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Amp for 105.2
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audiolabtower
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PostPosted: Mon Nov 25, 2013 9:20 pm    Post subject: Reply with quote

proffski wrote:
Seconded without a moments hesitation! Smile


and me!

I found the T52 to be one of the few tweeters to match the (famous?) treble quality of my BC1s, more so than T27s and 1" Audaxs of the time, and they still do today, so the surround problem cannot be a universal fault. Maybe it comes from non-use? I understood the 105.2s in that HiFi News review came from the "Kef Museum", maybe they had just been in storage for years??
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speakerguru
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PostPosted: Tue Nov 26, 2013 11:12 am    Post subject: Reply with quote

Thierry wrote:
Back to the T52 : unless I am completely wrong, what you describe would rather result in an excess of brilliance than a dull sound, wouldn't it ?Thierry

Yes that's correct. The only thing that I can think of which would dull the sound would be if the bond between voice coil and diaphragm was failing or the magnet had become partially de-magnestised. Both could possibly happen as a result of some abnormal gross transient overload.
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Thierry
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PostPosted: Tue Nov 26, 2013 11:45 am    Post subject: Reply with quote

speakerguru wrote:

Yes that's correct. The only thing that I can think of which would dull the sound would be if the bond between voice coil and diaphragm was failing or the magnet had become partially de-magnestised. Both could possibly happen as a result of some abnormal gross transient overload.


Completely agreed and very unlikely too - at least in my example - as when I measured the frequency / amplitude response of my own 105.2s the linearity was quite remarquable up to the last octave.
I should upload the results here some day.

De-magnetizing a ferrite magnet probably takes a fair amount of heat built in the unit (which I assume was likely to happen in the KM1, given the system capabilities). This is largely preferable over the old Al-Ni-Co technology anyway, Alnico magnets are known for de-magnetizing themselves as time goes by.


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speakerguru
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PostPosted: Tue Nov 26, 2013 11:56 am    Post subject: Reply with quote

Thierry wrote:
De-magnetizing a ferrite magnet probably takes a fair amount of heat built in the unit (which I assume was likely to happen in the KM1, given the system goals). This is largely preferable over the old Al-Ni-Co technology anyway, Alnico magnets are known for de-magnetizing themselves as time goes by.Thierry

Only the KM1 T52 used a (massive) ferrite magnet. The regular T52 (SP1049?) did indeed us an ALNICO magnet in a pot. Alnico magnets only de-mag easily if stored without a shorting "keeper". In a tweeter magnet assembly with a fairly small gap, a sufficiently long alnico magnet will be stable without a keeper but you can still de-mag them with a high burst of AC through the voice coil.
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Thierry
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PostPosted: Tue Nov 26, 2013 3:18 pm    Post subject: Reply with quote

speakerguru wrote:
Thierry wrote:
De-magnetizing a ferrite magnet probably takes a fair amount of heat built in the unit (which I assume was likely to happen in the KM1, given the system goals). This is largely preferable over the old Al-Ni-Co technology anyway, Alnico magnets are known for de-magnetizing themselves as time goes by.Thierry

Only the KM1 T52 used a (massive) ferrite magnet. The regular T52 (SP1049?) did indeed us an ALNICO magnet in a pot. Alnico magnets only de-mag easily if stored without a shorting "keeper". In a tweeter magnet assembly with a fairly small gap, a sufficiently long alnico magnet will be stable without a keeper but you can still de-mag them with a high burst of AC through the voice coil.


According to the document "A History of KEF Drive Units" the KEF 105.2 uses the T52 B SP1072.
The SP1049 is found in the first issue of KEF 105, but I assume that SP1072 has also an Alnico magnet anyway.

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audiolabtower
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PostPosted: Tue Nov 26, 2013 5:43 pm    Post subject: Reply with quote

I seem to remember that data sheet values for both 1072 and 1069 were exactly the same. I believe the moving parts and magnet were all the same. The 1072 was a cost cut because of the expensive thick metal "pot" of the 1069, replaced by a cheaper metal tube and screwed on back plate, and the expensive thick turned front plate replaced by a simple square to fit more easily into cabinets.
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speakerguru
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PostPosted: Tue Nov 26, 2013 10:55 pm    Post subject: Reply with quote

audiolabtower wrote:
I seem to remember that data sheet values for both 1072 and 1069 were exactly the same.


Yes, but SP1072 and SP1049. Found an old post of mine here http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=835&postdays=0&postorder=asc&highlight=pot+tube&start=15 (Fri Oct 23, 2009)
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iso
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PostPosted: Wed Nov 27, 2013 11:04 am    Post subject: T52 Reply with quote

speakerguru wrote:
The only thing that I can think of which would dull the sound would be if the bond between voice coil and diaphragm was failing or the magnet had become partially de-magnestised.

Can this failure mode be the reason for quite dramatic variation of VC inductance figures mentioned on my previous tread http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1299&highlight=t52

Efficiency figures have been so close to each other that I considerd that there is no reson to expect any major faults.

Best Regards

Kimmo
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audiolabtower
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PostPosted: Wed Nov 27, 2013 12:10 pm    Post subject: Reply with quote

sorry grey cells wearing out typo..1049. Yes that post was where I remembered the info from SG.
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speakerguru
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PostPosted: Thu Nov 28, 2013 9:58 am    Post subject: Re: T52 Reply with quote

iso wrote:

Can this failure mode be the reason for quite dramatic variation of VC inductance figures mentioned on my previous tread http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1299&highlight=t52


I think that very unlikely. More likely one of the reasons stated in my post Thu Jan 26, 2012 5:13 pm in that same thread.
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