Two techniques that I've used..
First is playing a low freq tone/music with a decent bass beat,and carefully aligning the cone so that the voice coil doesn't rub,while the glue is still wet/tacky.You'll have to slide the edge of the foam around on the basket to get it just right,and then hold it there..this can get tricky,and you've only got one shot at it. It is possible though.
Second is to remove the dust cap,and use some business card stock,or something of a similar thickness,and wedge it in between the voice coil and pole piece,to keep it centered.This is a common way of doing it,and is preferred over the method above.
First is playing a low freq tone/music with a decent bass beat,and carefully aligning the cone so that the voice coil doesn't rub,while the glue is still wet/tacky.You'll have to slide the edge of the foam around on the basket to get it just right,and then hold it there..this can get tricky,and you've only got one shot at it. It is possible though.
Second is to remove the dust cap,and use some business card stock,or something of a similar thickness,and wedge it in between the voice coil and pole piece,to keep it centered.This is a common way of doing it,and is preferred over the method above.
Not sure what your saying there, phivates. Foam surrounds are an optimal solution with acoustic suspension designs. They gently hold the cone in place while the air in the sealed box does the control. A rubber surround is less efficient in this application, and foam can sound very good.
For all that, It's a crying shame old foams rot so badly and create alignment problems and sagging. Acoustic Research never quite solved getting the surround soft enough either for optimal bass with heavy cones. Don't think I'd touch them with a bargepole myself. Happily surrounds are better these days in general.
typo
I meant "better than."
Now I can attempt to refoam some drivers that don't want the cap removed, in particular a pair of Technics flat diaphragm woofs I'm curious about, since the mids and tweets are also flat. The tweeter has a radial grid in front that resembles a phase plug sort of. Plane wave source maybe?
There's a guy in Eugene who says he's built a vc centering box but he won't say anything about it other than it speeds up the process a bunch
I meant "better than."
Now I can attempt to refoam some drivers that don't want the cap removed, in particular a pair of Technics flat diaphragm woofs I'm curious about, since the mids and tweets are also flat. The tweeter has a radial grid in front that resembles a phase plug sort of. Plane wave source maybe?
There's a guy in Eugene who says he's built a vc centering box but he won't say anything about it other than it speeds up the process a bunch
Avoiding the rub
It may be a little late,but I have a handy method of doing the centering thing on the Dyn's.
You know the 'windows' on the basket that are covered with that thin stretchy felt; well, if you remove that (I don't think it has any bearing on the sound anyway), you can gain access to the motor system with the cone/spider/surround installed. Simply put the required thickness shims into the V.C. gap, and tape them to the basket lower window (I use glossy type catalog paper). So, essentially you are gapping the outer face of the coil with the shims. When you drop the coil assy. in, the leading bottom edge of the coil pushes the shims into place. When you do a trial run to see if you've selected the right thickness shim, you can then use your preferred method of attachment. After the goo dries, simply remove tape and shims. BTW, I use Aleens Tack it Over and Over goo, it's excellent. You can always re-cover the thin felt later (if you really think you need to).
P.S. This method suits 24W75XL drivers, not all baskets are the same...
It may be a little late,but I have a handy method of doing the centering thing on the Dyn's.
You know the 'windows' on the basket that are covered with that thin stretchy felt; well, if you remove that (I don't think it has any bearing on the sound anyway), you can gain access to the motor system with the cone/spider/surround installed. Simply put the required thickness shims into the V.C. gap, and tape them to the basket lower window (I use glossy type catalog paper). So, essentially you are gapping the outer face of the coil with the shims. When you drop the coil assy. in, the leading bottom edge of the coil pushes the shims into place. When you do a trial run to see if you've selected the right thickness shim, you can then use your preferred method of attachment. After the goo dries, simply remove tape and shims. BTW, I use Aleens Tack it Over and Over goo, it's excellent. You can always re-cover the thin felt later (if you really think you need to)
.P.S. This method suits 24W75XL drivers, not all baskets are the same...
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This thread has my immediate attention.. and I'm in trouble.. I think
I need some real world science and also, the voice of experience.
I recently purchased a set of beautiful KEF 105/3 reference speakers. Being a qualified electrical systems engineer, I set about the normal upgrade process you migt imagine:- New foams, new OFC cabling, silver solder in place of silly connectors and also, a few replaced caps in the x/over.
