Crossover Diagram for KEF 103 Reference - diyAudio
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Old 14th November 2012, 06:32 AM   #1
bencat is offline bencat  United Kingdom
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Default Crossover Diagram for KEF 103 Reference

Does anyone have a full diagram for the crossover circuit for the original mid 70,s KEF 103 Reference speakers.

I have been able to find a good pair and would like to check the crossover and replace anything that is obviously a problem. Would be nice to be able to measure each resistor / capacitor and ensure they are still within spec .

Thanks in advance for any help .
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Old 14th November 2012, 10:39 AM   #2
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this link has the most KEF xo schematics I've seen on the WWW.

Technical Details of KEF Crossovers
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Old 14th November 2012, 12:21 PM   #3
bencat is offline bencat  United Kingdom
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Many thanks for that link the speaker I wanted was the very last one and has all the information I need to at least check the components out .

Regards

Andrew
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Old 14th November 2012, 12:29 PM   #4
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Be sure to check out your xovers against Colin's schematic. I recapped a Calinda xover recently and found a significant difference between what I had and what he showed. I sent him evidence of the difference, but he never responded. I don't know if he's still monitoring his site any longer. Most of the xover stuff he's posted pre-dates 2010.

Also, be sure to replace all of those Elcaps with the black bodies and red ends. They are not very good at this point in time.
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Old 17th November 2012, 10:42 AM   #5
bencat is offline bencat  United Kingdom
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Quote:
Originally Posted by speakerdoctor View Post
Be sure to check out your xovers against Colin's schematic. I recapped a Calinda xover recently and found a significant difference between what I had and what he showed. I sent him evidence of the difference, but he never responded. I don't know if he's still monitoring his site any longer. Most of the xover stuff he's posted pre-dates 2010.

Also, be sure to replace all of those Elcaps with the black bodies and red ends. They are not very good at this point in time.
Thanks for the advice will do a full check on the physical crossover when I open up the speakers before buying any components.
Was the crossover you found the original or could it have been changed by someone over the life of the speaker ?

I have been thinking about this work and there a couple of issues I need to think about. First one is that I like the sound the speakers produce now and I want to ensure any changes I make are just improvements to things like transparency and keep the original sound. I need to be sure that things are improved and not just different.

The other consideration I have is if I ever sell the units on any vale is going to be in the condition and the originality . So I have thought about making a pair of new crossovers using something like Clarity Caps , decent resistors and Inductors and putting them in a box out side the speakers then just disconnect the original c/overs in the speakers and connect the units direcly to the speaker sockets and the out board crossovers straight in to them. This will enable me to make any changes to the new units like increasing the resistor values to compensate for the greater efficiency of the new caps and try out what they sound like and fine tune them if need be. Then if I do not like the result or want to sell the Kefs then I just reconnect the inbox c/overs and they are back to the original state.
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Old 17th November 2012, 11:22 AM   #6
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The xover I worked on was original. Your plan sounds good.
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Old 28th November 2012, 09:57 AM   #7
bencat is offline bencat  United Kingdom
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Right I have now had the KEF 103,s in use for about ten days. They are in pretty good exterior condition and the driver cones appear to be fine with no dents . Have opened them up and confirmed that they are populated by electrolytic caps which are now over 30 years old.

I have decided to do a full capacitor substitution using polypropylene film items at the same values as the originals. I will leave the inductors and resistors as they are I measured the resistors and they all appear in specification.

Now as much as possible I want to keep the original sound and just return the crossover back to specification and use items that will last without any problems over time. Fitting polypropylene can lead to problems due to better electrical efficiency than the original and this can change the Q . I am told that the way to bring this back to spec is to place a resistor in series with the HF unit and about 100 / 150 R is the normal level.

Does anyone have experience of this sort of substitution ? Is there any formula you can apply to get the right value of the resistor or is it just trial and error ?

I have attached a copy of the Cross over circuit if this can assist anyone.
Attached Files
File Type: pdf REF 103_20121115092121.pdf (35.8 KB, 25 views)
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Old 28th November 2012, 10:49 AM   #8
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I'm a former KEF engineer.

Even when electrolytic capacitors were used they would be low loss with fairly low ESR. ESR (equivalent series resisitance) is all that you would want to add and in fact you would want to add the difference between the orriginal resistor (when new) and the replacement with its lower resistance. I wouldn't imagine the old caps had more than 0.2 to 0.3 ohms, so we aren't talking much in potential difference between that and a film cap.

The only potential difference would be a slight sensitivity change through the tweeters range. You would be as well off just changing to film caps (note the drawing's tolerances, pick something in the given range) and adding an ohm in series with the tweeter if you thought the end result was brighter than expected.

David S.
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Old 28th November 2012, 11:11 AM   #9
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Or, bencat could consider using only polyester film caps. This dielectric material provides an ESR somewhat higher than polypropylene film types and closer to electrolytics. I ran some tests a while back that confirms this.

Effect of Film dielectric type on capacitor ESR
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