I am rebuilding a pair of Kef K1 monitors (K2 version so-called studio monitors) I bought around 1965. Big cabinets with a B139 woofer which I am keeping, but I am replacing the original tweeter and eliptical mid-range which is no longer available with a T27 tweeter, B110 mid combination (as used in the LS35A).
The original mid and tweeter were housed in a sealed box of about 1.8 litres (not including the volume of the speakers themselves). I had hoped to use that old box for the new units. But I see that the spec sheet for the B110 specifies 4.25 litres for closed box volume when used within a larger enclosure.
So should I make a new 4.25 litre box (which will be sealed and heavily damped inside)? Can I follow the original design of putting the tweeter in with it? And should I allow anything for the volume that the speaker units themselves are taking up?
Any advice gratefully received.
Catawallah
The original mid and tweeter were housed in a sealed box of about 1.8 litres (not including the volume of the speakers themselves). I had hoped to use that old box for the new units. But I see that the spec sheet for the B110 specifies 4.25 litres for closed box volume when used within a larger enclosure.
So should I make a new 4.25 litre box (which will be sealed and heavily damped inside)? Can I follow the original design of putting the tweeter in with it? And should I allow anything for the volume that the speaker units themselves are taking up?
Any advice gratefully received.
Catawallah
Interesting! And I mean that in a Sir Humphrey Appleby sense! (Hope you're a "Yes Minister" fan.) Some of the answers are pretty straight forward. The B110 will much prefer the 4+litre enclosure, maybe even bigger. The tweeter and B110 can, and probably should go together. If you are just making a straightforward sealed box the volume won't be too critical. I.e. a few cubic inches displaced by the units themselves will not be crucial.
The bit that has me a little intrigued is the K1/K2 references. If you Google those two type number you get two different woofers (indeed two quite different speaker systems). According to my initial search the K1 had the earlier and bigger B1814 which must be pretty rare these days. But that does agree with your observations about the elliptical mid-range. The K2 seems to have the B139, no mid but the 1.5" T15 tweeter.
FYI there is mountain of material on the B110/T27 combination. I would have over half a dozen distinct cross-over designs (schematics) in my files which I have garnered from the Web. So there is plenty of material going around for you to work with. What you might find useful is a UK site entirely dedicated to older KEF designs and drive units. Just Google "hifiloudspeakers" . That might seem way to general but it will throw up the site I mentioned and there will dwell the "Serious KEF People". I also imagine they will be interested in your units as I can't recall they have anything on the K1. Colin Royle is a noted authority in your area and you can interact with him and others in their forum section.
Good luck, cheers, Jonathan.
(ex-UK now Aust')
The bit that has me a little intrigued is the K1/K2 references. If you Google those two type number you get two different woofers (indeed two quite different speaker systems). According to my initial search the K1 had the earlier and bigger B1814 which must be pretty rare these days. But that does agree with your observations about the elliptical mid-range. The K2 seems to have the B139, no mid but the 1.5" T15 tweeter.
FYI there is mountain of material on the B110/T27 combination. I would have over half a dozen distinct cross-over designs (schematics) in my files which I have garnered from the Web. So there is plenty of material going around for you to work with. What you might find useful is a UK site entirely dedicated to older KEF designs and drive units. Just Google "hifiloudspeakers" . That might seem way to general but it will throw up the site I mentioned and there will dwell the "Serious KEF People". I also imagine they will be interested in your units as I can't recall they have anything on the K1. Colin Royle is a noted authority in your area and you can interact with him and others in their forum section.
Good luck, cheers, Jonathan.
(ex-UK now Aust')
Thanks to Richie00boy and Jonathan for their rapid and interesting replies. The speakers will be crossing over at 400 and 3500 (I have DN12 SP1004 3-way crossovers), so Richie00boy's comment could save me quite a lot of time. Or I can make a new box, but less than the specified 4.25 litres, which would take quite a bit of fitting in.
Jonathan's reply was fascinating (and useful with the comment about tweeter and mid going well in the same box). I certainly made a mistake in using the term K2, since this is indeed a quite different and cheaper speaker system (also one of Kef's DIY baffles). My problem is that I've had to guess, since the labels on my speakers don't seem to bear any relation to anything much. For example:
Production label: writing looks like K2, though could be K1 - as established above, certainly not K2.
Tweeter: no name, handwritten 760 (could be 160 but probably not). Has a printed label 1286.6.
Mid-range eliptical: no name, handwritten red label marked 110. Also KEF, MADE IN ENGLAND, UK PAT 930.232.
Woofer: no name, a stencilled label saying B20. The unit configuration has 6 arms, like the enormous B1814 but it is very much less like 18 inches than the 13 inches which is the measurement for the B139 - 4-armed according to its picture. (I'm not quite sure where you measure from and the speaker is currently done up in plastic for safety.)