All seemed well except that clearly the previous owner had tried to get bass out of the system when the surrounds were shot and had mashed the edge of the alu voice coil. So, time for a repair.
Anyhow, I have just had the Unit (B200 SP1256) speaker returned from "the best in the country" re-winder. All appeared well until I realised that the voice-coil former had been glued into the spider mis-aligned.
:-(
i.e. the direction of alignment is a little off axis compared to the vertical motion of the cone.
The result ?
Well, in all of its travel, it does NOT rub - at all.
One side of the cone now sits higher than the other.
The difference is visible.. perhaps 1-2mm - at most.
Of course, the suspension at both edge and centre (i.e. the donut) is now slightly mis-aligned - whether this affects linearity, is open to question (I imagine that this mis-alignment might somehow stiffen the compliance as we are forcing the suspension to move unequally or non concentrically)
In my minds eye, one side of the VC is now slightly closer to the periphery of the airgap (AG) and the other is closer. So the net flux effect is probably zero or thereabouts. When the motive fore is applied, the (now misaligned) coil will attempt to right itself into a null position and also to drive forward. The vector sum will be that the motive force is reduced.
In effect, the N-S axis is now misaligned, causing an off centre force. However, the spider, cone surround and (uniquely to the B200 SP1256) do-nut centre suspension will all try to align the force in a perpendicular direction.
If this were mid range, I think it would be 100% "no thanks"... but as this is bass we are talking about.... perhaps there is room for this tolerance ?
My rewinder says "no problem" (he would though, wouldnt he !)
My foam surround supplier agrees..
But as an engineer, I know its wrong. It's just "how wrong" that matters now, and "what real world / practical / audible effect".
FYI, the 105's have 2 bass units working in anti-phase, each venting into a central chamber (cavity coupled bass). Perhaps this alone can help ?
More comments and (science based / experience based) feedback welcome !!!
THANKS !
My dad used to say:- "a blind man would be glad to see it".. but given the nature of the beast (2 drivers in anti-phase, lower registers only, no fouling of the air-gap) am I really in as much trouble as It looks. ?
I'm an engineer... if this is going to be a show stopper, I'd rather return to the rewinders and get them to do a proper job
What would you do ?
EDIT: - please see below, the answer from the repair company:
I need some real world science and also, the voice of experience.
I recently purchased a set of beautiful KEF 105/3 reference speakers. Being a qualified electrical systems engineer, I set about the normal upgrade process you migt imagine:- New foams, new OFC cabling, silver solder in place of silly connectors and also, a few replaced caps in the x/over.
All seemed well except that clearly the previous owner had tried to get bass out of the system when the surrounds were shot and had mashed the edge of the alu voice coil. So, time for a repair.
Anyhow, I have just had the Unit (B200 SP1256) speaker returned from "the best in the country" re-winder. All appeared well until I realised that the voice-coil former had been glued into the spider mis-aligned.
:-(
i.e. the direction of alignment is a little off axis compared to the vertical motion of the cone.
The result ?
Well, in all of its travel, it does NOT rub - at all.
One side of the cone now sits higher than the other.
The difference is visible.. perhaps 1-2mm - at most.
Of course, the suspension at both edge and centre (i.e. the donut) is now slightly mis-aligned - whether this affects linearity, is open to question (I imagine that this mis-alignment might somehow stiffen the compliance as we are forcing the suspension to move unequally or non concentrically)
In my minds eye, one side of the VC is now slightly closer to the periphery of the airgap (AG) and the other is closer. So the net flux effect is probably zero or thereabouts. When the motive fore is applied, the (now misaligned) coil will attempt to right itself into a null position and also to drive forward. The vector sum will be that the motive force is reduced.
In effect, the N-S axis is now misaligned, causing an off centre force. However, the spider, cone surround and (uniquely to the B200 SP1256) do-nut centre suspension will all try to align the force in a perpendicular direction.
If this were mid range, I think it would be 100% "no thanks"... but as this is bass we are talking about.... perhaps there is room for this tolerance ?
My rewinder says "no problem" (he would though, wouldnt he !)
My foam surround supplier agrees..
But as an engineer, I know its wrong. It's just "how wrong" that matters now, and "what real world / practical / audible effect".
