetic conductivity).
Then when a powerful H force is presented by the VC, it only modulates a small portion of the saturated iron at the tip near the VC up and down relative to saturated. The magnet is left isolated from that de-magnetizing VC H by the small amount of iron always saturated. Another minor curiosity, iron doesn’t have a straight line BH curve, it has little steps in it. So, not only can one modulate the flux up and down with voice coil current, it can do it in little steps. Here too an under hung coil would have a vastly smaller issue with this stuff as only part of the air gap has conductor in it.
So if you’re picturing this in the static condition, there are a few more issues when dealing with an AC signal like with loudspeakers.
The voice coil pushes and pulls against the lines of flux, in a magnet, those lines are more or less pinned to the surface but in iron, they can move around and in fact only their unwillingness to be concentrated next to each other is when they arrange themselves fairly evenly within. On the other hand, the H or push from the VC current can move the lines around and to the degree they move, the VC doesn’t,
The iron top plate and iron center pole are electrically conductive and if one moves the fluc up and down with a VC, that moving flux produces a Voltage in the top plate and center pole. Those parts are conductive and so current flows because of that Voltage induced by transformer coupling.
That Current flowing in those parts produces it’s own magnetic field and interestingly, it is in opposition to the field that is inducing the current. That effect is also called eddy current or a voltage induced in a large conductor which because of it’s “short circuit” nature opposes the magnetic field. If you have a small Neo magnet lying around, try dropping it into a normal copper water pipe (around here most water pipes are copper) and see a jaw dropping demo of eddy current flux pinning.
Anyway, iron is lame compared to aluminum and copper twice as good as aluminum. While neither is magnetically conductive, if you have the space for some saturated iron, you can hog out the gap a little bit more and put a copper sleeve on either or both the in or outside.
At high frequencies, the lines of flux the voice coil produces follow a tighter and tighter path and react less and less with the magnet’s magnetic flux. Electrically this is the series L which rolls off the response above Rmin in the impedance curve. With a copper sleeve (shorted turn) in very close proximity to the voice coil this will oppose the inducing field just as the top plate does at lower frequencies and so can reduce the Voice coils series inductance and roll off which can extend the hf corner –or- let you use a larger VC with more turns which would have otherwise imposed a lower inductive corner.
Anyway, I am rambling and will sign off.
Best,
Tom
I hope your doing well, I have an e-mail I never finished to you, maybe over Christmas holiday I can finish it and finally send it.
I think you and tinitus hit on some areas worth elaborating on since my first answer left many holes and “the magnet” is an interesting system which can be seen like an electric circuit.
In electricity we have Voltage and Current in a magnetic circuit we have the B or filed intensity or flux density and H the magnetizing force.
Air or Vacuum has a permeability or ability to conduct magnetic flux of 1, iron can have a permeability into the thousands meaning when presented with a given magnetizing force H, it has a flux density thousands of times higher than in air. One could say that higher permeability is something like lower resistance conductors are in an electrical circuit and is why we use iron to conduct the flux to the gap.
A coil with current passing through it in place of a magnet can accomplish the same thing as a magnet and if that coil has a magnetizing force proportional to the number of turns times the current with the magnetic path factored in. Thus a field coil with a current source is a purely constant H force (and often, the Rdc of the coil and Voltage source becomes in effect a constant current source).
The Voice coil is also a source of H, it has current and turns in the gap and so this H is on top of the constant force in the magnet. When it pushes against the magnetic flux the fly’s fall into the simple soup.
So, we have a magnetic gap, it has a thickness and surface area and like Voltage and Current, H and B, that area and thickness has what one can think of as magnetic impedance.
A magnet like ceramic or alnico has what you could think of as a magnetic impedance curve or BH product. The total load on the magnet is the leakage loss (lost flux not in the gap) and the load the gap presents. A magnet itself is ideally the right combination of magnet thickness AND area so that when connected to the gap (area and thickness) and leakage losses, the magnet is operating near or at the peak of it’s load line or “BH product”.
With magnets like ceramic and alnico, a large enough voice coil H force can shove the magnet’s operating point away from that maximum product and that modulates the field strength in the gap. Alnico magnets are rather fragile this way and so the way they made recharging them less of a problem was to use a large magnet volume than normal and deliberately increase the flux density at the pole tip to the point of saturation (where the irons permeability falls sharply and becomes more like air so far as magnetic conductivity).
