SpeakerTalk

Richie · Posted: Thu Feb 10, 2011 3:58 pm Post subject: T27's

Has any one ever actually tamed a pair of T27's, often desribed here as spitty, Ive done a number of upgrades/tweaks with better cables, soldering connectuons, re-capping etc and they have got better but still have an 'edge' to them, anything else I can do ?
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audiolabtower · VIP Contributor 500+ Joined: 06 Jan 2009 Posts: 686

have you already got the acoustic butterworth crossover for them? If so maybe a more sympathetic amp/dac etc in the high freqs might be better?

Trouble with the T27 was the 1.2 kHz resonance was close to 3kHz crossover, which the aB addressed. Somehow they never seemed so bad in good pairs of LS3/5as which always puzzled me.

Richie · Posted: Thu Feb 10, 2011 8:45 pm Post subject:

Yea I have the ab version. A Musical Fidelity A3.2 cd which I understand has a reasonably good DAC in it, I use a passive transformer volume control with an Arcam P35 power amp with Chord Odyssey cable which are all said to be good....

I plan to get some better mains cable, maybe that will help ?
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ColinR · Posted: Fri Feb 11, 2011 12:27 am Post subject:

I'm quite happy with my Quad 44/405-2/410-2/606/520 set-ups in passive and active modes.

My Rotel RA-1210, RX-850AL and free 55W JVC, plus my Crown DC300As sound ok too.

However a Quad 707/909 sounds poor in comparison to the 606.

I would suggest that (more recent) 0 to >80,000Hz audio bandwidth designs using power transistors with higher frequency resonance's requiring more negative feedback have less compatibility with T27s than older contemporary output devices with 0 to 30-40kHz bandwidths.

"Brighter" sources don't help either.

If you're really sad convert to "light bulbs".

Most valve amplifier output transformers top-out at 11.5kHz, better ones <14.5kHz.

If you want 0-40,000Hz output transformers they'll cost in the region of £400 each, only if you know someone who winds them!
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Richie · Posted: Fri Feb 25, 2011 8:08 pm Post subject:

Why do some T27's have a screw in the back whilst others have a flat plate ?
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speakerguru · Posted: Sat Feb 26, 2011 10:23 am Post subject:

.....because KEF used to screw magnet parts together before they got a "staking machine" (big press) and better adhesives became available in the mid 70s. Later when cheap forging became available from China the backplate and pole piece were combined into a "mushroom" and the back of the magnet assembly then looked smooth or just had a single location dimple.

Richie · Posted: Sat Feb 26, 2011 11:10 am Post subject:

Wow ! thanks ...
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Nimo_jon · Posted: Sun Feb 27, 2011 2:04 am Post subject: T27A Varaints

Hello SpeakerGuru

Greetings. How about these T27 from the constructor series.
They sounded more sensitive (yet to measure them) than the old 70s T27.
Were these made in Tovil? What period these belong to?

Thanks.

speakerguru · Posted: Sun Feb 27, 2011 11:25 am Post subject:

Those have the later one piece pole-backplate forgings from the far east but still made in Tovil. I can't remember exactly when they were introduced; mid to late 80s maybe? Is there a date stamp on the constructor series label?

The pole to backplate joint is the region of highest flux density in the whole magnet assembly, so when this joint was eliminated the gap flux increased by a few percent (~0.2dB)

Nimo_jon · Posted: Tue Mar 01, 2011 5:23 am Post subject:

Thank you Sir for the info!

There has been a big question mark hovering over my head for years Rolling Eyes

Nimo_jon · Posted: Tue Mar 01, 2011 5:38 am Post subject:

I ordered a pair of Hiquphon OW1 tweeters. I am considering replacing the T27A with the OW1. The reason being T27A becoming more of a collectors' item. I am sure the B200 deserves a superb tweeter.

By using the OW1, I suppose I can simplify the crossover since its resonance F is below 1khz at about 800hz. A simpler crossover means better resolution, I believe.

There are hundreds of LS 3/5a 104s KIA 'ed due to burnt T27A.
I have seen many attempts to replace the T27A. The Morel T29 being a common choice. and is not an ideal candidate.

I hope to get some advise and comments on this. Thanks in advance.

Richie · Posted: Tue Mar 01, 2011 11:11 am Post subject:

Its a pity there seems nobody expert enough to properly repair a T27.
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Nimo_jon · Posted: Tue Mar 01, 2011 12:14 pm Post subject: T27A repair

I have seen several T27A repaired, mostly by amateurs.
The voice coil removal is not easy, to start with.
The voice coil is linked to the external wiring via two very thin wires.
When the tweeter get fried, the thin wire that goes into the voice coil breaks.

This break can be repaired by adding a tiny bridging wire, or add bridging solder. But this may introduce severe distortion at certain frequencies.
Such repaired tweeters may be sold as a used unit. Yes, I bought one before.

The only professional repair I can think of is voice coil replacement.

In the old days where T27A were in abundance via KEF constructor resellers, a burnt tweeter could be replaced easily.

Shouldn't we look for alternatives using today's technologies? Having said that, if today's speakers are good, why should we look for KEF's speakers in the 70s. Rolling Eyes

Richie · Posted: Tue Mar 01, 2011 8:18 pm Post subject:

I wonder how many T27 were fried in 104ab's, I just noticed that the older 104 had no fuse.... the ab's do have a fuse to protect the tweeter.
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audiolabtower · VIP Contributor 500+ Joined: 06 Jan 2009 Posts: 686

The Kef design topic said that the T27 was in no danger in the system with music of all types and good amplifiers of greater than 100 Watts (I suppose "good" means amps that don't oscillate!!) Smile

However, it said fault transients and high speed tape spooling without turning down the volume burnt it out thus the fuse was added in the aB.