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In Reply to: RE: The nominal power for F12N field coil is 25 W. posted by amnesiac on July 12, 2007 at 05:47:18
As I posted above, 40 watts is WAY too much dissipation for a 12" jensen field coil. It is overheating and smelling bad because you are in the process of burning it out. The only 40 watt rating I have seen is the maximum dissipation rating on a Lansing 15" theatre woofer, which has a much larger motor than your Jensen.
I would recommend 14 watts as a good figure to work up to. Start with 10 watts for several hours, checking the temperature frequently, then proceed to 14 watts if the coil remains fairly cool. It should never become too hot to leave your hand on it comfortably.
Follow Ups:
Steve my poor speaker.It got really fried.Well maybe not really. I measured the dcr when it was hot last night and it was up from 980 to 1200ohms.I will measure it again today when I adjust the supply.At least the lower supply requirements will make it easyer to build .Steve have you experienced any difference in the sound when using minimal amount of capacitiace verses huge capacitance in fc supplys. And finally thank you your input.
I have seen a few references in the old theatre service literature to "cold" and "hot" field resistance readings. It is true that the resistance increases with temperature; sounds like it sure did in the case of your speaker! No matter, if the field coil still has continuity then it is okay.
So far I have not noticed much in the way of sonic changes due to the field supply used. Many folks insist that it makes a huge difference, but that has not been my experience so far. However, a friend and I made up some new tube field supplies for the Lansing Iconic two way speaker. When my friend installed the new supplies on his speakers he noticed a considerable improvement in sound quality over the solid state supplies he had used previously. To some extent it might depend on the supply voltage. The Iconic runs on 330VDC, while most of my work involves 12VDC field coils.
Hey Steve I just halved the voltage down to 20 watts.It hardly is getting warm.Musicly I little less power but perhaps more bottom end.And yes the coil did come back to 980 ohms once it had cooled.All so there is a coil that goes around the feild coil that the voice coil goes through before it enters the speaker.Is this a choke.I can wire around and see I supose.Which bring to wonder if choking it this way has advantages. Oh and I added on 100uf to my supply due to the now large resistance comprimising it.Perhaps it helped maybe they do need a good supply.I have 8 batterys there I will get around to trying them eventually.Thanks again.PS are they your altex feild coil compression drivers on ebay?
That thin coil under the top plate is a humbucking coil, and it is placed in series with the voice coil. Most of these speakers were used in situations where the field coil functioned as part of the smoothing circuit for the amplifier's power supply, so they were fed some gnarly DC. You can either leave it alone of disconnect it; I remove them when taking the drivers apart.
No, those weren't my field coil converted 288s on ebay.
If it were, then it was to be supplied with a hum signal out of phase with field coil hum.
The extra coil around the pole piece is a demodulator coil. Its purpose is to counter the modulation of magnetic field in the gap by the voice coil. It is connected in series and out of phase with the VC. It makes magnetic field in the gap immune to modulation by the field of the voice coil. Another advantage of the demodulator coil is reduction of driver's Le, which allows better high frequency extension.
With an underpowered field coil that you advocate, demodulation coil has most benefits and should not be disconnected. With high FC power, when the yoke is close to magnetic saturation, demodulation coil becomes magnetically uncoupled from the voice coil and all its benefits disappear.
Sser2, your comments are most interesting and thought provoking. I have never thought of that little extra coil in this way, but I like your thinking. Do you have any citings from the literature that indicate that this coil functions as a demodulator? It would be fascinating and would open up new avenues of thought if true, and I am all ears when it comes to improving field coil technology.
I have deduced that this coil is a humbucker mostly because the field coils in most of these Jensen units serve as filtering components in the amplifier's power supply. If the audio signal contained some hum components then their presence in this coil, if in opposite polarity to the field coil, would tend to cancel the hum in the speaker's output.
I am not sure why you state that I advocate an "underpowered" field coil. I have stated the recommendations of the manufacturer in response to what appeared to me to be your reckless recommendation of almost three times the nominal field dissipation that these units were designed for. As Paul Klipsch used to say, "Any mechanical or electrical device can be overdriven to the point where failure must occur."
Ok thank you.I will try with out it.It running nice and losening up at 20 watts. good bass impact.
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