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KEF 103.3 KEFCUBE

 
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Ianmac
Junior Contributor 10+


Joined: 07 Aug 2007
Posts: 11

PostPosted: Sat Aug 11, 2007 3:49 pm    Post subject: KEF 103.3 KEFCUBE Reply with quote

As I upgrade my system electronics the noise floor in todays amps is below that of the KEFCUBE.
I would like to try and reduce the noise in the cube by using lower noise Chips and other components, and improve the powersupply.

Am looking for a circuit diagram.

KEF have not replied to my emails.
Anyone out there have one
Ianmac
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proffski
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Joined: 22 Aug 2003
Posts: 1297
Location: Tewkesbury UK

PostPosted: Sat Aug 11, 2007 7:21 pm    Post subject: KEF 103.3 KEFCUBE Reply with quote

I cannot help you with the diagram, but...
Some advice.

I did once help and repair a KUBE. Unusually one of the op amps had popped.

Removing the 8 Pin DIL component I damaged the double sided PCB badly.
It did not take kindly to having bits removed.

There is a happy ending as in the end the repair was managed and a high quality turned pin socket soldered into the board.

Now with eyesight even worse than it was ten years ago I would refuse to offerany any such help. Maybe other KUBES used more robust tracks on the PCBs, I cannot even remember which one it was that I worked on.

These particular tracks lifted with the least provocation!
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Last edited by proffski on Mon Aug 20, 2007 10:38 am; edited 1 time in total
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speakerguru
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Joined: 18 Nov 2005
Posts: 959
Location: Green Hut, Tovil

PostPosted: Mon Aug 20, 2007 9:31 am    Post subject: Re: KEF 103.3 KEFCUBE Reply with quote

Just back from hols. I will look for a circuit diagram, but don't hold your breath. I don't recall the 103|3.

Ianmac wrote:
As I upgrade my system electronics the noise floor in todays amps is below that of the KEFCUBE.
Very unlikely, as the laws of physics have not changed recently Wink .

If you are experiencing noise when using the Kube it means that you are not operating it at a high enough signal level. I would suspect that you have it between the pre and power amp. This means that the full Kube output noise is always being fed to the power amp regardless of volume control setting.

Try putting the Kube in the tape monitor loop where it will normally be in circuit before the volume control. This means that the signal will be running through the Kube at a higher level and also that the signal and Kube noise will both be proportionatley attenuated by the volume control.

Ianmac wrote:
I would like to try and reduce the noise in the cube by using lower noise Chips and other components, and improve the powersupply.
Changing to lower noise chips won't do a thing, as the dominant noise source is the thermal or "Johnson" noise from the resistors. To get a significant noise reduction when changing from TL072 to NE5532 opamps you would also need to scale down the impedance by lowering all the resistors and increasing all the capacitors pro rata. You would also then need to buffer the input so that the new lower impedance Kube did not excessively load your source.
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vistisen
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Joined: 01 Sep 2006
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Location: Denmark

PostPosted: Mon Aug 20, 2007 4:26 pm    Post subject: Re: KEF 103.3 KEFCUBE Reply with quote

speakerguru wrote:

If you are experiencing noise when using the Kube it means that you are not operating it at a high enough signal level. I would suspect that you have it between the pre and power amp. This means that the full Kube output noise is always being fed to the power amp regardless of volume control setting.

Try putting the Kube in the tape monitor loop where it will normally be in circuit before the volume control. This means that the signal will be running through the Kube at a higher level and also that the signal and Kube noise will both be proportionatley attenuated by the volume control.



I tried doing this but found that some CD's were recorded at such a high level that this KUBE started popping violently, So be careful. I proved that this was the case by using a test disc, low frequency test signals at high levels, caused the popping straight away.
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Tue Aug 21, 2007 8:15 am    Post subject: Re: KEF 103.3 KEFCUBE Reply with quote

vistisen wrote:
I tried doing this but found that some CD's were recorded at such a high level that this KUBE started popping violently, So be careful. I proved that this was the case by using a test disc, low frequency test signals at high levels, caused the popping straight away.
In that case you would need to put attenuators in the Kube inputs or the tape monitor outputs. If you have no measuring gear then try 3dB to start with. Increase the attenuation until the popping stops.

