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KEF Kube

 
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dsmith
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Joined: 17 Mar 2010
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PostPosted: Sat Mar 27, 2010 10:15 pm    Post subject: KEF Kube Reply with quote

Here is a discussion at the Classic Speaker Forum (primarily focusing on the Boston AR brand) that covers some KEF Kube history.

http://www.classicspeakerpages.net/IP.Board/index.php?showtopic=5863

The Kube comes in at post #13. The Kube was a precision equalizer that "undid" the low frequency corner of a woofer (sealed box or coupled cavity) and then added a lower frequency corner back in.

Electronic bass boost but accurately done.

We also used a form of it to raise the bass corner. (Why would we do that?)

David
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audiolabtower
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PostPosted: Sun Mar 28, 2010 12:38 am    Post subject: Re: KEF Kube Reply with quote

dsmith wrote:
(Why would we do that?)


I guess bass extension is a tradeoff with efficiency so if the average program rolls off in the bass, you can boost the low bass at line level before the power amp just enough to avoid clipping or cone bottoming and extend the response at higher efficiency than otherwise for a given box size?

Or guess the bass might be tighter or faster or just subjectively better by overdamping the system in the box, ie earlier roll off, but get a flat response by some slight line boost to get back to the optimum Q shape for flat?
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dsmith
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PostPosted: Sun Mar 28, 2010 1:32 pm    Post subject: Re: KEF Kube Reply with quote

Quote:

I guess bass extension is a tradeoff with efficiency so if the average program rolls off in the bass, you can boost the low bass at line level before the power amp just enough to avoid clipping or cone bottoming and extend the response at higher efficiency than otherwise for a given box size?

Precisely. In fact you can make a good arguement for having a system with a raw response that follows the "maximum output of music" contour and use electrical EQ to make it flat. You could have much higher mid band sensitivity.
Quote:

Or guess the bass might be tighter or faster or just subjectively better by overdamping the system in the box, ie earlier roll off, but get a flat response by some slight line boost to get back to the optimum Q shape for flat?

Bit of trick question. Using Kube type technology to raise the bass cuttoff was done primarily in the testing process. We could force the system under test to have a faster decaying transient response. Measurements in the transient test room could then be truncated, prior to the first reflection, with no consequences. This gave near perfect "anechoic" measurements.

David
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audiolabtower
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PostPosted: Sun Mar 28, 2010 1:48 pm    Post subject: Reply with quote

I see, so that helped to get a more accurate frequency response from the Fourier result?

I'm sure one or more of the kubes had a Q knob, at least the original 107 I seem to remember, so that was offered to users, though dropped on later ones.

I'm guessing the smaller woofer reference systems had to be careful with cone overdriving, so that much bass equalisation flexibility was kept for systems with enormous bass power handling like the 107?

Was something similar built into the electronics of the KM1 to aid setup on site?
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dsmith
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PostPosted: Mon Mar 29, 2010 1:44 pm    Post subject: Reply with quote

audiolabtower wrote:
I see, so that helped to get a more accurate frequency response from the Fourier result?

Yes, although the impulse response looks like it is quickly over, ther is a long lasting LF ringing that may not be visible. Truncating it early will give an inaccurate picture of the speaker's bass response, usually with a shelved response extending to DC!
Quote:

I'm sure one or more of the kubes had a Q knob, at least the original 107 I seem to remember, so that was offered to users, though dropped on later ones.

I remember the deluxe one for the 107 and a plastic cased version used for the 103/3 (?) but recently saw a picture of one with more knobs and buttons fro frequency and Q. Maybe speakerguru remembers the various models better?
Quote:

I'm guessing the smaller woofer reference systems had to be careful with cone overdriving, so that much bass equalisation flexibility was kept for systems with enormous bass power handling like the 107?

Was something similar built into the electronics of the KM1 to aid setup on site?

There were two distinct variations on the KM1. On the second generation I worked on an elaborate vented LF section. We found that the best power handling came with a rather high venting frequency. Although the natural response became highly bumped (6 to 8 dB, by recollection) the power handling went up to the vent frequency and below. A fairly elaborate cancelation notch was then added and the final response was smooth and well extended. Not exactly the Kube approach but enabled by the various filter optimization schemes we had at hand.

David
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audiolabtower
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PostPosted: Mon Mar 29, 2010 5:08 pm    Post subject: Reply with quote

thank you. That's fascinating about the KM1.

I always thought about the "diminishing returns" of the laws of physics if the only way to get more bass and higher output than something like a 105 (which is a kind of updated BBC LS5/5 or LS5/8 ) is to use FOUR 12in units etc etc and have a huge cabinet which needed special thought and ingenuity to get a sharp stereo image out of the large baffle Smile

Another thought - why so relatively few Kef designs were true reflex, and seemed to prefer a passive bass radiator when required (Cadenza, Carlton, 104, Calinda etc)as opposed to the BBC/Rodgers/Spendor philosophy which always seemed to use reflex when the cabinet was large enough for it to be useful?

