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104ab sensitivty rating question

 
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Mike G
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Joined: 04 Apr 2009
Posts: 19

PostPosted: Wed Apr 29, 2009 3:10 am    Post subject: 104ab sensitivty rating question Reply with quote

I am wondering if anyone can translate the sensitivity rating from the 104ab spec sheet. It reads as follows.
12.5 watts into nominal 8 ohms produces 96dB at one meter and 400 Hz in anechoic conditions
How does this relate to the current standards of measurement? The most common rating I see is...
Sensitivity (2.83V/1m) 88dB
Any takers?

I have looked over the specs on the drivers and if my memory serves me right the B200 SP1039 is 86db at 1watt and the rest was in German. But I am assuming that it is at 1m. The T27 is rated at 84db. So it seems to me that the 104's are very inefficient. Is this correct?
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speakerguru
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Joined: 18 Nov 2005
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PostPosted: Wed Apr 29, 2009 8:10 am    Post subject: Reply with quote

2.83 V gives 1W into 8 Ohms. 10 V would give 12.5 W into 8 Ohms. 20log(10/2.83)=11dB. 10log(12.5/1) gives the same result but I'm an audio engineer rather than RF.

So after all that, the 104 is 96-11= 85 dB (2.83V/1m)

You can get 1dB back by running the MF control at "+". This position is "flat" at 1 to 2m on the tweeter axis. The mid position gives a hollow back, "low colouration" final balance (a la Spendor/BBC BC1).

Pedant's corner: we're talking sensitivity here. Efficiency is something else.
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Mike G
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PostPosted: Wed Apr 29, 2009 6:29 pm    Post subject: Reply with quote

Thank you very much for the breakdown on that one! I will have to look at that for quite some time to fully understand it. In fact it would take more than looking at it. Sheeze that is what you get for flunking math Rolling Eyes
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SaSi
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Joined: 24 Aug 2008
Posts: 223

PostPosted: Thu Apr 30, 2009 10:03 pm    Post subject: Reply with quote

In case you decide you ... don't understand it, here is an alternative layman's method.

12.5W gives 96dB and every doubling of power gives an extra 3dB.

So 6.25W goes to 93dB and 3.12W gives you 89dB and 1.56W gives you 86dB. From there, 0.78W turns out 83dB.

From there, approximation and guesswork (we are talking layman here) ends up figuring out that 1W should be 84 or 85dB.

But, understanding maths is more fun!
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rushkoff
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Joined: 22 Jul 2009
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PostPosted: Wed Jul 22, 2009 8:13 pm    Post subject: what about power Reply with quote

How much power do I need to feed to my 104ABs?

I've been using a Yamaha CR1020, which is about 75 watts per channel at 8ohms.

But every few years I have to take it in for a new power supply or something similar, and the HiFi guy told me I should be using an amp with 150watts per channel or more for my 104ABs.

This is the receiver the stereo store sold to me in 1979 with the speakers as the best match. But maybe they were wrong?

If they are insufficient, I wonder if I can still just use the the Yamaha as a pre-amp, and then get something else for more power?
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KEF 104ab
Yamaha CR 1020
Denon DP 760 ttable
Grado G+
Cal Audio Labs D1 CD-player
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ColinR
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Joined: 31 Jul 2004
Posts: 1175
Location: Staffordshire

PostPosted: Wed Jul 22, 2009 10:32 pm    Post subject: Reply with quote

Quote:
every few years I have to take it in for a new power supply or something similar


Sounds as if some incompetent has replaced the correct items with either an underated bridge rectifier or substandard smoothing capacitors.

It's a Yamaha not Kelvin monoblocs so shouldn't need such treatment.

Quote:
the HiFi guy told me I should be using an amp with 150watts per channel or more


Bollocks, for a speaker rated between 15 and 100 Watts rms, 75 Yamaha Watts should be plenty enough as Yammies were always conservatively rated.

So find a technician who has the service manual and doesn't contain so much rubbish.
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rushkoff
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Joined: 22 Jul 2009
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PostPosted: Thu Jul 23, 2009 2:29 am    Post subject: Reply with quote

Thanks, and blessings be upon you.

I had no problems for the first ten years or so, and even if there's a bad coil or something in a speaker (I don't hear anything wrong, myself) I wouldn't think that would be enough to fry the Yamaha.

Someone else said to play with the low pass filter on all the time for safety, but that seems a little silly, too.
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KEF 104ab
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krimney
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Joined: 25 Apr 2003
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Location: Alberta

PostPosted: Wed Aug 19, 2009 6:51 am    Post subject: 104ab and power Reply with quote

I had a set running here for a while and used some 30 watt tube gear and some smaller SS gear. I enjoyed the sound as it was very good. One day I decided to connect a 225wpc amp to the 104ab, and did they ever come alive. they lost some of the warm laid back "british" sound and had more detail and more bass.

If you are able to experiment a bit you may be surprised at the change.

FWIW, I use the tube amp daily with other speakers right now but still have the 104 and the "big amp" for those days when I want a change.
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