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Old 13th June 2002, 06:36 PM   #1
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Unhappy HiFi Hell..please help with Kefs

First, thanks for the opportunity to speak, and second, I apologize if this has been discussed previously (I searched).

I have purchased Kef Q55.2 loudspeakers and blown the drivers twice. On the first occasion, I was driving them with an Onkyo TX-SV444 receiver with the volume at about 11:00. I know...I think I learned my lesson on clean power and after getting a replacement speaker, I moved to a Carver HR752 receiver that I borrowed from a friend. Guess what. With the volume at about 10:00-10:30, I blew a driver again.

This time, I have been informed by the dealer that I must move to a more substantial power source - a dedicated amplifier - and therefore separates. The theory was backed by his demonstration of my speakers connected to his MCINTOSH setup and running my speakers at a volume that I could not imagine was possible for any loudspeaker for about 10 minutes.

It is my fault for not understanding the Kef requirements better and the dealer's fault for not asking what I was driving them with. But in the end, I am looking at a serious outlay of cash to solve the problem. Am I headed in the right direction, or are the Kefs just the wrong speaker for me?

I will answer all questions and will appreciate any help/suggestions.

Lamepops
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Old 13th June 2002, 07:52 PM   #2
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Hmmm. Even a POS Carver receiver shouldn't be blowing drivers at any sort of humanly tolerable volume level unless there is something wrong with the receiver. Your chances of having two receivers broken in the same way seems a bit unlikely.

Were you listening to the same source (CD player, tape machine, etc.) when the drivers blew? Is it possible that your source is putting out some ultrasonic crap that is going through your receiver and into the speakers? Which driver (woofer, mid, tweeter) blew? If your source has a problem, when you bring home that new, expensive amplifier (did he try to sell you some $500 cables to go with it? Those KEFs are pretty particular about cables, too) you're going to have the same problem.

Check your sources and if they look OK, check the amplifiers. If you have access to an oscilloscope you can see what is happening at the output. Large DC offset (anything >100mV or so) at the output of the amp indicates a problem. If the offset goes to just a few volts it could blow a woofer, depending on how the crossover is wired in the speaker. Ultrasonics would kill tweeters and maybe midranges.

As to listening to to your audio dealer who says you MUST go to separate components... HAH! If your dealer really cared about YOU, he'd be trying to figure out what's wrong with your system, not trying to sell you more junk. If we all listened to our dealers we'd all have $100k+ systems (complete with mpingo disks and CD demagnetizers), and our dealers would all drive Lamborghini's to work. Why do you suppose an audio dealer would tell you to buy more equipment- it couldn't be because he wants to get into your pants (specifically, the pocket where you carry your wallet) is it?

The speakers don't require any special electronics to drive them. If you took them apart you'd find a crossover and coil and magnet drivers just like 99% of all the other speakers out there. The impedance of the speakers is going to be about the same as 99% of all speakers. They are designed to present a benign load to almost any amplifier (though I'll admit anything but a resistor is asking a Carver to work awfully hard).

MR
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Old 13th June 2002, 08:11 PM   #3
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Just a suggestion, till you find the source of your problems get an inline fuse attached to your pos terminal. A 1 amp fuse should do it. It won't help the sound having that attached but it could save your drivers. Talk to somebody about getting a fuse or circuit breaker built into your xover untill this is all figured out. My speaks have fuses and I have blown them 3 times using high powered SS.
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Old 13th June 2002, 08:13 PM   #4
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Default Kefs - cont'd

Thanks MR, I know this is a DIY site, but I'm a bit desparate.

1) On you first questions, yes - same source, a CD. In one case, it was Phil Collins which I know is a fairly dynamic recording. In the other case - Puddle of Mud. And yes, I was playing them fairly loud, but not as loud as I have cranked the Advents and certainly nowehere near as loud as what I heard through the MCINTOSH setup (yikes).