Then when a powerful H force is presented by the VC, it only modulates a small portion of the saturated iron at the tip near the VC up and down relative to saturated. The magnet is left isolated from that de-magnetizing VC H by the small amount of iron always saturated. Another minor curiosity, iron doesn’t have a straight line BH curve, it has little steps in it. So, not only can one modulate the flux up and down with voice coil current, it can do it in little steps. Here too an under hung coil would have a vastly smaller issue with this stuff as only part of the air gap has conductor in it.
So if you’re picturing this in the static condition, there are a few more issues when dealing with an AC signal like with loudspeakers.
The voice coil pushes and pulls against the lines of flux, in a magnet, those lines are more or less pinned to the surface but in iron, they can move around and in fact only their unwillingness to be concentrated next to each other is when they arrange themselves fairly evenly within. On the other hand, the H or push from the VC current can move the lines around and to the degree they move, the VC doesn’t,
The iron top plate and iron center pole are electrically conductive and if one moves the fluc up and down with a VC, that moving flux produces a Voltage in the top plate and center pole. Those parts are conductive and so current flows because of that Voltage induced by transformer coupling.
That Current flowing in those parts produces it’s own magnetic field and interestingly, it is in opposition to the field that is inducing the current. That effect is also called eddy current or a voltage induced in a large conductor which because of it’s “short circuit” nature opposes the magnetic field. If you have a small Neo magnet lying around, try dropping it into a normal copper water pipe (around here most water pipes are copper) and see a jaw dropping demo of eddy current flux pinning.
Anyway, iron is lame compared to aluminum and copper twice as good as aluminum. While neither is magnetically conductive, if you have the space for some saturated iron, you can hog out the gap a little bit more and put a copper sleeve on either or both the in or outside.
At high frequencies, the lines of flux the voice coil produces follow a tighter and tighter path and react less and less with the magnet’s magnetic flux. Electrically this is the series L which rolls off the response above Rmin in the impedance curve. With a copper sleeve (shorted turn) in very close proximity to the voice coil this will oppose the inducing field just as the top plate does at lower frequencies and so can reduce the Voice coils series inductance and roll off which can extend the hf corner –or- let you use a larger VC with more turns which would have otherwise imposed a lower inductive corner.
Anyway, I am rambling and will sign off.
Best,
Tom
they could've used colored pictures in the fem results 😛 and the images too small to see values.
Even if there is an improvement with a shorted VC instead of ring the complexity of manufacturing is increased by a large factor for now either you make the top of the central pole piece removable (which is not good for magnetic circuit) to fit the coil or you will have a process to wind the coil on the pole piece.
What i know and used is high field strength, tight gap and as less copper in the VC as possible. If i add a shorting ring in the gap i extended either above the central pole piece and add width or below the gap and add width as its effect depends on coupling (position and distances) and DC resistance of the ring (thickness the bigger the better).
Even if there is an improvement with a shorted VC instead of ring the complexity of manufacturing is increased by a large factor for now either you make the top of the central pole piece removable (which is not good for magnetic circuit) to fit the coil or you will have a process to wind the coil on the pole piece.
What i know and used is high field strength, tight gap and as less copper in the VC as possible. If i add a shorting ring in the gap i extended either above the central pole piece and add width or below the gap and add width as its effect depends on coupling (position and distances) and DC resistance of the ring (thickness the bigger the better).
Electromagnets do behave better because you can rule out the effects of magnetic materials in presence of external magnetizing forces.
Yes, manufacturing complexity is increased, but it isn't just a shorted coil. The whole point is that the fixed coil is driven in parallel.
Hi
I guess there are 2 questions ,
one experiment for the second qs , why not use a copper or al (shorted) vc former .
A) take any "loose suspension" driver / woofer press or lay a weight on it , measure the "x" traveled / feel the pressure required to move the cone . B) Remove the weight C) Short the speaker terminal (Voice coil now acts as a shorted Vc former) D) Move the driver as earlier ---- A thin Vc shorted Vc former will have a similar effect though the effect will be lower than shorting out the Vc
the first qs how to stabilize Ac flux (over a large x) ,
A) Underhung motor structures with suitable flux modulating rings stabilizes the "transformer effects on a vc" over most of its travel
B) Purchase Dan Wiggens software for stabilizing Ac flux in drivers
C) One could try the Accelerometer Dsp as shown by Tranquility Bass in the Dsp Servo Sub thread .
Toms post is a very good read for detailed theoretical perspective presented in a easy to picture way .
