MERGED: KEF Q15.2 Speakers

[Old Sparky]

On measuring the DC resistance across the terminals of a pair of these bookshelf units, I was surprised to get a reading of only about 1.7 ohms!  Surely this can't be right.  These have four wires coming from  the terminal panel, which, in the case of the units to hand are terminated in two 4mm plugs, with red & white wires joined together in one plug, and black & green wires paralleled in the other.  Removing the teminal plate reveals a conventional L/C crossover circuit for a 2-way system, the red wire being HF+ , the black HF-, the white LF+, and the Green LF-., whilst the drivers are KEF B160s (SQ1445), which incorporate a tweeter within the bass/midrange speaker.  The tagstip on the actual speaker has 4 terminals marked MF+, HF+, HF- & MF-.  The odd thing, to me, is that the MF+ & HF+ are soldered together at the tagstrip, which means that, with the external leads joined together as they are, the Bass/Midrange and tweeter are effectively in parallell.  THere is no DC reading across the tweeter terminals, but, with a DMM on its 20Meg range, a capacitor appears to be charging.  Could the tweeter, which is buried inside the wooffer, be eleectrostatic?  (I've not come across these speakers before, and haven't found much info. on the web about them)  It seems to me that four external wires are provided to allow for biamplification.   The B160s are marked 4 ohms, but, on the terminal plate it states '8 ohms 10-100watts'   Any idea why the DC resistance shoul be less than 2 ohms?

[Old Sparky]

Following on from the above, I have found more info on the Q15.2s, including the fact that the Tweeters are moving coil 4 ohm types type NT25 or similar, which fit  inside the bass drivers, so it would appear that at least one is o/c (haven't checked the other yet!).  Replacements seem to be around £50 each, too!

[Refugee]

Is there not a capacitor in series with the tweeter?
There is no need for the bass signal to be sent to the tweeter so the LF signal will be reduced by having the capacitor connected in series with the tweeter.

[Mark Hennessy]

Are you sure these haven't been modified?

[Old Sparky]

THese speakers have a conventional L/C crossover network (8.5uf series & a shunt inductor for the tweeters, and 1.2uh in series, 12uf shunt cap. for the Bass/Midrange. The Bass Drivers are original KEF B160s, and the Tweeters KEF NT19C mounted in the centre of the Bass Units, where they are fitted through the middle. As far as I can tell, the drivers are all original. Both Tweeters are O/C, which I believe has been caused by bas & mibddle frequencies passing through them. I havent been able to find a circuit diagram for the Q15.2 but maybe someone replaced the tweeters in the past and wrongly solderd the LF & HF + terminals together?

[Mark Hennessy]

Yes, it sounds like someone has been at them. New tweeters and removing that link between the two driver +ve connections. Your description of the crossover sounds entirely plausible to me, BTW.

Now, whether those are worth the price of the new tweeters plus your time is another question. They're pretty OK loudspeakers, but they are ~15 years old now.

There's a YouTube video showing the procedure - it looks pretty easy. Needless to say, opening them up and exposing the woofer's voice coil to the elements carries risks. Steel wool is the classic way to ruin loudspeakers!

[Old Sparky]

I've actually opened the speaker cabinets, removed the wooffers, disconnected and removed the Tweeters, which is how I know they're NT19Cs, not NT25s, as in a specification I found on the 'net., though what the difference is between the 25s and 19s I know not. The Woofer voice coils aren't exposed to the elements, other than the fact that they're in my workshop. One, minus it's tweeter is back in it's cabinet with the grille on, the other is face side down on my bench, but will be put back in it's cabinet tomorrow. Whether I shoult re-insert the tweeters pending replacements is open to question, as is whether it's worth spending £108 on two new tweeters!

[Mark Hennessy]

19mm and 25mm are popular sizes for tweeters (3/4" and 1"), but I've only ever seen Uni-Q drivers with 1" tweeters, so those numbers are probably coincidental...

When I said "voice coil" above, it would have been more correct to say "magnetic gap". Either way, I'm sure they'll be fine. In fact, with these, there is a residual risk when they are fully assembled because of the small gap between the inner side of the voice coil former and the tweeter. It's no worse than the many units out there that have a "phase plug" rather than a conventional dust cover (whoever thought that was a good idea???). I've seen many bass units destroyed because of magnetic debris finding its way into the gap - even in a domestic environment - so it's almost a "reflex action" to mention the risk whenever I see people taking loudspeakers apart

Let us know what happens next...

Mark

[Old Sparky]

Furthe googling has confirmed what i suspected, which is that these have been 'got at'. The original screw terminal connectors on both have at some time been replaced with about 3 metres of substatial multistrand wire (red, White, green, & black) wsoldered to the crossover PCB, and led out through the holes in the terminal plates, which once held the screw terminals. AS these speakers were designed for Bi-wiring, the leads have beeen (correctly) bridged by joining the two +ve ones together in one 4mm plug, and the two -ve leads in another. This is OK, and it eliminats connections at the speaker end. However connecting the tweeter & wooffer +ve terminals together at the speaker itself was obviously incorrect. However, having seen a pair of Q15.2s on eBay for £75 (Buy-it-Now price), it begins to look as if repairing these is uneconomic! They're not mine, so it's down to the owner. If he wants to pay me £108 plus my time, then I'll repair them, otherwise they're only good for spares.

[Mark Hennessy]

Yes, the eBay pair is the obvious answer. The buyer protection works pretty well these days, so if they have an issue, you won't end up out of pocket.

Naturally, I believe that it would be wise to restore the crossover connections to stock. There's every chance that air leaks around the wire exit points are causing more problems than the "bi-wiring" connection is alleged to improve.

When it comes to bi-wiring, I always spell it "buy wiring". Because that's what it means. Like all the best audiophile beliefs, it's founded on a grain of truth, but when you crunch the numbers and/or perform objective measurements, the truth is pretty clear to see