Kef 104ab with XT25 - looking for info/advice with crossovers

[alan-1-b]

Hi Jonathan,

Hiquphon OW1 will not work well with either the Kef 104 or 104aB crossovers, because:
its middle of bandwidth Impedance is a bit lower than a T27,
and its Fs is lower in both Frequency and Magnitude,
thus it will need some changes to the crossover or you will be losing a significant amount of signal in the 3kHz <--> 6kHz region at least,
and maybe broader band loss than that, but with a little louder in the higher treble region.

The aB crossover was specifically tuned to the 1100 Hz Fs of T27A/SP1032 version, and with respect to the magnitude of the Impedance maximum there.
This crossover will only work with a tweeter that has its Fs close to 1100 Hz {1.1kHz} and with a similar Q of Impedance and SPL magnitude there.

The earlier 104 crossover did not ideally match the SP1032, but was likely the closest Kef could get to ideal then at reasonable cost with the limited options of readily available capacitor magnitudes then.
It seems to have been developed from the very similar crossover Kef used for their earlier 900 Hz Fs T27 type 6535.

OW1 has a lower Q of Impedance and SPL magnitude at its specified 850Hz Fs, and as such it does not need a tuned notch in an aB style crossover for the 3kHz crossing.
A simpler crossover of:
Capacitor - Inductor - Capacitor { 3rd Order filter as in original 104}
plus two resistors in L-pad format around the tweeter to get its midband impedance to a suitable magnitude for your Tube amp, and it SPL to match the B200, is sufficient.

I'll post values for all those components if you want to proceed with OW1.


If you convert the crossover, it would be a good idea to add the aB version's resistor of 10 ohms in parallel with the .6mH inductor in the bass/mid filter, because that was part of Kef's revision to get the crossover point to be exactly 3kHz.
The actual crossover point seems to have been a little lower than 3kHz in the original 104, and with not as smooth response through the overlap region where both drivers contribute as was achieved with the aB revision.

If you have sufficient number of T27A now, may as well stay with that.

Are all your T27s the A/SP1032 version ?
or do you have some of 6535 ?
6535 is visually different - it has a very short horn type flare around the dome, and no SP**** number on the back.

There are new unused original T27A in original packaging box available from a Canadian seller for USD$99 + Shipping if you want to pay that.

Alternately, if you want a new model of hard dome you could try Visaton's Ceramic dome tweeter,
though it has a smaller and different shaped chassis than a T27A.
I have heard this Visaton ceramic in two very well designed l'speakers.
The sound is very clear, but these used modern, fast sounding carbon coated paper coned, Scanpeak bass/mid drivers.
With the older B200 bextrene cones the fast response ceramic dome may cause an audible discrepancy you might not like.

I will be experimenting with different tweeters for B200 myself later this year ... when I eventually get time to ...
as I now have two working B200, and four 104 cabinets with good condition ABRs,
though only two working T27A and I'm not sure how close to a pair those T27s sound.


Your OW1 are from your Linn Kan if I have understood correctly .. ?

I have forgotten the sequence of tweeters in the various Kan models.
The original Kan had either T27A or a Scanspeak .. ?
or the second version had the Scanspeak or the Hiquphon .. ?
or possibly a third version had the Hiquphon prior to discontinuation of Kan ..?

A different type of Kan was introduced a few years later, and with reported very different sound to that of the earlier series of models
- of which I heard and liked two pairs, and one more-so than the other, though I forget the tweeters in either.
I have never heard the re-introduced Kan.


Kef 105.4 had a simpler crossover to 105.2, and a reported more transparent sound as result.
Both used the same drivers, thus if you can get your bass drivers repaired or replaced it would likely be worth re-conditioning the crossover to .4 specification.


The KEF LS3/5A you refer to as owning, are these the "Reference 101" or the "CS1A" ?

[nimo_jon]

Hello Allan

I never expected a dead T27A to pull me back into KEF 104's history, and development into the 104aB, and subsequent speaker designs. I own several KEF speakers of the 70s for over 25 years. Nevertheless, it has been a very enjoyable re-leaning experience.