And the site Home Page hifiloudspeakers.info with its forum is a notable discovery. Thanks again.
Jonathan's reply was fascinating (and useful with the comment about tweeter and mid going well in the same box). I certainly made a mistake in using the term K2, since this is indeed a quite different and cheaper speaker system (also one of Kef's DIY baffles). My problem is that I've had to guess, since the labels on my speakers don't seem to bear any relation to anything much. For example:
Production label: writing looks like K2, though could be K1 - as established above, certainly not K2.
Tweeter: no name, handwritten 760 (could be 160 but probably not). Has a printed label 1286.6.
Mid-range eliptical: no name, handwritten red label marked 110. Also KEF, MADE IN ENGLAND, UK PAT 930.232.
Woofer: no name, a stencilled label saying B20. The unit configuration has 6 arms, like the enormous B1814 but it is very much less like 18 inches than the 13 inches which is the measurement for the B139 - 4-armed according to its picture. (I'm not quite sure where you measure from and the speaker is currently done up in plastic for safety.)
And the site Home Page hifiloudspeakers.info with its forum is a notable discovery. Thanks again.
A History of Kef Drive Units from the 1960s and 70s - A History of Kef Drive Units issue 3
http://www.hifiloudspeakers.info/Anatomy/KEFDocuments/OldModels.pdf
http://www.kef.com/uploads/files/en/museum_pdf/60s/K1_Series_Brochure.pdf
http://www.hifiloudspeakers.info/Anatomy/SpeakerSystems/Concerto/ConcertoCrossoverSchematic.jpg
The B139 SP1044 working with the DN12 SP1004 crossover is a 8 Ohm driver. Older units of the B139 and B1814 where 15 Oms. If interchanging crossovers/drivers make sure you are using adequate impedances or redesign the crossover for that.
http://www.hifiloudspeakers.info/Anatomy/KEFDocuments/OldModels.pdf
http://www.kef.com/uploads/files/en/museum_pdf/60s/K1_Series_Brochure.pdf
http://www.hifiloudspeakers.info/Anatomy/SpeakerSystems/Concerto/ConcertoCrossoverSchematic.jpg
The B139 SP1044 working with the DN12 SP1004 crossover is a 8 Ohm driver. Older units of the B139 and B1814 where 15 Oms. If interchanging crossovers/drivers make sure you are using adequate impedances or redesign the crossover for that.
Yes, the B139 is 15 ohm and the other units I am putting in are 8. The SP1004 DN12 crossover is all 8 ohm. Alas, though, modifying a crossover circuit is beyond me. As it is, the LF part consists of a 5mH choke (1 ohm internal resistance) with an 80u capacitor (50v) going to earth from the end of it. Any sugestions gratefully received.
You can double the inductor and half the cap. Adjust, because probably you don't need as much full value components maybe less more between the two. Having access to an old crossover for the 15 Ohm might give you the answer. Post with new info.
Catawallah, you really need to pull a SP or A number off the B139. There were so many versions! You're also going to have to work out what the original crossover values were for comparison even if you need to buy a £60 multimeter with an inductance scale. That should tell you something, especially since bafflestep is hard to guess, though I'd think these were designed to mount against a wall in big cabinets, hence not so big coils, but bigger rolloff capacitors.
Personally I'd be wondering if you can't find some midrange and tweeter units to match, even if not KEF. You can find modern stuff like this SB Acoustics polycone that look rather better than the B110:
Madisound Speaker Store
Level matching and estimating precise crossover points is going to be the bitch.
These schematics by Colin Royle and Steve Bell as usual.
Personally I'd be wondering if you can't find some midrange and tweeter units to match, even if not KEF. You can find modern stuff like this SB Acoustics polycone that look rather better than the B110:
Madisound Speaker Store
Level matching and estimating precise crossover points is going to be the bitch.
These schematics by Colin Royle and Steve Bell as usual.
Attachments
I would call Falcon Acoustics Falcon Acoustics | The Leading DIY Speaker Parts and Kit Supplier since 1972 and would ask their advice or if they have the crossover for the Kef K1 monitor but mentioning that is the same as the concerto with the old Kef B139 15 Ohms.
Thank you Inductor and System7 for your helpfulness. It's all quite a learning curve and during it I have discovered that I have been pretty naive about these speakers and some of my assumptions about these speakers seem to have been wrong from the start. It may be that I have something like a couple of rare museum pieces on my hands and that I should consider restoring them as they were and not modifying them as I was intending. I'll explore this on Colin Royle's site, which you introduced me to. But I am very grateful for your information, which has taught me a lot.
Some top designers use the KEF B110 on a cheese-wedge shaped transmission line, open at the back:
transmission line speakers at KE Engineering - YouTube
speakerman19422 told us about this.
You misunderstand. It's being used as a midrange. Transmission line is a way of applying optimal resistive loading to any unit.
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