Suranjan
I guess there are 2 questions ,
one experiment for the second qs , why not use a copper or al (shorted) vc former .
A) take any "loose suspension" driver / woofer press or lay a weight on it , measure the "x" traveled / feel the pressure required to move the cone . B) Remove the weight C) Short the speaker terminal (Voice coil now acts as a shorted Vc former) D) Move the driver as earlier ---- A thin Vc shorted Vc former will have a similar effect though the effect will be lower than shorting out the Vc
the first qs how to stabilize Ac flux (over a large x) ,
A) Underhung motor structures with suitable flux modulating rings stabilizes the "transformer effects on a vc" over most of its travel
B) Purchase Dan Wiggens software for stabilizing Ac flux in drivers
C) One could try the Accelerometer Dsp as shown by Tranquility Bass in the Dsp Servo Sub thread .
Toms post is a very good read for detailed theoretical perspective presented in a easy to picture way .
Suranjan
Hi
Use of a saturated pole tip will not reduce / address the flux modulation effects in an Ac field as "The transformer or in other words the induced back emf on the steel parts by the Vc is not shorted out or shielded out to reduce interference .
Suranjan
Use of a saturated pole tip will not reduce / address the flux modulation effects in an Ac field as "The transformer or in other words the induced back emf on the steel parts by the Vc is not shorted out or shielded out to reduce interference .
Suranjan
Hi Guys
454Casull, I had one of those drivers at one point but from what I recall, the main advantage was it reduced the series L. Depending what one wanted to do with the driver, that governed if it was an advantage or not. Since nearly everything I deal with at work are horns of one sort or another, the driver didn’t get used where I had in mind. It worked by taking some amplifier signal and adding an opposing field to the voice coil which was free to move.
In the old days, I used to work at a company called Intersonics and used to work on acoustic levitation sound sources and the very first one was a St Clair sound source which used a centerpole wound coil and shorted turn on the end of a resonant bar. In this case, it was the induced current and it’s opposing magnetic force that “drove” the bar at resonance. Later I developed my own sound source which was much more efficient and powerful. These live on today in laboratories;
http://www.matsdev.com/sitebuildercontent/sitebuilderfiles/levad4.pdf
Acoustic levitation - YouTube
After the space shuttle disaster and the company went under, I went back into audio full time;
Live Sound: In Profile: Tom Danley, Exploring The Possibilities Of Audio Technology - Pro Sound Web
Hentai is correct, an electromagnet has a magnetizing force (H) related to the number of turns and the current flowing through the wire. As such, the BH curve is a straight line unlike the BH curves for most materials which have a “knee” and so also have an ideal operating load point. Also many magnetic materials have a BH curve which if examined closely is more like a very large number of steps or increments and neither a “knee” or steps in a transfer function curve are good signs.
http://www.magnesy.com.pl/cms.php?img=5
Neo magnets (most of them) are a duck of a somewhat different feather, they also have a straight line BH curve and pretty much act like a constant H magnetizing source.
http://www.magna-tokyo.com/eng/service/img/tokusei/BH-Curve2.gif
Hunter, your shorted VC is exactly why a shorted turn on the moving system is not normally used. When connected to an amplifier (which also appears to be a short circuit (with a Voltage superimposed if producing a signal) from the loudspeakers perspective, that same damping of motion is mostly the Qe (the cumulative part of the loudspeaker. With an open circuit, if you tap on the cone, if will damp out much more slowly at a rate defined by the Qm and acoustic radiation. Linearity is cost, loudspeaker drivers have to be cheap (as a rule) a huge deep under hung magnet is big, heavy and expensive (smaller if it can be made with neo) but operationally ideal.
To me, the closed loop servo offers a double edged sword solution, most of the ones I have played with were pretty easy to get stable up to 10 or 15 dB off feedback but very quickly one faces the fact that you are adding the properties of the feedback sensor to your system. They have their own noise level, frequency limits, distortion, maximum signal levels and so on and your drivers response is seen through those eyes. Dealing with the abrupt end of linear motion is not easy to deal with gracefully either. With loudspeakers one sees that the bad things they do (free sound, not part of the input signal) increases faster than the real signal with increasing level and so to me it makes a case for the old adage “headroom is your friend” and fixing the problem at the source before it’s radiated vs fixing it (in one location) with DSP afterwards..
To be clear there are several interconnected things going on. The saturated pole tips help to stabilize the magnet’s operating point with magnet materials like Alnico, they could be partially demagnetized by changing the load point enough (with a VC) or especially if the magnet is physically removed from the magnetic circuit. In the old days, one would use “keepers” to short the magnet out (magnetically) before moving or removing it.
