Centering rings in small format drivers

[Russellc]

Well in a way, I was also wondering about the 800 series, and you have answered that question, none of the 800 series had the centering rings. I did not know that! I just assumed that some did and some didnt. Your explaination of the way the two different series drivers magnet structures are situated was informative as well. Thanks also for giving me a way to check for the existence of such rings in the various 900 series drivers.

regards,
Russellc

[bfish]

Big time my bad! Sorry for the misleading info I posted, and thanks to Jim D. for setting the record straight. I based my assumption on the cutaway views on the front pages of the spec sheets for several 800 series drivers. Is the ring the thin piece apparently retained by the inner screws in this picture?;

http://www.lansingheritage.org/image...-808/page1.jpg

And is it the item between the phase plug and rear plate in this one?;

http://www.lansingheritage.org/image...8-8b/page1.jpg

[Jim D]

All the 800 series drivers have centering rings, but they are perforated to let the voice coil breathe into the pot casting. In these drivers, the only thing holding the pole in place is magnetic force, so the centering ring, which is screwed, and sometimes glued, to the back of the top plate, performs the function of keeping the gap centered. Not to worry too much, when you drop one of these drivers, they often bend and the driver goes out of center anyway. The only ALTEC drivers I am aware of that have solid centering rings are: 288, 288B, 902-8B and 908-8B with the 7/8" exit, and early 1" exit 902-8B, 908-8B, 909-8A. Of the later three, I know most do not have centering rings. To find out what you have, remove the diaphragn and see how far you can push a piece of #22 copper wire down the gap. In the case of those with perforated centering rings, it may take a try or two to find one of the perf holes. If the wire only goes in a short ways, you have a solid centering ring, if it continues into the magnet assy, you don't have one, or you have a perforated ring. JIM

[Russellc]

Thanks all and bfish, your info was still helpful, for my puposes the perferated would accoustically be the same as NO ring, and your method of determining the existence of the ring in 900 series drivers was correct.

Thanks all,
Russellc

[Steve Schell]

Here's a little history, tangential to the discussion. In the original Lansing and early Altec Lansing field coil compression drivers, the concentricity of the voice coil gap was maintained through an ingenious though costly design.

The center pole maintained the same diameter from the gap area to the back plate (exit side of the driver). On the thread mount drivers a reduced diameter of the center pole traveled through the hole in the back plate and was secured on the outside with a threaded ring that also was threaded on its exterior for screwing the driver onto the horn. On the stud mount drivers, a reduced diameter of the center pole still fit tightly into the hole in the back plate and the two pieces were attached rigidly with small machine screws. Both methods locked the center pole securely to the pot structure, through both tight machining tolerances and the fasteners.

The top plates (outer pole pieces) of all these drivers had a step machined into their circumferences. The lip of the pot was also machined so that the top plate fit tightly when they were joined. There was no centering ring, as none was needed with this method; the pot structure would have to bend to allow relative movement between the center pole and top plate. The step was deep enough that the voice coil would not contact the center pole when the top plate/diaphragm/rear cover assembly was removed or installed. This assembly was replaced as a unit in the field coil days.

When the small and large format drivers were converted to permanent magnets, neither the center pole nor top plate were rigidly held to the pot with machined junctions. Instead, a centering ring was placed beneath the voice coil gap to hold the parts in alignment. The removable diaphragm assemblies also debuted at this time, as the top plates could no longer be removed for diaphragm replacement due to the charging of the permanent magnet. The pop out diaphragm assembly was also certainly a practical advance from a servicing standpoint.