Later on of course Kef believed coupled cavity had advantages, though it seems to me to have its own problems when the foam surrounds needed to get low free air resonance all seem to disintegrate over time, when much older drivers still work ok Wink
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speakerguru
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PostPosted: Thu Apr 01, 2010 8:02 pm    Post subject: Reply with quote

audiolabtower wrote:

Later on of course Kef believed coupled cavity had advantages, though it seems to me to have its own problems when the foam surrounds needed to get low free air resonance all seem to disintegrate over time, when much older drivers still work ok Wink


Yes I think it's fair to say, the rapid rotting of foam surrounds took most people by surprise. They were widely used because of the low moving mass, which was very suitable for bandpass designs.
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geoffwood
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Joined: 24 Sep 2009
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Location: Wellington, New Zealand

PostPosted: Thu Apr 01, 2010 9:16 pm    Post subject: Reply with quote

I'm still desparately seeking proper KUBE 107 setup procedure (and schematiocs if poss). !

geoff
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geoffwood
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Location: Wellington, New Zealand

PostPosted: Tue Jun 22, 2010 2:06 am    Post subject: Reply with quote

I'm still after whatever info of KUBE 107.


KUBE alternative could be a Behringer DEQ 2496, but the auto EQ does not work (or attempt to) at LF IIRC...

Re the famous R107 rotting foam surrounds - I wonder if current replacement foam now has 'antibiotics' injected ?!!! Or will I need to replace in another 15 years ?

geoff
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exkefman
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Joined: 19 Jun 2015
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Location: Cheshire, England

PostPosted: Tue Jun 23, 2015 2:06 am    Post subject: Reply with quote

audiolabtower wrote:
...
snip
.
.
Another thought - why so relatively few Kef designs were true reflex, and seemed to prefer a passive bass radiator when required (Cadenza, Carlton, 104, Calinda etc)as opposed to the BBC/Rodgers/Spendor philosophy which always seemed to use reflex when the cabinet was large enough for it to be useful?

Later on of course Kef believed coupled cavity had advantages, though it seems to me to have its own problems when the foam surrounds needed to get low free air resonance all seem to disintegrate over time, when much older drivers still work ok Wink


I had it explained to me by Laurie Fincham, that adding an ABR was done for one or two reasons, depending on the date when the speaker was designed.

Firstly: it was for UK tax reasons, as there was a time (and I can't remember when this "operated" but mid-1960's) when 3-way speaker systems were regarded as "professional" and hence did not attract purchase tax (PT was something like 25% at the time - PT is what VAT was called at the time). I seem to recall another brand used to "paint" a picture of a drive unit on the front baffle to imply it had more drive units than were fitted.

In later years, adding an ABR made the speaker appear to be of "higher value" to customers as it had more drive units fitted (even if the ABR was not actually active, though it did actually help extend bass response, just as well as a reflex port would).

Finally, Laurie had great success when he was at Celestion, designing the Ditton 15XR, which used an 8" ABR. So, when he moved to KEF, it was logical he would follow the same principles.
_________________
regards
Tim

(I used to work for KEF from 1988-1995 - you can see my "profile" here: http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1706)
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exkefman
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Joined: 19 Jun 2015
Posts: 25
Location: Cheshire, England

PostPosted: Tue Jun 23, 2015 2:07 am    Post subject: Reply with quote

audiolabtower wrote:
snip
.
.
Another thought - why so relatively few Kef designs were true reflex, and seemed to prefer a passive bass radiator when required (Cadenza, Carlton, 104, Calinda etc)as opposed to the BBC/Rodgers/Spendor philosophy which always seemed to use reflex when the cabinet was large enough for it to be useful?


I had it explained to me by Laurie Fincham, that adding an ABR was done for one or two reasons, depending on the date when the speaker was designed.

Firstly: it was for UK tax reasons, as there was a time (and I can't remember when this "operated" but mid-1960's) when 3-way speaker systems were regarded as "professional" and hence did not attract purchase tax (PT was something like 25% at the time - PT is what VAT was called at the time). I seem to recall another brand used to "paint" a picture of a drive unit on the front baffle to imply it had more drive units than were fitted.

In later years, adding an ABR made the speaker appear to be of "higher value" to customers as it had more drive units fitted (even if the ABR was not actually active, though it did actually help extend bass response, just as well as a reflex port would).

Finally, Laurie had great success when he was at Celestion, designing the Ditton 15XR, which used an 8" ABR. So, when he moved to KEF, it was logical he would follow the same principles.
_________________
regards
Tim

(I used to work for KEF from 1988-1995 - you can see my "profile" here: http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1706)
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