2) On cables...I have a set of Monster Cables (not sure about AWG - but guessing about 12) and I definitely checked phase - RED to RED, BLACK to BLACK (not bi-wired, and about 10ft in matched lengths).

3) The driver that is being blown is the Uni-Q driver - a combined tweeter and midrange. In both cases it sounded like everything above 100 Hz was gone.

In fact I do have access to an oscilloscope, and that sounds like a fairly easy thing to check out. So if everything is ok, I should see an AC signal with very little DC offset. If the DC offset increases with volume, then there is definitely an issue, right?
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Old 13th June 2002, 08:15 PM   #5
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Default Thanks - Thatch-Ear

Good idea on the fuses - will do.

I will have an Adcom 200W/CH amplifier to play with this weekend and will run some tests to try to determine the source of the problem.
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Old 13th June 2002, 08:25 PM   #6
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Sound like there is more to the story.

Maybe a solution is hidden in the answers to the questions below.

1) Which driver was being getting damaged, tweeter, woofer, or mid?

2) What was the source device, CD, tape, turntable, etc?

3) What was the source material, Rap, rock, orchestral, fusion etc?

4) How did you have the tone controls set? Do you have a Graphic equaliser?

5) Is there a second set of speakers hooked up to the Speaker B output?

6) What type of speaker cable are you using?

7) How long is the speaker cable?

8) A trickier area: How are the componts powered? Are you using a power strip? Is the system powered from 2 different AC wall plates?

Let us know, maybe the forum can suggest some fixes.

Aud_Mot
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Old 13th June 2002, 08:25 PM   #7
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If you can check with a scope, do so. Other thing to notice, are there any strong HF sources around. It looks like you might have HF getting in the amp and to the speakers. A DC ofset might be, but less likely as you blow the high/mid.

Other thing to try: if you have a pc with soundcard and a bit of soft, you could connect that to your output (through a suitable attenuator) and use your pc as a real-time (audio) spectrum analyser, that would make it easy to spot HF breakthrough.
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Old 13th June 2002, 08:34 PM   #8
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Default To Aud_Mot - Replies, and thanks for your time

1) Which driver was being getting damaged, tweeter, woofer, or mid?

Lamepops: The combined tweeter/mid, everything above about 100 Hz (I think this is the Uni-Q driver).

2) What was the source device, CD, tape, turntable, etc?

Lamepops: CD in both cases

3) What was the source material, Rap, rock, orchestral, fusion etc?

Lamepops: Phil Collins in one case, Puddle of Mud (Rock) in the other

4) How did you have the tone controls set? Do you have a Graphic equaliser?

Lamepops: No equalizer. Tone controls - first time set at treble +20% and bass flat. Second time (using Carver) both flat.

5) Is there a second set of speakers hooked up to the Speaker B output?

Lamepops: No

6) What type of speaker cable are you using?

Lamepops: Monster Cable, 10-12 AWG

7) How long is the speaker cable?

Lamepops: 10 ft

8) A trickier area: How are the componts powered? Are you using a power strip? Is the system
powered from 2 different AC wall plates?

Lamepops: Receiver plugged into wall plate, CD player plugged into Receiver's outlet.


Let us know, maybe the forum can suggest some fixes.

Aud_Mot
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Old 13th June 2002, 08:40 PM   #9
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Default Thank you Havoc

Interesting that you mention HF. That reminds me that there is a digital cable receiver running off the same outlet for the television.

Could it be?

I have picked up some of that rf noise while playing my guitar in that room and eliminated it by turning off the tv and cable box. Could that same rf energy find its way into my stereo???
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Old 13th June 2002, 09:41 PM   #10
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Could be that...
Another thing which I thought of and is the reason I have become disenchanted with Kef's in recent years is the complex x/overs they use on some of their speakers with uni-Q drivers. These can be a notorious load (low imp & capacitive/inductive) for some amps to drive, thus causing them to clip momentarily even at medium vol levels which could also be the cause...
tomcat
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