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Good day, my first post!

WHY IT HAPPENED:
I have a set of KEF C7's I use in my dorm at my university residence either at a really low volume (to study) or REALLY loud (special occasions party vibe...) In our home's living room they can't fill the room with deep bass (although being quite a large room, it took 2 12inch kicker subs to get the place shaking up), but you can imagine what the KEF's do to my small university room... What a pleasure. I run them with two massive old-school 15kg RS-MOSFET ANNIHILATOR amps (unfortunately, that's about as much info I have on these).

Anyway, the one night I was jamming them out with my friends at the max volume my laptop will go. From previous experience with the amps, different inputs etc. I'm pretty sure the amps will still go way louder when given a louder line signal, but the speakers seemed to be at the edge of their abilities at this volume, so I deemed my laptop the perfect source so I or anyone else won't be tempted to go any louder and damage my speakers. Not that they didn't make my ears beg for mercy at this volume...

Yet, that night, for some very unknown reason;), I decided my speakers are the best in the world and can take anything and I've recently installed an EQ on my laptop allowing preamplification and so why not bump up the volume by 9dB (when I saw this number the following morning I was completely dumbfounded). This was until my one friend in a much better condition than myself:D, told me, "Listen here, I'm smelling something burning and your speakers don't sound normal." I reluctantly put the laptop's volume down to 70% with the 9dB increase still in place and continued to enjoy the night.

I don't know if this is in my defence or only makes matters worse - but I promise you, I am usually extremely good at distinguishing when speakers are clipping/distorting and the first to notice when anything's off with a speaker system.

THE PROBLEM:
The following morning I was surprised and relieved to find the speakers sounding good, but after a few songs I noticed something's out of place. I proceeded to place the speakers next to each other and played the same piece of music on the one, then the other. Sure enough, there was definitely a difference, but it wasn't even easy to say which one sounded right, vs which one sounded wrong.

THE CULPRIT:
So I swapped the tweeters, but the sound stayed the same, so I used a multimeter to test the resistance of the one speaker and the other. The one was like 3.5ohms and the other 2.7. These are rated 8ohms (and I know dc resistance is different but not by this much.) So I took them apart and read the resistance of all 4 woofers (2x / speaker) - thank goodness, 3 read 4.6ohms and one 4.5ohms. Seems good! Then I take a look at the crossovers... Sure enough, on both speakers' crossover, the one inductor (rated 1mH) seems to have burnt a bit. I went ahead and desolder the inductors and measured their DC resistance, the one speaker's inductor read 0.1ohm and the other speaker's 0.9ohm...

THE SURPRISE:
Then I saw something that baffled me real bad. Woofers with DC resistance of 4.5 ohms are probably rated at like 6 ohms, right? Then I saw that the two woofers are wired in PARALLEL. Wth? So that's 3 ohms. And they rate it 8ohm! Now my hypothesis is they use an inductor (which is in series) with high resistance (not like my blown ones) to act as a protection mechanism, up the ohm rating and possibly allow them to give some insane wattage rating... Idk.

THE SOLUTION:
The real question is with what do I replace these inductors, cause normal audio ones seem to have a much lower dc resistance than these should have? Or do I rewire the woofers in series and try to build my own crossover :confused:

I have some photos of the crossover and inductors, as well as a circuit diagram I made of it, but don't know if that's allowed, as per forum rules quote "Reverse engineering and/or materials analysis of audio and home theater components/accessories parts thereof, whether patented or unpatented, shall not be discussed on these forums."
 

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It's difficult to blow an inductor since they are only wire. Are you sure about the readings and they weren't .1 and .09? I don't think the inductor's resistance increases when heated. I suppose the solder joints could melt and then create cold solder joints during cooling, but I doubt it. Also impedance is not the same as resistance, impedance has a frequency element. Since the drivers check OK, I think it's more likely a capacitor. The schematic shows three in the xover. A change in capacitance could cause a change in frequency response for a driver, making the sound different.
 

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It's difficult to blow an inductor since they are only wire. Are you sure about the readings and they weren't .1 and .09? I don't think the inductor's resistance increases when heated. I suppose the solder joints could melt and then create cold solder joints during cooling, but I doubt it. Also impedance is not the same as resistance, impedance has a frequency element. Since the drivers check OK, I think it's more likely a capacitor. The schematic shows three in the xover. A change in capacitance could cause a change in frequency response for a driver, making the sound different.
The resitance of copper has a coefficient of 0.00393/°C, so a 100C tempearture rise above reference would give 1.393X the reference resistance. Measurable, but unimportant. And not permanent; the effect goes away when the copper is cooled again.

If you heat the inductor enough, I imagine that you'd burn the insulation, shorting some of the turns. Looking over "Jacques W's" post, I'm not clear on which coil has the lower DC resistance: the burnt one or the other. (Both may be burnt, one more that the other.)

I'm not competent to comment on the likelihood of burning an inductor vs. blowing the caps. Some capacitors make diagnosis easier by literally blowing apart.
 

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Discussion Starter #4
I double-checked the resistances and they are correct. (even soldered some short clean wire to them to make sure it's not just a poor connection).

Sorry for the ambiguity - there are two inductors on each crossover and each crossover seems to have their L1 inductor blown/burnt.

kjfalls, you mention a schematic. Could you maybe refer me to the one you're talking about? Cause the only one I found, manualslib.com/manual/986030/Kef-C-7.html?page=6, has a different crossover layout, but all the parts' specs look exactly the same EXCEPT for the inductor I'm talking about. So weird, but the part number for L1 in this service manual is 1806-2792+0, yet on my inductors, it has an added B at the end (see pics). Also, at the beginning of this service manual they state a rated 125w for the speaker, whereas mine is clearly rated 15-200w.

So now my guess is that they had an early c7 version rated 125w, but needed to get a 200w version on the market. Instead of upgrading the woofers (the woofer seem to be the same as in the service manual - the same WF+C7 printed on my woofers as in the service manual "Rated Impedance 6+/-1.2 ohms at 1V 400 Hz"), they just used a 3 or 4 ohm inductor to take some of the load and increase the rated impedance to 8ohms.

I know this sounds a bit like a conspiracy theory, but how do they get to an 8ohm rated speaker otherwise with two 6ohms in parallel?

Thanks for the answers so far!
 

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Well, at least the coils are labelled (1.0 milliHenrys). A quick look at Digikey doesn't show any obvious exact replacements.

I wonder whether Kef can do anything for you?
 

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I didn't see any other versions after performing a google search. The schematic shows the two inductors as L1 1mh and L2 as 11mh. There are also two low resistance high-power resistors (5W, 11W), so it still doesn't surprise me the coils got hot. Have you checked the caps?
 
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