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KEF Carlton III resurrection with Seas 22T/AF

 
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Nimo_jon
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PostPosted: Tue Jul 05, 2011 2:50 pm    Post subject: KEF Carlton III resurrection with Seas 22T/AF Reply with quote

The KEF Carlton III (1983) speakers are now more than three decades old. After the long years, the T33 tweeters may suffer from performance degradation due to the thickening of the ferrofluid in the voice coil gap. Replacing the T33 tweeters or cleaning the voice coil on several pairs of Carlton (II & III) yielded no real benefits. The T33s weren’t particularly impressive tweeters to start with. The T33s may had put an end to most Carltons’ serviceability, perhaps already so in the 90s. It could have been a major factor to the Carlton’s short product life span.

The Carlton III is the continuation of the very successful KEF 104 aB design. But it did not surpass the predecessor. The cabinets were more solidly built, with better damping. The layout of the drivers reduced edge diffractions. The new B200 units were computer designed and have better consistency. (And yes, these drivers are still going strong until today.) The crossovers employ the acoustic Butterworth design, and the snubber circuit took into consideration the spread of resonance frequency (+/-100 Hz) of the tweeter units at 900 Hz. Still, the Carlton III did not enjoy a better fame than the 104s. I still put the blame on the T33 SP1074 fabric dome tweeter. If this problem did not exist, then Carlton III may very well enjoy a bigger fame than the 104s. Sadly, I know of many of them been mutilated for parts to be sold in the second used market.

The modern speaker manufacturers do offer a vast number of tweeter units, but it is not easy to find a good replacement for the Carlton. Cost is a factor, more so are electrical characteristics and facet dimensions. Most amateurs would look for a drop in unit without any work done on the crossover. This could be mission impossible because the crossover was built to suit a particular tweeter, and no two tweeters were built the same, electrically and mechanically, even by the same manufacturer. Unless, we are talking about very cheap crossovers that use only first order filtering. As such, to revive the Carlton III, a complete redesign/reconsideration of the crossover is absolutely necessary after the choice of the replacement tweeter.

My plan was to redo the Carlton III such that it has the high frequency performance of the alloy dome T27a found on the KEF 104 aB. Not using the T27a, as it does have some particular "ringi-ness" or undesirable irritation. Also, it is impossible to find a good pair of T27a these days. The tweeter may be able to fit into the rectangular recess onto the speaker facet without much modification. The sound should retain the smoothness of the fabric dome, and yet able to play cymbals nicely and crisp like the 3/4" metal dome tweeter. The Carlton III should live on.

I started off with the Hiquphon OW-1 tweeters, and a number of other popular choices, but eventually I finalized my choice with the Seas 22TAF/G. This is a hybrid dome tweeter that has a 3/4" alloy dome supported by very large fabric skirt. It turned out to be the best sounding tweeter, and able to blend into the KEF’s sound characteristic seamlessly. Do not let the price fool the judgment; the 22TAF/G is an excellent performer.

Original Crossver

The Carlton III’s high pass section uses the similar 3rd order Butterworth design as the 104aB. The snubbing circuit uses a resistor to slightly widen the snubbing frequency range around 900 Hz, without sacrificing the deepness (effectiveness) of the notch. A Zobel circuit tames the tweeter’s rising impedance above 15 kHz.

The low pass section consists of a RLC resonance that provides a peak near ~2 kHz and also determines the Q of the system. The 600uf capacitor not only isolates the speaker from the danger of DC from the older transistor amplifier, it provides a peak at 100 Hz to give a deeper and quality bass.

Consistent with most KEF’s 70s/80s products, the crossover components were adequate but never overkill. The inductor sizes were modest, and the capacitors were from Alcap. Although the passive components still measure fine after 30 years in service, the capacitor performances degrade over the years. The PCB tracks became slightly corroded, partly due to remnant flux from soldering (?). Many Carlton III sounded as if it could only produce mid range after all these years.

One particular concern was the choice of a single 600uF 50V NP capacitor. It seems too small for the job, and on most Carlton III crossovers the capacitors show signs of fatigue, as most look bloated at the ends. In some cases, the speakers became totally mute because the caps had become open circuit. (many who tried to measure the Carlton III’s impedance usually would have a shock ; ) OC? anyway…).