OTOH if you decide to stick with the Kube between pre and power amp, put attenuators between the Kube and power amp. Then you can run your preamp and Kube at a higher level (relative to the Kube noise).
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Ianmac
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Joined: 07 Aug 2007
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PostPosted: Sat Aug 25, 2007 6:02 pm    Post subject: KEFCUBE 103.3 Reply with quote

Sorry for my delay in replying have been away for 2 weeks
Have already tried the various connection modes with both my integrated and separate pre amp/power amp (Cyrus) It does not cure the problem. I'm certain it is due to the signal to noise ratio of the Kube being some db's worse than a modern amplifier. I have already recieved a PM from a member who had the same problem and he replaced the NE5532 op amps in the Kube with OPA2132's and reduced the power rails. This resulted in a considerable improvement to both noise and sonic performance.
I would like to see the benefits I could gain by doing a similar excercise, I envisage a complete new build using current component and power supply technology,
( leaving my existing kube untouched )
thus my quest for a circuit diagram.

If nothing is forthcoming I will have to trace the circuit from my existing Cube
Ian
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speakerguru
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PostPosted: Tue Aug 28, 2007 9:12 am    Post subject: Reply with quote

Sorry, I've had a search but I can't find a schematic of the 103|3 Kube Sad . You'll have to trace out the existing circuit. If you send me your work I can check the topologies and help with any impedance scaling redesign, particularly the Friend biquad sections.

Unless you have a rare ProKube, your Kube will have TL072 opamps, so anything you've been told about changing NE5532s is not really relevant, is it? Confused . Assuming you don't have a circuit simulator I am willing to run designs through one for you so you'll be able to assess if any worthwhile noise reductions are possible.

BTW, the noise for both NE5532 and OPA2132 is 8nV/rtHz, so the only increase in signal to noise you can get by using OPA2132s is by increasing (you said decrease in your post; that's wrong) the supply volts and running them at a higher signal level. The NE5532s will run on +/-22V supplies and the OPAs will run on +/-36V, so there is a potential 4dB improvement there, if, and only if, you reduce the resistor noise by scaling the circuit impedances and you run the Kubes at a higher signal level (and then attenuate both signal and noise before the power amp).
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Ianmac
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Joined: 07 Aug 2007
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PostPosted: Wed Aug 29, 2007 1:55 pm    Post subject: Kef 103.3 Cube Reply with quote

Many thanks. I still have a few feelers out trying to get a circuit. If these turn up blanks I will try to trace it from the PCB.
Ian
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speakerguru
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PostPosted: Thu Aug 30, 2007 11:24 am    Post subject: Reply with quote

What is the SP number of the 103|3 Kube?
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Ianmac
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Joined: 07 Aug 2007
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PostPosted: Thu Aug 30, 2007 4:17 pm    Post subject: Kef 103.3 Kefcube Reply with quote

Cube No SP2080BA S/N 3930
Spkrs S/N 002925A and B
Ian
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Tue Sep 18, 2007 8:46 am    Post subject: Reply with quote

Well, well, well, who's a lucky boy then Mr. Green Ianmac.

I have managed to get a copy of a set of 103|3 Kube drawings. Don't ask.

Send me a pm with your email and I will send you a zip file.
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Ianmac
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Joined: 07 Aug 2007
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PostPosted: Wed Sep 19, 2007 8:06 pm    Post subject: 103.3 cube Reply with quote

Many thanks for the diagrams. have sent a PM
Ianmac Very Happy
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erik_bjorge@hotmail.com
Introductory Contributor


Joined: 04 Aug 2016
Posts: 3

PostPosted: Fri Aug 05, 2016 6:43 pm    Post subject: Schematics kube 103/3 Reply with quote

Hi
Im looking for schematics to my kube 103/3 - sp 2115 - no 001355
Can somebody help me.......
Erik
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Sun Aug 14, 2016 3:36 pm    Post subject: Reply with quote

I've not found them. Can you help, Ianmac?
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