Re-Coners Question... Does the voice coil winding height matter?

[guitician]

Hey, I just bought some parts from WeberVST, and the voice coils are the right diameter but the winding height is more than the originals. The dead speakers are an old G12m-70 and the other a P12R type. They are both about 1/8" more wind than original. The P12R coil is two rows of wind, but the original has three, which would explain less spread, but the gauge of wire on the VST one is a lot finer. The gauge of the G12M coil is the same as original. My question is... how much of a difference will these coils make in the performance over the originals? I bought cones and spiders too, which have there differences, but none that concern me as much as the coils. Thanks

[jrfrond]

Both of those speakers have OEM recone kits available. How come you didn't go with those?

No matter. More wind on the coil equals longer possible excursion, by the same amount as the difference in wind height.

[guitician]

OEM kits would work if they were $15ea. And, I don't need anything exact, just for the speaker to work again. I measured the windings with calipers, and the P12R coils are .1" longer and the G12M is .05", not much, and the P12R coils wire looks to be wound with the same gauge wire. One thing that I now noticed about the G12M coil, as I placed it over dial of my calipers, is that it isn't round! It's a Kapton coil, can I get it round somehow? Do these go "out-of-round" easily? I remember accidentally dropping it to the floor from about 2 feet. Thanks

[guitician]

I got it round. It just needed to be flexed a little, using the 1.5" diameter dial on my calipers as a guide. Phew

[gbono]

Was the magnet structure designed for "overhung" or "underhung" VC placement? I bet this is a speaker design with not a lot of Xmax? You may end up with a "non-linear" motor action = distortion if you have a over hung VC in a under hung magnet circuit. Probably not an issue for MI.

[cbarrow7625]

As well as the other things mentikned, a longer-than-intended voice coil in the same gap with about the same resistance / impedance will have a different sensitivity than the orginal. If the original was an overhung or equal height voice coil, a new longer but similar voice coil will have a lower sensitivity (i.e., it will have lower output with the same original drive signal) but will be more linear as it moves throught the magnetic gap. If the orignal was an underhung design then chances are the new coil will increase the sensitivity while makjng it more non-linear.

In short, yes it matters. Which way it will affect your speaker can't be determined without knowing more about your speaker magnet and design. Weight also matters. Additional weight vs. the original will also reduce sensitivity

[Steve Conner]

I remember seeing some guitar speaker with a quoted Xmax of 0. Does that mean that it's neither overhung nor underhung? If the voice coil is the same height as the gap, then any movement at all will cause nonlinearity.

[cbarrow7625]

That would be an equal height voice coil. Xmax is not technically related solely to whether the voice coil is in the gap the entire time; it also has to take into account physical limitations. Stating xmax of 0 sounds like an odd claim to make.

Non-linearity really refers to making sure that the forward and rearward "swing" of the cone / voice coil is the same in both directions of movement. Just because only half of the voice coil is in the gap on the positive swing does not automatically make it non-linear. If it reacts the same way in the other direction, it may be very linear. Of course, the amount of voice coil in the gap does not tell you if the driver is linear or not, there are still physical considerations to take into account.

I was just trying to give a simple explanation of why the voice coil height difference does matter. I plead guilty to over-simplification for demonstration purposes!

[guitician]

OK, the P12R's VC's winding originally started at the top of the gap, like shown here, but the coil is wider than the gap's plate and extends down past it about 1/16". I weighed the new cone and voice coils compared to the old one and they are lighter, which may offset some loss of efficiency due to the coils spread of two winds vs. three. I was thinking of setting the gap in the middle of the coil because of the shape of the lines of flux in the gap. Keeping the most flux where it will do the most good as the cone moves.

[cbarrow7625]

There is non sense putting it in there offset if you can get it centered in the gap.

[guitician]

I'm thinking now that the lines may not be even (top-to-bottom) above and below the gap. Because the pole has more surface on the bottom of the plates gap to attract flux lines to. Which would explain why they originally positioned it that way. Not shown in the illustration is a cup between the pole piece and the alnico magnet that seals off the area from the outside... and I'm guessing centers the pole in the gap.