New Crossover

The 22T/AF has lower impedance than the T33 and has a higher sensitivity of 93dB. The snubber circuit in the high pass is necessary to produce the smooth KEF sound. This simple KEF specialty is not found in most modern HF designs. Many DIY speaker builders using 22T/AF may be surprised how important this snubber is, even if they cross the tweeter way above the 1.05 kHz resonance F.

Using the same crossover F point, the notch filter RC was adjusted to match the same roll off. The sensitivity matching is provided by the simple R pad network. The attenuation network table can be found on the internet for both 4 and 8 ohms speakers. The value of 6 ohms unit can be derived by interpolation of 4 and 8 ohms values. To match the sensitivity of the B200B unit, the tweeter was attenuated from 93db down to about 88db. Although the R values are somewhat awkward, they can be formed by series parallel resistors of standard values. There is no need to add a bypass cap on the series attenuation R (verified by SPICE and listening), but the R should be non-inductive in nature. Zobel is best included to provide good response way above 15 kHz.



There is no need for the high pass section to go thru the series 600uf cap at the input terminals because the cap is basically an AC short circuit. As such, the high pass can tap directly from the input, leaving the 600uf to do the low pass job only. Simulation confirms there are no phase issues.
The following is the SPICE simulation showing how the modified crossover mimics the original design by KEF. The simulated frequency sweep results closely match the measured SPL below 15khz. The knees of the original circuit went away. The tweeter is capable of frequencies above 20 kHz, an important capability when playing LP.

SPICE simulation of high pass - green trace Seas red trace KEF


The original inductors for the low pass section may be undersized. Using air cored units has both size and cost penalty, but both are preferred in the pursuit of better sound. The 12 ohms in the RLC resonance circuit may be reduced to 10 ohms to add more mid range, its up to individual.

As the air cored inductors add slight series resistance to the low pass section, this can be compensated by adding more capacitance in the series large valued capacitor to 650uf. The peak is shifted lower to below 100 Hz, thereby pushing up the magnitude of overall response curve near/above 100 Hz.



Using a single large capacitance may not be a good idea as most capacitors, even modern switch mode supply type, cannot withstand too much ripple current. The capacitor will heat up quickly and degrade rapidly. Some may even explode if stressed. The 600uf cap was replaced by 6 paralleled low loss 100uf 100V non polar for audio units, bypassed with 10-50uf of polypropylene fast caps. This combo is a lot more expensive of course. What make sense in mass production may not be welcomed for the end user. Changing this 600uf cap has a greater return than using a much better amplifier in practical terms. By simulation, the ripple current in individual cap is well below 1 A @ max load (I cannot image passing a few amps thru a single small cap). The sound becomes more open, detailed and the bass can go deeper in listening tests.

SPICE simulation of low pass - not much difference


The crossover was built on three perf boards by hard wiring and secured by hot glue. The original crossovers were retained and can be used for comparison if required. Internal wiring was old stock Western Electric tinned solid core copper wire, measuring 1mm in diameter. These cotton sleeved wire sounds amazingly smooth and good than most multi stranded wires with all the fancy names. The speaker terminals were replaced.

All parts were sourced from Jantzen parts of Denmark. Total cost for new crossover parts was about US$200++. The tweeters cost less than US$50 each. A good alternative would be Mundorff parts. For the high pass section, using Solen fast cap did not give a very good return in comparison. (I only use Solen for snubbing). The Jantzen standard or Mundorff white caps are simply better. Alcaps would give a more mellow sound. Those who prefer the Alcaps can by all means do so, its personal choice.

Woodwork

Most KEF speaker cabinets of the 70s/80s were built using chip boards. They do not like water, moisture, and they do not like to be mishandled. Some basic wood working is necessary to improve the overall presentation of the worn cabinet. The recess holding the drivers may become loose or bad, which can be touched up by wood fillers and repainted.

I have sealed the cabinet by applying a layer of polyurethane at the
internals. It turned out to be a nightmare because the oil based polyurethane would continue to smell very badly for weeks and choke any one near the speakers. Even placing the cabinets under the sun did not help much. It took me 3 weeks to totally get rid of the smell, so it is best to apply spray paint or lacquer that is rapid drying.



I have discarded the wool damping materials in the speakers. The internal was reinforced by a layer of hard wood filler and 1 inch noise absorbing materials padded to the walls (just like the instruction found in KEF constructor sheet). This is a time consuming job. KEF in their mass production schedule will have no time for this laborious process of internal damping.