Part B is stray flux and the effect of the voice coil’s series inductance and both of these sum to act like an electrical low pass filter. In a magnet, the lines of flux are more or less pinned to the surface where they emerge but in other materials like air or iron, the lines of flux distribute themselves according to the need to find the shortest path length, while not wanting to be close together if that makes sense. To the degree the VC current can push them around, to that degree the cone didn’t move
Iron is also electrically conductive BUT it isn’t as good as copper or aluminum and as a stationary shorted turn anchored to the magnet already resists the motion of the flux some.
A copper stationary shorted turn in the magnet gap helps pin the lines of flux even more and electrically it reduces the series L extending the hf response. Since the lines of flux can form loops without interacting with the magnetic flux (such as the turns which are above and below the gap in an overhung coil) putting a shorted turn around the entire center pole (and or outside as well) will help reduce leakage flux that as well. The problem with all that is one is depending on transformer coupling where it’s own leakage flux increases dramatically as the distance between the primary and secondary increase (which leads to mulitfilar winding in regular transformers).
Best,
Tom
454Casull, I had one of those drivers at one point but from what I recall, the main advantage was it reduced the series L. Depending what one wanted to do with the driver, that governed if it was an advantage or not. Since nearly everything I deal with at work are horns of one sort or another, the driver didn’t get used where I had in mind. It worked by taking some amplifier signal and adding an opposing field to the voice coil which was free to move.
In the old days, I used to work at a company called Intersonics and used to work on acoustic levitation sound sources and the very first one was a St Clair sound source which used a centerpole wound coil and shorted turn on the end of a resonant bar. In this case, it was the induced current and it’s opposing magnetic force that “drove” the bar at resonance. Later I developed my own sound source which was much more efficient and powerful. These live on today in laboratories;
http://www.matsdev.com/sitebuildercontent/sitebuilderfiles/levad4.pdf
Acoustic levitation - YouTube
After the space shuttle disaster and the company went under, I went back into audio full time;
Live Sound: In Profile: Tom Danley, Exploring The Possibilities Of Audio Technology - Pro Sound Web
Hentai is correct, an electromagnet has a magnetizing force (H) related to the number of turns and the current flowing through the wire. As such, the BH curve is a straight line unlike the BH curves for most materials which have a “knee” and so also have an ideal operating load point. Also many magnetic materials have a BH curve which if examined closely is more like a very large number of steps or increments and neither a “knee” or steps in a transfer function curve are good signs.
http://www.magnesy.com.pl/cms.php?img=5
Neo magnets (most of them) are a duck of a somewhat different feather, they also have a straight line BH curve and pretty much act like a constant H magnetizing source.
http://www.magna-tokyo.com/eng/service/img/tokusei/BH-Curve2.gif
Hunter, your shorted VC is exactly why a shorted turn on the moving system is not normally used. When connected to an amplifier (which also appears to be a short circuit (with a Voltage superimposed if producing a signal) from the loudspeakers perspective, that same damping of motion is mostly the Qe (the cumulative part of the loudspeaker. With an open circuit, if you tap on the cone, if will damp out much more slowly at a rate defined by the Qm and acoustic radiation. Linearity is cost, loudspeaker drivers have to be cheap (as a rule) a huge deep under hung magnet is big, heavy and expensive (smaller if it can be made with neo) but operationally ideal.
To me, the closed loop servo offers a double edged sword solution, most of the ones I have played with were pretty easy to get stable up to 10 or 15 dB off feedback but very quickly one faces the fact that you are adding the properties of the feedback sensor to your system. They have their own noise level, frequency limits, distortion, maximum signal levels and so on and your drivers response is seen through those eyes. Dealing with the abrupt end of linear motion is not easy to deal with gracefully either. With loudspeakers one sees that the bad things they do (free sound, not part of the input signal) increases faster than the real signal with increasing level and so to me it makes a case for the old adage “headroom is your friend” and fixing the problem at the source before it’s radiated vs fixing it (in one location) with DSP afterwards..
To be clear there are several interconnected things going on. The saturated pole tips help to stabilize the magnet’s operating point with magnet materials like Alnico, they could be partially demagnetized by changing the load point enough (with a VC) or especially if the magnet is physically removed from the magnetic circuit. In the old days, one would use “keepers” to short the magnet out (magnetically) before moving or removing it.
