Determining correct VC depth
- Thread starter 62sunbeam
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The idea for voice coil depth, is for when the speaker is at rest- for almost all speakers- the amount of exposed voice coil winding above the top plate, and the amount of exposed voice coil winding below the top plate, should be equal.
The easy way to do that- is:
1) measure the total height of the winding on the voice coil, and the thickness of the top plate (between the magnet and the woofer frame).
2) Subtract the height of the top plate from the winding height.
3) Divide the difference by two. That's the amount of winding that should be showing above the top plate, when the voice coil is in the correct rest position.
4) Place the voice coil and appropriate shim (inside the coil, between the coil and the pole piece, to hold the coil square in the gap) into the gap, and press it downward until that amount of coil winding is showing.
5) Assemble/glue the spider and cone onto the voice coil and frame, making sure that the coil does NOT shift upward or downward while you are doing this. I sometimes, if the shim doesn't snugly hold the coil tightly, use a piece of masking tape, to tape the inside of the shim to the inside of the voice coil (you will obviously remove this tape once the parts are glued together, so that you can remove the shim, before gluing on the dustcap).
Once you put everything together and let it cure, put in the lead wires, put on the dustcap, and you should be ready to play.
Yes, it's a picky, delicate, exacting process that needs to be done JUST RIGHT, or the speaker will distort or otherwise not work correctly. It's not that hard; you just have to be very careful and pay attention...
Regards,
Gordon.
The easy way to do that- is:
1) measure the total height of the winding on the voice coil, and the thickness of the top plate (between the magnet and the woofer frame).
2) Subtract the height of the top plate from the winding height.
3) Divide the difference by two. That's the amount of winding that should be showing above the top plate, when the voice coil is in the correct rest position.
4) Place the voice coil and appropriate shim (inside the coil, between the coil and the pole piece, to hold the coil square in the gap) into the gap, and press it downward until that amount of coil winding is showing.
5) Assemble/glue the spider and cone onto the voice coil and frame, making sure that the coil does NOT shift upward or downward while you are doing this. I sometimes, if the shim doesn't snugly hold the coil tightly, use a piece of masking tape, to tape the inside of the shim to the inside of the voice coil (you will obviously remove this tape once the parts are glued together, so that you can remove the shim, before gluing on the dustcap).
Once you put everything together and let it cure, put in the lead wires, put on the dustcap, and you should be ready to play.
Yes, it's a picky, delicate, exacting process that needs to be done JUST RIGHT, or the speaker will distort or otherwise not work correctly. It's not that hard; you just have to be very careful and pay attention...
Regards,
Gordon.
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BTW: The amount of coil "showing" above the gap will vary GREATLY depending on what speaker you are working on. Some have almost none showing (and indeed- "underhung" woofers- where the coil is shorter than the top plate thickness, will have the coil completely inside the plate- in those cases, you do the opposite subtraction and division- the goal is, for the coil to be centered vertically inside the top plate), and some will have a lot showing. It completely depends on how long the voice coil winding is, compared the the thickness of the top plate...
Regards,
Gordon.
Regards,
Gordon.
I know this is a very old thread but the guidance is helpful. You answered my question on another thread and I'm unable to source a voice coil that has the same height as the original. In that case I'm guessing the driver won't have the same amount of "throw" but I don't know what that translates to in practice. Would it be SPL or transient response or unable to determine? Im replacing the spider and surrounds as well so I guess it's all a mishmash and impossible to tell what the result will be until done.
Transient response would be very slightly affected (lighter mass of coil, ostensibly), but the biggest difference will be a loss of low-frequency output, before distortion, due to the lessened travel available before the coil starts to leave the magnet plate gap. The lower the frequency, the more excursion is needed to maintain a constant output level- hence, the frequency where max excursion will occur, is greater with a shorter travel length.
Midrange and highs will likely be mostly unaffected- it's just the lowest bass that will suffer first, at higher volumes.
Regards,
Gordon.
