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T27 6535 versus T27 SP1032
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habrune
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PostPosted: Wed Nov 19, 2014 12:12 am    Post subject: Reply with quote

FWIW, owned some (say 5 pairs) SP1032's all within the 7-8 Ohms range DC, and fs between 1100-1250 Hz. I had one pair 6535, fs indeed around 900 Hz, but DC resistance both between 4-5 Ohms. Some doubt if this last pair was 'as should be' or due to earlier overcharge. They sounded OK. The strange thing was that I could exchange both types without change in loudness. (one would expect lower impedance = more current = more dB's).

To my taste the SP1032 sounds better in the newer KEF aB XO (as in Corelli and 104aB), whereas the 6535 definitely sounded better in the 2 x 4u7 / 0,25mH T network, without the harsh edge the SP 1032 exhibits in this simple network.
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proffski
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PostPosted: Wed Nov 19, 2014 12:26 am    Post subject: Reply with quote

Interesting, if my memory is not letting me down the earlier T27 was more "shouty" and in your face than the following replacement.
It may have been the way I was using them of course, but they ended up as HF units in a car eventually!

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proffski
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PostPosted: Wed Nov 19, 2014 12:29 am    Post subject: Reply with quote

habrune wrote:
FWIW, owned some (say 5 pairs) SP1032's all within the 7-8 Ohms range DC, and fs between 1100-1250 Hz. I had one pair 6535, fs indeed around 900 Hz, but DC resistance both between 4-5 Ohms. Some doubt if this last pair was 'as should be' or due to earlier overcharge. They sounded OK. The strange thing was that I could exchange both types without change in loudness. (one would expect lower impedance = more current = more dB's).

To my taste the SP1032 sounds better in the newer KEF aB XO (as in Corelli and 104aB), whereas the 6535 definitely sounded better in the 2 x 4u7 / 0,25mH T network, without the harsh edge the SP 1032 exhibits in this simple network.


Some good valid points, thank you!
Look at the fs of the T27A here... http://www.hifiloudspeakers.info/Anatomy/KEFDocuments/KEFGermanGallery/pages/p34.htm
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speakerguru
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PostPosted: Wed Nov 19, 2014 9:36 am    Post subject: Reply with quote

habrune wrote:
...one would expect lower impedance = more current = more dB's....


...not if this is due to a reduced number of turns of wire, then the "L" in BLi would be reduced in the same proportion as "i" had increased; result the same spl. (spl ∝ acceleration ∝ force ∝ BLi)

In the late 70s, one of my major projects was to develop a winding strategy to reduce the tolerance of production KEF voice coils from +/-10% to +/-5% while still using standard winding tensions and British Standard tolerance winding wire. Before that, KEF coil winding lengths had been much more variable.
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audiolabtower
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PostPosted: Wed Nov 19, 2014 10:13 am    Post subject: Reply with quote

I thought the aB network was necessary (and worked) because the T27 resonance was 1.2kHz and close enough to the crossover frequency to interfere with the rolloff etc. If the resonance was lower, eg 900Hz, the effectiveness was not so much, and a T section might well be just as good subjectively. This would bear out Habrune's experience...

As an aside I always thought it strange the LS3/5a never seemed to get a lot of criticism about treble quality despite not having an aB section.
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SaSi
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PostPosted: Wed Nov 19, 2014 6:51 pm    Post subject: Reply with quote

According to KEF, the "original" T27 was A6340 and not 6535. Also, looking at the history of the systems, Cresta came with A6340 and Chorale came later in 1970 fitted with the SP1032.

The only system I could find that used either tweeters was the Concerto, long lived between 1969 and 77, and it used both A6340 in the earlier builds and SP1032 in the latter ones. However it came with different xo; version 2 came with SP1002 (using B110 A6362/T27 A6340) and version 3 came with SP1004 (using B110 SP1003/T27 SP1032), so I can't be sure which of the two drivers was the reason why the xo was modified - perhaps both drivers had something to do with it.
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habrune
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PostPosted: Fri Nov 21, 2014 12:13 pm    Post subject: Reply with quote

Proffski, to be honest the old ones where in the Cambridge kit, with Coles on top. So that might have sweetened them up. Dazzled by your T27 application! I'd like the rest of the scheme, I never used a T27 as a car horn. What mids? Laughing
Idea Speakerguru, smart explanation for the coilburns/dB(lack of)loss, immediately accepted.
Audioabtower, I have the same experience: I've a DIY rebuild 11 Ohm version LS3/5A, very sweet, hardly recognizable as a T27, even more so with the special T27 grid in place (cost me a fortune on E-prey). Tried the trick to place the grid on a T27 in the B139/B110/t27 R50 XO combi, but that didn't smoothen the T27 up as it did in the LS3/5a's.
Maybe it has also something to do with the different mid-top integration in the XO.
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proffski
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PostPosted: Mon Nov 24, 2014 10:41 am    Post subject: Reply with quote

That was a long time ago, sometime in the 1970s!
The car was ab 'Austin 1100' and the B110s were loaded into the boot.
The few measly watts were provided by a Sony TC-10 cassette player.

Surprisingly at most speeds with good smooth roads the system gave ample volume and excellent bass, not the BOOM BOOM variety one hears these days.
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