Part B is stray flux and the effect of the voice coil’s series inductance and both of these sum to act like an electrical low pass filter. In a magnet, the lines of flux are more or less pinned to the surface where they emerge but in other materials like air or iron, the lines of flux distribute themselves according to the need to find the shortest path length, while not wanting to be close together if that makes sense. To the degree the VC current can push them around, to that degree the cone didn’t move
Iron is also electrically conductive BUT it isn’t as good as copper or aluminum and as a stationary shorted turn anchored to the magnet already resists the motion of the flux some.
A copper stationary shorted turn in the magnet gap helps pin the lines of flux even more and electrically it reduces the series L extending the hf response. Since the lines of flux can form loops without interacting with the magnetic flux (such as the turns which are above and below the gap in an overhung coil) putting a shorted turn around the entire center pole (and or outside as well) will help reduce leakage flux that as well. The problem with all that is one is depending on transformer coupling where it’s own leakage flux increases dramatically as the distance between the primary and secondary increase (which leads to mulitfilar winding in regular transformers).
Best,
Tom
Tom , you are right there are several cause and effects in a magnetic system , attempting to summarize them
DC Field Non Linearity
AC Field (When loudspeaker is in operation and Vc engineered)
AA) Eddy Current and secondary field development from the direction of current flowing BB) Voice coil inductance increasing when traveling towards the back plate and reducing when traveling out of the pole piece
CC) Secondary field caused by Eddy current on the pole tips imposing a back emf on the Vc
DD) Lines of force in pole plates shift , increasing fringe field interfering / reducing concentration of field at the magnetic gap
EE) The amount of DD and the ability of the permanent magnet system to over come it .
some solutions to the above , DC Field - Sfg , AA) Shorting ring around the pole and or copper cap to cover the pole BB) Pole Sleeve , Magnet ID sleeve and Pole cap or Pole shorting ring . CC) Shielding by pole sleeve the most suited DD) Hf drivers copper shorting cap underneath the Vc shorting the pole and top plate EE) BH curve selection of magnet , Fea
Thanks for the info on servo correction with or wo dsp , it makes sense .
I did not understand the following
and
A full pole sleeve of about 2mm shows better results over magnet ID sleeve and Pole cap
Suranjan
DC Field Non Linearity
AC Field (When loudspeaker is in operation and Vc engineered)
AA) Eddy Current and secondary field development from the direction of current flowing BB) Voice coil inductance increasing when traveling towards the back plate and reducing when traveling out of the pole piece
CC) Secondary field caused by Eddy current on the pole tips imposing a back emf on the Vc
DD) Lines of force in pole plates shift , increasing fringe field interfering / reducing concentration of field at the magnetic gap
EE) The amount of DD and the ability of the permanent magnet system to over come it .
some solutions to the above , DC Field - Sfg , AA) Shorting ring around the pole and or copper cap to cover the pole BB) Pole Sleeve , Magnet ID sleeve and Pole cap or Pole shorting ring . CC) Shielding by pole sleeve the most suited DD) Hf drivers copper shorting cap underneath the Vc shorting the pole and top plate EE) BH curve selection of magnet , Fea
Thanks for the info on servo correction with or wo dsp , it makes sense .
I did not understand the following
and
A full pole sleeve of about 2mm shows better results over magnet ID sleeve and Pole cap
Suranjan
Sorry for the necro of this old thread, but does anybody have any info on how the thickness of a pole sleeve correlates with its effect on different frequencies? e.g. 80% of the maximum inductance control (diminishing returns) can be attained with a 1 mm sleeve at 500 Hz, etc.
There are no rules of thumb for this as there are many factors involved. Electroplating the pole piece is not very effective as the layer will be very thin. There will normally be a significant difference between using a 0,1mm and a 0,2mm copper sleve. Sometimes one will need to go up to 0,5mm or more to get an effective copper sleve. This is many, many times thicker than electroplating.
I did read this thread quickly, however, it seems like the gentleman starting the thread overlooked the fact that the corrective coil allways will provice force in one direction, regardless of polarity. In order to avoid this, it is crucial that the corrective coil does not provide a field that is stronger than just what is needed to keep the pole from modulating.
I did read this thread quickly, however, it seems like the gentleman starting the thread overlooked the fact that the corrective coil allways will provice force in one direction, regardless of polarity. In order to avoid this, it is crucial that the corrective coil does not provide a field that is stronger than just what is needed to keep the pole from modulating.
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