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Old 16th October 2013, 11:33 PM   #1
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Default Kef Concerto Crossover Rebuild


In my easy listening room, I have a pair of Kef Concerto's driven by a Kenwood Supreme 500. I love the way they sound, but they have the original crossovers and I'm feeling that they may have deteriorated somewhat since they were manufactured somewhere between 1969 and 1977. Winter is on it's way, and I have decided it is a good time to rebuild the crossovers. I'll put in my ESS AMT 1 mini towers or my Wharfedale Dovedale 3's while the recap is being done.

Now, the question is, do I rebuild the crossovers to the original Kef design, or do I try to emulate the improvements that some say were done by Falcon Accoustics, Wilmslow Audio, or some other design? To be frank, I am looking for your help on this decision.

I will document the full recap with pictures every step of the way.

I really love these speakers, so I want to put in the best components possible to within reason for these beautiful sounding classics.

I've been reading a lot on rebuilding the crossovers on the Concerto's over the last few days. Some say the caps were seriously under rated for the power handling of the drivers. If that is the case, I want to put in higher power components so that I can benefit from the additional output. But I don't want to risk blowing the drivers.

What are your thoughts?

I also don't want to lose the balance between the drivers. As it stands right now, these speakers project beautifully at all volume levels I use in this room. This is where I tend to spend most of my listening to music time and entertaining. I have no plans to modify the cabinets unless they will be hidden changes.

I'm thinking quality caps and aircore inductors, and I also intend to build a whole new circuit board so that I can compare the original with the upgraded crossovers. What recommendations do you have?

Should I follow the schematic specs or the in-unit specs?
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Old 17th October 2013, 03:09 AM   #2
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I do certainly remember Concertos. Sold many. No need to replace inductors, those don't wear out. Going to "better" caps will change the sound, as they will have less series resistance. I am headed towards a similar re-cap on my Reference 101s, not all that much newer. I'm going to replace with same parts.
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Old 17th October 2013, 07:49 PM   #3
system7 is offline system7  United Kingdom
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I must be a world expert on KEF speakers. Either that or I'm just plain talented, you have hit paydirt anyway...

KEF Concertos:

Click the image to open in full size.

And the crossover:

Click the image to open in full size.

I'd replace those tweeter capacitors with 4.7uF polypropylenes:
4 7 fd MKP Capacitor

And those old 104mm mylar tweeters with the excellent Morel CAT 298 soft domes:
Morel Classic CAT298

You could replace those old tired electrolytics on the midrange and bass with new ones like this 80uF:

It's always a bit iffy to replace electrolytics with polypropylenes on bass and midrange due to the significant DC resistance of electrolytics that improves damping.

Last edited by system7; 17th October 2013 at 07:52 PM.
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Old 17th October 2013, 10:17 PM   #4
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Thanks for the replies. Regarding the audible change in speakers after replacing the caps, there is an excellent thread here about recapping Celestion Ditton 44 speakers.

Crossover nightmare!!!!!!!

alan-1-b gave a number of us assistance through the whole process with attention on keeping the original sound. When I did mine, I only recapped one speaker and then listened to the two of them together. Huge difference, although subtle. There was distortion on my original 44 that I didn't hear before until I had one of them recapped. Didn't take me long to recap the other. At that point I listened to the 44s and the Kef's together, and decided I preferred the Kef's over the 44s.

In my case here, I want to keep the original sound as much as possible too. So I am planning on compensating for the lower ESR in the new caps with very small mil resisters following the example in the Crossover Nightmare thread.

I have also read on a thread somewhere in internetlandia that Air Core Inductors also make a big difference.

As for the tweeter, I am planning on keeping the original T27s, at least for now. That may be a project for the future. I absolutely love the bass that these speakers present. Amazing!

One thing I find interestingly different in the Kef Concerto threads versus the Ditton 44 threads is that Kef seemed to keep the same components in their crossovers and Celestion seemed to adjust them with the times. Has anybody seen different cap values than the specs call for?

Last edited by TechnoDweeb; 17th October 2013 at 10:36 PM. Reason: fixed spelling
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Old 17th October 2013, 10:41 PM   #5
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I thought I would start the research in advance of actually taking the speakers out of operation until I had a plan going forward. I have two sets of speakers I am considering putting in their place. One set are ESS AMT 1 pedestals, and the other set are Wharfedale Dovedale 3s. They are very different from each other, but the location I will be placing them in is very limiting. It will be interesting which pair I like better in that location.