FYI, the 105's have 2 bass units working in anti-phase, each venting into a central chamber (cavity coupled bass). Perhaps this alone can help ?
More comments and (science based / experience based) feedback welcome !!!
THANKS !
My dad used to say:- "a blind man would be glad to see it".. but given the nature of the beast (2 drivers in anti-phase, lower registers only, no fouling of the air-gap) am I really in as much trouble as It looks. ?
I'm an engineer... if this is going to be a show stopper, I'd rather return to the rewinders and get them to do a proper job
What would you do ?
EDIT: - please see below, the answer from the repair company:
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I found this thread looking for "voice coil alignment". There is another axis to consider. My underlying concern is for centering an overhung voice coil within the gap. Klippel can describe it better than I:
http://www.klippel.de/uploads/media/AN_01_Optimal_Voice_Coil_Rest_Position.pdf
I'll be assembling a JBL 2235 cone, surround, spider and a longer flat wound voice coil into a 2226 frame. This concept comes from GordonW over on audiokharma:
Building a JBL 15" "Super Sub" driver on the relative cheap! - AudioKarma.org Home Audio Stereo Discussion Forums
The surround is supplier mounted. I'll be taking some measurements as the bobbin must be shortened to get the coil centered. My plan is to consider the gap field to be symmetric, based on JBL's claim's. Any input otherwise from your experiences would be welcome.
Zero has mail
http://www.klippel.de/uploads/media/AN_01_Optimal_Voice_Coil_Rest_Position.pdf
I'll be assembling a JBL 2235 cone, surround, spider and a longer flat wound voice coil into a 2226 frame. This concept comes from GordonW over on audiokharma:
Building a JBL 15" "Super Sub" driver on the relative cheap! - AudioKarma.org Home Audio Stereo Discussion Forums
The surround is supplier mounted. I'll be taking some measurements as the bobbin must be shortened to get the coil centered. My plan is to consider the gap field to be symmetric, based on JBL's claim's. Any input otherwise from your experiences would be welcome.
Zero has mail
@ Ed - many thanks for the information. Since I posted, I discovered (from the person that actually did the work, and not the person who wrote the mail to me) that the CV is indeed aligned using a combination of HF and DC to optimise the airgap. It seems some of the formers onto which the cone is fixed is in fact higher on one side than the other !
in other words, its only the cone that is on mis-aligned.. the VC is perfectly centred (or as much as it ever can be in this imperfect world !)
Thanks again
in other words, its only the cone that is on mis-aligned.. the VC is perfectly centred (or as much as it ever can be in this imperfect world !)
Thanks again
I have refoamed many woofers, as well as recones and voice coil replacements. For surround replacement, I dont even boter removing the dustcap or applying signal to it, I just sit it down and glue the surround on. I glue the inside first and let it dry, and then glue the outside. Then give the cone a gently push in and out with my hands to check for rubbing, if there is any rubbing then I just shift the surrounds position very slightly before the glue has fully set and thats about it. And then I always run a sine wave through it at various frequencies to check for rubbing or any buzzes. If I can't get it right then I will remove the dustcap but its not generally necessary.
Tapping the cone around the edge can tell you if the coil is off-centre in the gap, if you tap one edge of the cone and get a scrape and tap the opposite edge and get nothing, then the coil is off centre towards the side where tapping the edge caused it not to scrape.
Most speakers I have refoamed have not needed any adjustment after gluing, and when they did just simply moving the outer edge of the surround while the glue was still soft has been enough to fix it, with no buzzing or rattles even when driven beyond xmax.
Tapping the cone around the edge can tell you if the coil is off-centre in the gap, if you tap one edge of the cone and get a scrape and tap the opposite edge and get nothing, then the coil is off centre towards the side where tapping the edge caused it not to scrape.
Most speakers I have refoamed have not needed any adjustment after gluing, and when they did just simply moving the outer edge of the surround while the glue was still soft has been enough to fix it, with no buzzing or rattles even when driven beyond xmax.
Thanks for sharing your technique, i wanna try it out with the DC generator.
I'm scared to damage the voice coil due to bad heat dissipation, though.
I mean... We are talking about some deciVolts, right? So, it's not a high current indeed. But i was wondering how much time i can give a DC signal without being scared to burn the voice coil...
