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T52 coil inductance

 
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iso
Senior Contributor 200+


Joined: 29 Aug 2011
Posts: 215
Location: Finland

PostPosted: Wed Jan 25, 2012 10:33 am    Post subject: T52 coil inductance Reply with quote

I have measured DCR:s and inductances of 4 pcs T52:s from 1978-79 with inexpensive LCR meter. I have used LCR meter earlier to check coils. Figures I have recorded have been believable, but how accurate they have been I don´t know. Probably within 5-10%...

DCR figures were 6,8-6,9 ohm. But... inductance figures did have considerable variation. 1979 units measure at 0,143 and 0,117 mH. 1978 units made 0,265 ánd 0,500 mH.

Kef specs for voice coils were probably measured from coils alone, ie. not measured from complete tweeters.

It is hard to believe that coils themselves are so different... but are magnet assemblies so far to each other and does this really matter anything?
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audiolabtower
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Joined: 06 Jan 2009
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PostPosted: Wed Jan 25, 2012 1:03 pm    Post subject: Reply with quote

With a datasheet spec of 0.085 mH I would suspect you meter accuracy?
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iso
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Joined: 29 Aug 2011
Posts: 215
Location: Finland

PostPosted: Wed Jan 25, 2012 3:43 pm    Post subject: LCR accuracy Reply with quote

I would have suspected LCR accuracy on 0,1-0,15 units, but other units measured several times 0,265 and 0,500 mH...

I also have tested several T27 and they seems to clock too high figures... two T27 from 1976 I have now, measure at 0,75 mH and 0,88 mH. As far as I remember, spec say something like 0,05 mH.

Best Regards
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Thu Jan 26, 2012 6:13 pm    Post subject: Reply with quote

The impedance of a moving coil rises at 3dB/Oct at the higher audio frequencies due to the movement and eddy current losses in the pole. i.e. the "inductance" varies with frequency. So is not a true inductor but a "semi-inductor". It can be simulated in a piecewise linear fashion using a resistor-inductor network. The more sections, the more acurate the impedance. That's the reason why KEF resisted quoting a single value "inductance" for years. Marketing pressure when Constructor Series was launched forced them to put something in the datasheets, but it's not a lot of use for xo design. You need to measure the driver impedance and use that as the load.
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