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105.2 tweeter crossover questions

 
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RKV1
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Location: Washington State

PostPosted: Sun Dec 02, 2012 2:12 am    Post subject: 105.2 tweeter crossover questions Reply with quote

Hi, I'm a long time KEF owner(1970's to present) but first time poster to this site. I mostly lurk.

My questions are:
1) What is the function of the small inductor L6 (.03mH) in series to the T52 crossover? It's present in the earlier versions but shorted in the later versions.
2)When replacing the 3.3 mfd capacitors with low loss film types, would it be better to put the additional series resistance required before the capacitors(at L6's location) or after the capacitors?

Thanks
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speakerguru
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PostPosted: Sun Dec 02, 2012 10:32 am    Post subject: Reply with quote

1. The inductor was there to eq a rise in response before cut-off which was present in early T52s. I seem to remember that later there were changes to the tweeter surround damping ring and adhesive which changed the top end response and made the crossover inductor unecessary.

2. The order of components in a series circuit branch makes no difference. The impedances add, so X + R = R + X
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audiolabtower
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PostPosted: Sun Dec 02, 2012 2:10 pm    Post subject: Reply with quote

I put a 1 ohm 10W resistor at the output of the crossover to T52 positive.
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RKV1
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PostPosted: Sun Dec 02, 2012 6:57 pm    Post subject: Reply with quote

Thanks speakerguru.

1) So the two versions of the T52 are slightly different, frequency response wise. With my old ears a slight rise in response before cutoff would probably be a good thing.
2)I would think that resistance at the L6 location would interact,slightly, with the DC resistance of L7, like a L-pad.Putting the resistance at the tweeter would effect the crossover frequency slightly.I suppose the value of R3 could be changed too to compensate.

Anyway, the spot on the PCB that L6 occupies would be a convenient location to put a 1 ohm 5-10 watt resistor that audiolabtower suggests.
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RKV1
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Location: Washington State

PostPosted: Sun Dec 02, 2012 6:59 pm    Post subject: Reply with quote

Thanks speakerguru.

1) So the two versions of the T52 are slightly different, frequency response wise. With my old ears a slight rise in response before cutoff would probably be a good thing.
2)I would think that resistance at the L6 location would interact,slightly, with the DC resistance of L7, like a L-pad.Putting the resistance at the tweeter would effect the crossover frequency slightly.I suppose the value of R3 could be changed too to compensate.

Anyway, the spot on the PCB that L6 occupies would be a convenient location to put a 1 ohm 5-10 watt resistor that audiolabtower suggests.
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audiolabtower
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PostPosted: Sun Dec 02, 2012 9:06 pm    Post subject: Reply with quote

1 my input caps were both 3.5uF measured, with the 0.028mH of the series inductor the first order resonance is the order of 16kHz, so I reckon the attenuation may have been around there.

2 there is no dc L pad because there is a capacitor in the way. I had no R3 because some selected T52s did not require the zobel network to meet target function.
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RKV1
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PostPosted: Mon Dec 03, 2012 12:00 am    Post subject: Reply with quote

There definitely shouldn't be any DC in this circuit. javascript:emoticon('Laughing')

Since my hearing is shot above 12Khz it doesn't matter if my speakers peak at 16KHz. So, I've removed L6 and replaced it with a 1/2 ohm 5 watt resistor.Soldering the resistor to the PCB seems more "elegant" than dangling it on the tweeter's blue positive line.

Speaking of "target function" it sure would be nice to know the part values KEF actually used for each serial number in their older Reference series line. I understand that they kept this information so they could send out replacement crossovers to match the actual drivers used when a repair was needed.

Does anyone know if this information still exists? Would it still be considered proprietary after all these years?

Thanks
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audiolabtower
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PostPosted: Thu Dec 06, 2012 5:03 pm    Post subject: Reply with quote

I asked about my build data many years ago but they had already dumped it as out of date, Mike Gough or Laurie Fincham sent me copies of their crossover diagrams instead.
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