Poor sound from KEF coaxial speaker

[Javachip]

I have a 12 year-old KEF Model 100 center channel speaker with a Uni-Q coaxial driver. It produces a sound with a raspy, buzzy edge to it. A stereo salesman listened to it and said that the woofer was blown but the tweeter seemed OK. (He separated the two components by placing his thumbs firmly on the woofer cone, stopping its motion.) I disassembled the coaxial driver. The tweeter fits inside the former of the woofer with a close tolerance (<1mm) and there are a few small rub marks/abrasions on the outside of the tweeter, as if the inner surface of the former has been rubbing against the tweeter. I played the tweeter and the woofer apart from each other and didn't hear any of the raspiness.

Has anyone had a similar problem with a KEF Uni-Q or with any similar coaxial driver? Could the raspy, buzzy sound be caused by the inner surface of the former rubbing against the tweeter? What would cause that -- a burned, deformed voice coil changing the shape of the former, or design/manufacturing flaw? I can buy a pair of used replacement woofers for $20 or so, and put one of them into the cabinet with the original tweeter. Would that be reasonable to try as a fix, or should I just scrap the whole thing and build my own speaker? Thanks for any comments!

[Javachip]

I decided to keep the cabinet but discard the coaxial driver and crossover and install one or two new full-range drivers. The cabinet has an internal volume of 7.2 liters and has 2 port tubes of 1.25" diameter by 2.5" length.

The drivers I am considering are the CSS FR125S, the Tang Bang W4-657SB/1337S/1320SB (aluminum, titanium, or bamboo, respectively) and the Fostex FE127E.

Can anyone help me with the following questions:

1) How high in frequency does the human voice go? Is 10 kHz high enough for a center channel speaker intended for movies, or do I need 20kHz?

2) Is there a way to predict or calculate maximum undistorted SPL output for a driver? Would any of the drivers I mentioned be acceptable (or unacceptable) for a medium-sized room at a listening distance of 10 feet? Would adding a second driver (without increasing cabinet size) be a good way to increase SPL output?

Thank you for any comments you can provide.

[ToE]

One of my Q15 (uni Q) drivers had this problem. It was indeed the center of the woofer rubbing on the tweeter. My theory was the spider and surround were sagging slightly. I forget whether I flipped the driver or just pushed on it vigorously, but the problem went away. If you have it apart you could probably shave a wee bit of woofer and see if it helps, especially if you feel they are a write off anyway.

I'm all for full range drivers, but rather than kluge them into a box that might not be optimum, why not just build one of the excellent designs going on in the forums...

[Javachip]

Hugh, you're awesome! I tried your suggestion and widened the inside of the voice coil former about 0.5mm using fine sandpaper, to create more clearance around the tweeter. I reassembled the speaker, inverting the driver (your idea -- in case sagging was a factor) and now it sounds much improved. The raspy buzzing sound is gone, for now anyway.

You da man!

[ToE]

Javachip,

Glad it worked out, but don't let that stop you from exploring a fullrange design. My Kef Q15's do good service as my computer speakers, but I'd rather listen to a pair of 8" alnico drivers with wizzer cones torn from a junk magnavox console I found in an alley. I kluged them into some old JBL BR boxes and they are my shop speakers. Response is all over the place and they beam like crazy, but when they are on, they are really on.

Now I am building some Fostex Fe103 BiB's ( Bigger is Better) just to hear what they can do. Check the threads, lots of effort from very talented people. All you need is some glue and a saw to make some speakers that are amazing at any price...

[Javachip]

Hugh,

I do plan to build my own center channel speaker at some point, probably using full-range driver(s) with no crossover, and I hope it will sound better than my old KEF. But first I want to come up with a striking visual design that I can get excited about. I also have to decide where a center channel speaker will sit once I get a flat panel HD monitor in another year or two. For example, would the speaker hang on the wall above/below the monitor, or would it be on a stand/pedestal in front of the monitor, or would it be in a wall niche/recess? The intended location of the speaker will influence its visual design. So for now, I will live with what I have.

[ToE]

Ah well, I did my best to turn another listener to the single driver side... ( :

If your bored and have a couple bucks try dumping the electrolytic caps in the Kef for some film types. No need to get too fancy; Solens or Axons... For Twelve bucks or so from Speaker City( http://www.speakercity.com ) I did both my Q15's.
Presentation is more forward with greater clarity in vocals. Could be just the thing for a center channel.

[bottomfeeder]

Originally posted by ToE
Ah well, I did my best to turn another listener to the single driver side... ( :

If your bored and have a couple bucks try dumping the electrolytic caps in the Kef for some film types. No need to get too fancy; Solens or Axons... For Twelve bucks or so from Speaker City( http://www.speakercity.com ) I did both my Q15's.
Presentation is more forward with greater clarity in vocals. Could be just the thing for a center channel.

It took me 2 years to find something that sounded as smooth as my kef 90 (sure you can spend $600+ and it MAY sound better) but the off axis frequency response is awesome , plus multiple driver centers seem to get a off axis deley not to mention a problem making a S sound seem to resonate .
I hope my kef never has that problem the previous writer complained about !
But I want to try your capacitor upgrade , it sounds great now but it could sound better !!
Did your Q15's sound better with that upgrade ?