Here is an interesting idea for centering a voice coil. This was found in a patent (attached) assigned to Harman Industries no less. I'm afraid it's only of academic interest because it requires a rig that independently holds the frame and voice coil in the optimum position while gluing... once that position has been determined.
But, it was the method of finding that optimum position which intrigued me.
The proposal was to use capacitance measured between the frame and the coil. The coil position resulting in minimum capacitance corresponds to the ideal lateral position of the coil.
It appears that their assembly rig allows precise and repeatable movement of the coil in an XY/horizontal plane... and that they record the capacitance along both X and Y (within the constraints imposed by the gap). Knowing this capacitance profile the rig can then return the coil to the point of minimum capacitance on both the X and Y axis.
With enough data they can detect out-of-round and tilted coils also.
But, it was the method of finding that optimum position which intrigued me.
The proposal was to use capacitance measured between the frame and the coil. The coil position resulting in minimum capacitance corresponds to the ideal lateral position of the coil.
It appears that their assembly rig allows precise and repeatable movement of the coil in an XY/horizontal plane... and that they record the capacitance along both X and Y (within the constraints imposed by the gap). Knowing this capacitance profile the rig can then return the coil to the point of minimum capacitance on both the X and Y axis.
With enough data they can detect out-of-round and tilted coils also.
I know how Dynaudio does it. They use some very stiff steel wire to center the voice coil. When you have a look at a typical one-piece Dynaudio cone you can see some tiny holes where the voice coil sits that are filled with some drops of glue or varnish after the assembly is finished and the wires removed.
Regards
Charles
Regards
Charles
Yes, assuming the spider has not been damaged first glue the surround to the cone. No need to be too precise because getting the centring right is the next step. The surround is then resting unglued on the frame.
Note (or with masking tape mark) four "corners" of the assembly. With the surround shoved off-centre in each direction, push on the dust cap lightly and then further pin-point the spot just where the rubbing peaks when the assembly is urged in that direction the least. Split the difference and glue the surround to the frame so as to keep away from the wrong spot.
Done carefully - or even measuring with a ruler how much off-centring leads to rubbing in exactly what direction - this method can be quite precise.
At the worst, you may have to trim the surround on one side a bit with a scissors.
Easy to overlook the importance of having both the spider and surround "at rest" with no signal. Otherwise, they aren't operating in neutral at near-zero signal levels (in fact, at all levels). The surround has to rest gently on the cone when the spider is at rest when gluing. Fs will be off and distortion will rise.
B.
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Scanspeak Revelator surround replacement?
I am replacing the surrounds on some Scanspeak Revelator 7" mid/woofers & have a few questions. I plan to use the DC voltage or 50Hz sinewave method to help center the VC. The question is about the gluing process. These surrounds fit tightly into the rim of the frame with very, if any, wiggle room for final alignment. If I am very careful when gluing the cone would this lack of rim adjustability be a concern? On one hand I can see that this situation may benefit the accuracy of my initial gluing of the cone & on the other hand.... well it may make it more difficult. Any thoughts are appreciated. Also, when using a regulated power supply what voltage should I set it to & should I worry about limiting the current? Power handling specs state: 100h RMS noise test = 60W & Long term Max Power = 70W
Also x2: Is the DC voltage as effective at aligning the VC or is it just a convenience for lifting the cone into position? Guess & could use both to offset the cone while still having it travel in the gap. My main concern tho, is the amount of DC to apply for the time it takes to complete the job.
Thanks,
Kev
I am replacing the surrounds on some Scanspeak Revelator 7" mid/woofers & have a few questions. I plan to use the DC voltage or 50Hz sinewave method to help center the VC. The question is about the gluing process. These surrounds fit tightly into the rim of the frame with very, if any, wiggle room for final alignment. If I am very careful when gluing the cone would this lack of rim adjustability be a concern? On one hand I can see that this situation may benefit the accuracy of my initial gluing of the cone & on the other hand.... well it may make it more difficult. Any thoughts are appreciated. Also, when using a regulated power supply what voltage should I set it to & should I worry about limiting the current? Power handling specs state: 100h RMS noise test = 60W & Long term Max Power = 70W
Also x2: Is the DC voltage as effective at aligning the VC or is it just a convenience for lifting the cone into position? Guess & could use both to offset the cone while still having it travel in the gap. My main concern tho, is the amount of DC to apply for the time it takes to complete the job.
Thanks,
Kev