How did past recap projects on the Concerto deal with the larger capsand positioning them in the cabinets?
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Old 18th October 2013, 05:32 AM   #6
system7 is offline system7  United Kingdom
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Between the idea and the reality lies THE SHADOW, IIRC. I certainly hope this thread doesn't get as longwinded as that Celestion Ditton 44 thread.

Keep it simple and it is all doable. The reason shunt capacitor ESR matters is it affect phase alignment greatly. So don't mess with it.

You don't have much room on the circuit board for huge changes. Do bear in mind that newer lead-free solder needs a hot iron and some quick work.

Speakers aren't hard if you have a clear mind. I would confine myself to upgrading the top end here, as mentioned earlier, along with renewing dried-out electrolytics elsewhere. That treble filter IS a bit messy. It would be more usual to use a 4uF and a 12uF for a smoother rolloff. 4.7uF/4.7uF is a bit peaky, but may be a zingy treble sound you like.
3-Way Classic

The Morel CAT 298 is considered a slot in replacement. I don't know if it'll need attenuation or a 7.5R/1uF Zobel at this stage. Probably not. 250V polypropylene would seem to be much easier than bigger 630V types too.
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Old 18th October 2013, 06:29 AM   #7
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Originally Posted by system7 View Post
.....I would confine myself to upgrading the top end here, as mentioned earlier, along with renewing dried-out electrolytics elsewhere. That treble filter IS a bit messy. It would be more usual to use a 4uF and a 12uF for a smoother rolloff. 4.7uF/4.7uF is a bit peaky, but may be a zingy treble sound you like.
3-Way Classic
I agree....just do the tweeter filter (4.7uF + 4.7uF, and/or try the 4uF + 12uF as suggested also), and then the mid filter.
For the mid two 15uF in parallel and a 4.7//2.2 uF for the 7 uF cap.
I would use 100V Polypropylene caps..higher voltage get too big and too expensive, and you wont need higher voltage rating in this application.
I have heard Concertos done this way and the result works out quite fine.

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Old 18th October 2013, 10:01 PM   #8
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Talking Crossover nightmare

Originally Posted by system7 View Post
Between the idea and the reality lies THE SHADOW, IIRC. I certainly hope this thread doesn't get as longwinded as that Celestion Ditton 44 thread.
I appreciate your input system7, but that thread was invaluable to myself and many others. The only reason I was willing to attempt a cap replacement on my Ditton 44s was due to the detailed explanations from alan-1-b and many others. It had been many years since I had used a soldering iron, and then only on digital circuits. To be frank, analog has always mystified me. alan-1-b gave detailed explanations that gave me the confidence to approach the task of recapping my speakers.

Following your last post, I went on the Internet and did a search on the tweeter you mention. I found a number of threads that provided insight to me on tweeter replacements, in the unfortunate situation where I might find myself replacing my T27s if disaster were to occur, but I really hope that never happens. I really do love the balance and tone of the Concerto's. These are very special speakers in my eyes. My personal opinion is that the people who have suffered from tweeter failure in their Kef T27s are a result of clipping. I have come to this conclusion as a result of testing many speakers and amplifiers over the last year in many different places. I have heard the differences of an underpowered speaker, and then a properly driven speaker and when it is underpowered, I find the highs are shrill to the point of hurting my ears, and the bass seems uncontrolled. I was originally driving my Kefs with a hefty MAC1900 and both my Ditton 44s and my Concerto's sounded harsh if I turned the volume higher. Now, I regularly turn my Kenwood Supreme to louder volume levels than my MAC1900, with no harshness or uncontrolled bass at all. Hopefully, I won't be eating my words for this statement.

The concerto's like lots of current to sound their best. It may also help that I am driving them with a Kenwood Supreme 500. It has been described as having a "tube like" sound, and I have to agree, never having owned a Tube Amp.

Speaking of alan-1-b, I sincerely hope he drops over to this thread to provide his insight. When I completed the recap on my ditton 44s, I got into a few private messages with Alan over my intent to recap my Kef Concerto's.

With his permission, here is an excerpt from the correspondence.

Originally Posted by alan-1-b
... there may have been some variations in the drive units used over the long production life of the Concerto.

If your Concerto has the same crossover as the one I have seen , then proceed as follows:

In Tweeter section , use either 5uF or 5.1uF - whichever the seller you choose has.
No resistors are needed here unless it sounds too bright after you install the new polypropylene caps ,
and if so then install 0.47 or 0.5 ohm in Series with the Input cap - that is the first 5uF after the wire from input terminal on the cabinet.

