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KEF R107 head crossover specifications?

 
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Parts
Senior Contributor 100+


Joined: 13 Jan 2006
Posts: 174
Location: Edinburgh

PostPosted: Tue Nov 25, 2008 11:16 pm    Post subject: KEF R107 head crossover specifications? Reply with quote

Hi all,

I am curious. What sort of signal is sent via the main cab crossover to the head crossover? Is it modified in any way? IE like a high out.

Is the mid crossover point in the bass or head crossover?

Is the Kef R107 head capable of being driven on its own? Or is the crossover and drivers susceptible to damage in that configuration?

Steve
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nat8808
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Joined: 14 Nov 2008
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PostPosted: Fri May 08, 2009 2:52 am    Post subject: Reply with quote

Take a look at the circuit diagram of the cross-over in the bass unit here:

http://www.diyaudio.com/forums/attachment.php?s=&postid=1763069&stamp=1236128685

It looks like there are three components on the crossover in the bass cab that are part of the high-pass filter to the mid/treble unit i.e. C7, C8 and L5. p.s. all those tables are to do with the matching of the drivers to the crossovers.

See the whole thread for more info, including input from the original designer!

http://www.diyaudio.com/forums/showthread.php?s=&threadid=99056
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ColinR
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Joined: 31 Jul 2004
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Location: Staffordshire

PostPosted: Fri May 08, 2009 9:49 am    Post subject: Reply with quote

Quote:
Take a look at the circuit diagram of the cross-over in the bass unit here:

http://www.diyaudio.com/forums/attachment.php?s=&postid=1763069&stamp=1236128685



Bit pointless as you have to register to see the information.
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nat8808
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Joined: 14 Nov 2008
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PostPosted: Thu May 14, 2009 6:54 pm    Post subject: Reply with quote

Pointless?!

The link shows you where this information can be found, the ONLY place it can be found freely and publicly available online as far as I have ever found! If you don't want the info then don't worry about it...

If you do want the crossover diagram then I am sure you will bother to register as it is free and quick.

The most interesting points are in the text of the thread anyway. AndrewJ explains the design principles of the Kef 107 crossover, the Kube and the problems that had to be overcome that HE HIMSELF designed (among others Im sure)! No speculation here as is normally the case in forums, this is from the horse's mouth.

Finally something concrete to work from when trying to improve these already amazing Kef designs! A great deal of point when, like me, your Kube was blown (or lost) by the previous owner..
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Parts
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Joined: 13 Jan 2006
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Location: Edinburgh

PostPosted: Thu May 14, 2009 11:14 pm    Post subject: Reply with quote

I still have a circuit diagram of my Kube in the loft somewhere.

Currently though, I am having noise issues with my Kef R107 when used with my Krell and Classe amps, as I get buzzing with it in circuit, need to play around a bit more and try it in the tape loops. Not sure if it is due to the amplifiers balanced topology or the Kube needing refurbished?

The biamping with a Kube doing the bass only duties sounds great, but I am sure I read somewhere that the Kube also affects the mid tonal balance?

Also teh original Mk1 Kube differed from the MkII kube.

Could you not also just run the head directly?
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nat8808
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PostPosted: Fri May 15, 2009 3:55 am    Post subject: Reply with quote

Looking at the measurement sections in Stereophile's reviews for both versions of the speaker:

HERE for Mk1

and HERE for Mk2

you can see the difference in Kube EQs - the Mk1 has a dip in the midrange to offset a peak in the midrange of the speaker. The mk2 is flat in this area when set to 0.

In the DIYAudio thread, the ex-Kef designer says:

Quote:
Yes, the KUBE's did not only address the bass tuning, but in certain models were also used to provide midrange and high frequency EQ. This is why when the KUBE is not used, the response will not only lack low bass, but will also have a very skewed sound balance.
The reasons for this is that with some amount of active EQ combined with the passive EQ in the network, the overall efficiency of the system can be maximised. A passive network alone will always result in lowest overall efficiency.
The down side is that if the KUBE is eliminated, the speaker will not have a flat sound balance.
Now we did not do this for all kubes. Some were add-ons, therefore they were designed for systems that had already been neutrally balanced, so the Kube jsy extended the bass response.
Of course, with the example of the R107, if the kube is bypassed, then because of the huge change in sysem response then of course you will hear a big difference in sound, though most of this will be due to the change in frequency response, NOT the elimination of the op-amps.....


I reckon (i.e. not a statement of fact) that by the time they made the 107/2 they simply threw out the idea of individually voiced Kubes and added the user HF adjustment. I've mk1s and the Kube is serial number matched with the speakers.

In the Mk1 you can't just plug straight into the head as you'll be feeding it low frequencies that it's not designed to handle. There is a filter in the bass cabinet crossover that filters these out before going to the head unit - if you join the DIY Audio forum and look at the .pdf (or ask for a copy of it from Kef) you get to see from the circuit diagram and physical layout diagram which components these are.

Not sure about the Mk2, maybe it's the same..

I'd say that you would need to fiddle with the head unit crossover to get rid of that dip if you have the Mk1s and want to go striaght into the head unit without the Kube. The bloke mwalters10 in the DIYAudio discussion has bi-passed the head crossover completely and replaced it with just a simple one and it measures flat to him in his room. What he gains perhaps in having a simple crossover he probably looses in efficiency, uneven impedance and stuff.
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exkefman
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Joined: 19 Jun 2015
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Location: Cheshire, England

PostPosted: Mon Jun 22, 2015 1:11 am    Post subject: Reply with quote

Hi,

For the original Model 107's, the matching KUBE:
a) as noted, extended the bass and also provided some mid-band cut - so without the KUBE in circuit, the bass will be weak and the midband will be over emphasised.
b) was factory adjusted for specific pairs of speakers, hence a pair of speakers with S/No. 1234 A/B had a KUBE with S/No. 1234.

If an original KUBE 107 failed when in use, then the original settings were found and a new KUBE was supplied.


With R107/2's, the LF extension side was still handled by the KUBE, but there was no "mid-band cut" nor was the KUBE adjusted for specific speakers (as by then drive unit consistency was far more improved.

As far as the cross-over network was concerned, some elements of the cross-over in the LF enclosure acted as high pass filter, and any frequencies over (IIRC) 150 Hz were then sent out to the head assembly, where a further network divided the signals between HF and MF units.

In theory, one could bypass the HF input terminals on the bass enclosure and feed the head section directly, via the XLR input...but you'd need to have a passive (or active) filter to stop any signals below 150 Hz getting through to the mid-driver, else it would be over-driven.
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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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