Many modern speaker designs still continue the fundamentals of KEF 104 type C-R-C third order cross over. With an R-R dividing net work and possibly impedance compensation Zobel net work as addition. Also. most of modern crossover uses poly caps than non polar electrolytics.

Certainly the Hiquphon was not working with the correct crossover. What struck me was the sound of soft dome fabric tweeters as opposed to hard dome ¾ inch tweeters. Their sound characteristics are basically different by nature. While the fabric type is smooth like silk, they lack the metallic bite and brilliance like those of hard dome tweeters. There is nothing wrong with the Hiquphon OW1. I am sure it can be a superb upgrade. But I still am looking for a close resemblance of the T27A. The T27A makes the 104 magical.

These Hiquphon were bough as used units when I was traveling in Japan recently. They were taken from old Linn speakers, so I presume it was from the old Kans. The seller wasn’t very clear in his descriptions, and I did not understand many things he said. Anyway, I can always use them again in a separate project.

I have sold my old Linn speakers many years ago. I remember I preferred the LS3/5a or KEF 101 than the Linn, so I sold it for a good price. Yes, the Kans had a series of different tweeters. I wonder how they managed to keep the flavor consistent.


When I was in Japan I heard a pair of knock off KEF LS3/5a from China. The shocking thing was it was able to achieve about 90% of the sound of a real pair of Rogers LS3/5 (side by side), BUT with only 1/10 the price of the Rogers. I arranged a pair to ship back to US, and need to spend some time studying it. I have the KEF LS3/5a with the falcon crossover installed. I have the technical data sheet of the CS1A. Yes, the CS1A, 101 and LS3/5 all sound very different.

One car audio fanatic suggested to me the use of the Seas prestige series 22TAF/G H1283 as T27A replacement. It has a 3/4inch metallic dome, and also exhibits a 1.2 kHz free air resonance like the T27. Faceplate is at 98mm, close to the 108mm T27. Frequency response is well up to 30 kHz. The sensitivity can be a little high, but can be attenuated by a series resistor. The sensitivity was measured by a 2.83V input. I may get a pair and try them out.
http://www.europe-audio.com/document.asp?document_id=1700&link=datasheets\seas \h1283.pdf

All of my T27A are either of 1975 vintage or latter from KEF constructor kits. There are still a good number floating around from the dreadful Epay site. I do not favour the 6535 because of their age, and also those I found in the market now usually may have some flaws one way or another.

I find the 104 cabinets shabby. The veneer can be tarty after being exposed to moisture. I spent more time trying to restore the wood work, than working on the electrical aspects.

KEF105.2


Sometime in the 80s, I surgically removed the big caps in the 105.2. I thought they were for DC blocking and would rob away the details of the speakers. I think I made a mistake as those caps were there not just for protection; they also were bass boost for the bass units. So I listened to a pair of lame 105.2 all my life. WTF . I also removed the S Stop circuit as I did not like the ‘devil’s eyes’ flashing at times.

Yes, it will be good to restore to 105.4. But I remember the 105.4 were using 2 x 8 inch woofers?


It will be good if you can share the crossover details using the OWI.
What about the original aB crossover?


Thanks and regards.
Jonathan

[Mike-Toronto]

OK back again!

Thanks Allan for your amazing post. People pay good money for that quality of advice! So here I am about a year and a half later up and working on this one again. I was searching around looking to confirm my readings of the crossover and found this post. And I thought dam this is exactly what I need to read, then I realized that it was a thread that I had started. You know you spend a lot of time on the Internet when..

OK so I have come to the conclusion that there is no way in hell I am going to build my own crossover from scratch no matter how much and how generous the help. So I have decided to buy an active crossover and a pair of amps and go with that. At the moment the temporary setup is an ART SLA2 and Alesis RA150 coupled with the DBX Driverack PX. I have been going over crossover settings and tweaking away. It feels pretty close at this point. The system is going to get changed up in about two weeks. I plan on buying another RA150 to replace the SLA2 and the Driverack PX will be replaced with a Behringer Ultradrive Pro.

I have to say that on first impressions this rig is sounding pretty dam good.

That is all for now but Ill keep updating this thread along to way with any helpful information I might come up with.