In Midrange section , if you want an exact 7.0uF cap you will have to buy from:
Sonic Craft High-End DIY Audio Parts
Look in their Sonicap GenI series.
Otherwise , use a 6.8uF cap as that is close enough ,
and install a 2.7 ohm resistor in Series with this cap or you will get an upper midrange resonance.

For the 30uF Input cap the choice of resistor is yours to decide , depending on how much midrange level you want.
1.8 ohm will likely be the most required ,
but you may prefer less - 1.5 , 1.2 , or even 1 ohm if you like a lot of midrange level ,
but that may then dominate over the bass sound you like -
- listen with no resistor to the 30uF cap and you will hear what I mean.

For the woofer section , there are no 80uF caps , but 82uF is fine if you want to pay for the large Solen PA Series 250 volt cap ,
though much cheaper is to buy Axon True Cap in 39uF and 41uF and connect them in Parallel ,
and then add 2.0 ohms minimum , or 2.2 ohms if no stock of 2.0 .
This resistor is necessary to reduce a lower midrange resonance , as the woofer plays about half the lower midrange.

All resistors to be in 5 watt rating - the large 10 watters are not necessary , and take up excess space.
I recommend Mills MRA-5 , which are available from the Axon TRue caps seller:
Parts ConneXion - The authority on hi-fi DIY parts and components
and also from SonicCraft , address as above.

Minimum quality caps for mids and treble I recommend are ClarityCap PX , from:
Madisound Speaker Components: distributor of loudspeaker drivers and parts for speaker builders.
though the Sonicap GenI may be sufficiently better to justify their price.

There is a ClarityCap PX 82uF , but it is quite expensive , and not necessary in the bass ,
but use it if you want to.

The most important aspect is the Series resistors to be connected to the 7uF and 82uF caps.

Look at the photo in #429 on Page 43 of the thread:
celestion 66 needs Midrange
to see Mills MRA-5 connected to ClarityCap PX { Blue } , and the Black Axon caps.
Click on the photo to enlarge it.

Post describing your impressions of the sound after you have listened for a while with the new parts.

Last edited by TechnoDweeb; 18th October 2013 at 10:05 PM. Reason: Added some additional detail
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Old 19th October 2013, 01:02 AM   #9
system7 is offline system7  United Kingdom
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My neighbour in London used to run KEF Concertos along with Quad amplification. I fixed them up for him one day back in the eighties. He wondered what I had done that so improved matters. In fact I had just cleaned up the tarnished connectors. We were enjoying Paul Simon's "Graceland" on them as I recall.

For all that, I always found bextrene and mylar drivers dull and recessed sounding, albeit polite and smooth. I have a pair of soft silk dome ferrofluid Morel CAT 298 tweeters running in one of my projects currently, and they are remarkably good and smooth. We used them in Mr. Ives' repair KEF Cresta projects to good effect:
T27 tweeter replacement in Kef Cresta (1968)

He blogged it all here:
mending things: Kef Cresta speaker renovation

It's my personal opinion that people get too prissy about restore projects, paying huge sums on eBay for old driver replacements. The KEF T27 and Celestion HF 2000 weren't all that good really. Just the best around at the time. The Concerto's doubtless like amps with good current reserves being heavy reflex bass units, that's how it works.
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Old 19th October 2013, 01:39 AM   #10
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Vintage audio is new to me, although I loved that kind of equipment growing up. The realities at the time were that I couldn't afford any of this stuff then, being young, and then young and in a relationship, and then young with 3 kids. I had to wait until my kids grew up before I returned to this passion.

I agree, the Concerto's do sound a little dull at first after listening to Celestion Ditton's. However it doesn't take long to realize it isn't dull at all. It is just your ears tuning to a different ... feeling? I ended up picking the Kef's over the Dittons. And I have listened to many many other different speakers since. I am cursed with the golden ear, and the majority of speakers I've listened to don't sound anything special. I have also had the pleasure of listening to some really excellent speakers. Top of the line Missions, Polk SBAs, and Sound Lab A-3s! The A3s were astounding! However, I have to be realistic, and the A3s are way out of my league.

And I love the sound and look of vintage audio.

I did read that thread. It was quite interesting, and I went on to look at the Mr. Ives site. I think it was that one anyway. Sometimes I read way too much way too quickly and get confused on what I've read and where it came from. Another part of the fun is the research!

Has anyone seen any Concerto crossovers that were different from the schematics? Just curious if they did adjustments like Celestion did in their crossovers..
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