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<blockquote data-quote="JohnEJanowitz" data-source="post: 4531129" data-attributes="member: 584636">With the full copper sleeve around the pole, any eddy currents are instantly shorted out before they have a chance to do anything.&nbsp; This only works if you have a rigidly affixed highly conductive material on the pole.&nbsp; Copper, aluminum, or other materials can all work.&nbsp; The thickness required is determined by how conductive the material is.&nbsp; Alum needs to be about 2x as thick as copper to get the same effect.&nbsp; The thicker the material, the lower in frequency it has effect.&nbsp;&nbsp; We&#039;ve found that .025&quot; thickness all around the pole is about ideal.&nbsp; Thicker will be more effective, but you are also widening the gap between the top plate and pole then and reducing flux.&nbsp; Again it&#039;s a tradeoff.&nbsp; The more conductive the material the better.&nbsp; The other option is if you make the entire pole less conductive.&nbsp; Powdered iron is cool as it is magnetically permeable but not electrically conductive.&nbsp; You essentially kill the inductance and the coil looks purely like an air core.&nbsp; The problem is that it is not as highly magnetically permeable as low carbon steel.&nbsp; You lose about 20% of the flux in the gap.&nbsp; And it isn&#039;t that cheap either.As far as power compression goes, you&#039;d be quite surprised.&nbsp; On a 2.5&quot; coil driver with no copper, our high temp coil on black anodized alum former, we ran it at 300W for a period of 1 hour.&nbsp; This was a sealed box power test 20Hz-200Hz filtered pink noise, 6dB crest factor per some AES Standard I can&#039;t remember the number of right now.&nbsp; Anyway, the coil resistance went from 3.2ohm up to over 7ohm after that period of time with only 300W.&nbsp; This is equivalent to over 4dB in power compression.&nbsp;&nbsp; And this was a driver that had good physical cooling built in.&nbsp; You&#039;d be surprised how much power compression becomes a factor.&nbsp; Measure the DCR of a driver, then play it at somewhat high levels for an hour and measure again before letting it cool.&nbsp; The pro audio companies spec power compression usually at -10, -3, and 0dB(full power).&nbsp; Look at even some of the large 4&quot; coil pro audio drivers and even then you&#039;ll see compression of 3dB or more at 500W or less often times.I remember all the packers/lions rivalry from when I was up at tech.&nbsp; It seemed to be split about 50/50.&nbsp; Of course I was up there from 95-99.&nbsp; The packers had 3 good season in there... did the lions even win a game those years?&nbsp; //content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif&nbsp; It&#039;s funny though because I was a John Elway fan since the mid 80&#039;s.&nbsp; I had my Elway jersey on amidst a sea of Packer jerseys when the broncos beat the packers in the superbowl.John</blockquote>

[QUOTE="JohnEJanowitz, post: 4531129, member: 584636"] With the full copper sleeve around the pole, any eddy currents are instantly shorted out before they have a chance to do anything. This only works if you have a rigidly affixed highly conductive material on the pole. Copper, aluminum, or other materials can all work. The thickness required is determined by how conductive the material is. Alum needs to be about 2x as thick as copper to get the same effect. The thicker the material, the lower in frequency it has effect. We've found that .025" thickness all around the pole is about ideal. Thicker will be more effective, but you are also widening the gap between the top plate and pole then and reducing flux. Again it's a tradeoff. The more conductive the material the better. The other option is if you make the entire pole less conductive. Powdered iron is cool as it is magnetically permeable but not electrically conductive. You essentially kill the inductance and the coil looks purely like an air core. The problem is that it is not as highly magnetically permeable as low carbon steel. You lose about 20% of the flux in the gap. And it isn't that cheap either. As far as power compression goes, you'd be quite surprised. On a 2.5" coil driver with no copper, our high temp coil on black anodized alum former, we ran it at 300W for a period of 1 hour. This was a sealed box power test 20Hz-200Hz filtered pink noise, 6dB crest factor per some AES Standard I can't remember the number of right now. Anyway, the coil resistance went from 3.2ohm up to over 7ohm after that period of time with only 300W. This is equivalent to over 4dB in power compression. And this was a driver that had good physical cooling built in. You'd be surprised how much power compression becomes a factor. Measure the DCR of a driver, then play it at somewhat high levels for an hour and measure again before letting it cool. The pro audio companies spec power compression usually at -10, -3, and 0dB(full power). Look at even some of the large 4" coil pro audio drivers and even then you'll see compression of 3dB or more at 500W or less often times. I remember all the packers/lions rivalry from when I was up at tech. It seemed to be split about 50/50. Of course I was up there from 95-99. The packers had 3 good season in there... did the lions even win a game those years? [IMG]//content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif[/IMG] It's funny though because I was a John Elway fan since the mid 80's. I had my Elway jersey on amidst a sea of Packer jerseys when the broncos beat the packers in the superbowl. John [/QUOTE]

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