The Seas 22T/AF has a plastic or fiber glass facet that can be easily modified. Just filing off 1mm off each round edge, the speaker would fit nicely onto the speaker cabinet. Certainly, we need to open the insides of the ‘eye’ on the tweeter recess so that the wire terminals may go in smooth.



Still, there are no major works involved on the cabinets. Opening up the speaker recess and work on the tweeters should not take more than 1-2 hours by a novice. The tweeter fits snugly into the cabinet. Using silicone sealant to make a gasket should make it air tight easily.

The completed speakers require at least 50 hours of breaking in. Before that, the bass can be lacking and the high frequency closed in. After 100 hours of running, the speaker performance stabilizes and can allow serious listening.



Final Sound

The Seas 22T/AF on the Carlton III has a similar extension, richness in high frequency range as the T27a, but with none of the harshness. The cymbals sound more refined and delicate, but with no loss in metallic brightness. The sound may not be as ‘exciting’ as the T27. The sound is indeed a cross between a metal dome tweeter and a fabric tweeter, but more towards the metallic characteristics. The tweeter integrates well with the B200 and would not break into two different type of sound as the frequency range change over. Amazingly, it is exceptionally smooth like silk and I guarantee you there will be no fatigue after hours of listening (but on some other speakers that use the 22T/AF, my ears could bleed). Yet, there are no loss in any details whether listening to vocals, jazz or classical music. It is now a class above my pair of overhauled 104aB in terms of high frequency reproduction.

Mid range wise, I was somewhat disappointed because it is not as colored as the 104aB. The newer B200B (5 ohms) is more damped that the B200 SP1039 and has a different characteristics than the SP1039. The Carlton III sounds more blend and less emotional than the 104aB in this area. But still, it is accurate and sound staging is better than the 104aB. The mid range is indeed very open and detailed. The source and amplification would be more a limiting factor.

For the bass section, I was very pleased because the new combo caps indicate a marked improvement. Not only the speakers have better control, the bass went deeper as if a quality sub woofer was added. (I experimented with a 1000uf 50v (polarized) cap in place, and the nice bass was gone). So the combo works. We are still talking about the same cabinet dimensions, slightly improved damping and the same driver units 30 years of age. The air core coil would not saturate as the ferrite types, and that provides a signification improvement in the quality of the voice of any singer. Even the cello never sounded so good before. The speakers may not be very efficiency, by could be driven loud by either a 50W solid state integrated or 5W single ended amplifier.



As the crossovers continue to break in, the sound becomes more and more transparent and open. It comes to a point that some decent recordings can sound as good as live performances. I have finally decided to let this pair of Carlton III take center stage instead of the overhauled KEF104aB (with new drivers and Mundorff parts). I am now contemplating to incorporate the Seas 22T/AF into my second pair of KEF 104aB, instead of using the T27a. That pair of speakers are now under work and I hope to finish before this summer ends. But there would be more work on simulation and measurement. I am confident the work done now elevates the old Carlton III to a level that can rival anything in the market less than GBP1000, or maybe more.




If you have a pair of Carlton, stop messing around with replacement T33. Do consider putting in a pair of Seas 22T/AF with the corresponding crossover parts. If you want to keep the original working crossover, include the R attenuation network, change the snubber RC network and the Zobel values. Listen and tell me what you think. Cheers.
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audiolabtower
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PostPosted: Tue Jul 05, 2011 4:54 pm    Post subject: Reply with quote

Congratulations, you've done a lot of work to improve your speakers Smile

A small point, probably due to translation Wink , there was only one T27 SP1032 as far as I know (the type with wires visible on the faceplate), and the same was used in 104 and 104aB. It was a plastic dome made of melinex.
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Nimo_jon
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PostPosted: Wed Jul 06, 2011 3:30 am    Post subject: Reply with quote

Thank you Sir. You are right. Smile
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mulane
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PostPosted: Tue Aug 30, 2011 12:02 am    Post subject: Reply with quote

Another very interesting thread Nimo_jon. What progress have you made with the Seas in the 104aB?

Very interested in the circuit you come up with as I also use T27s in my transmission lines and if you could develop an upgrade for the T27 you will be doing the classic KEF community a great service. Once completed, any chance of running the circuit added to the P.Atkinson SOTA crossover through the SPICE program to see if it is appropriate there also?
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Nimo_jon
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PostPosted: Tue Aug 30, 2011 12:44 am    Post subject: Reply with quote

mulane wrote:
Another very interesting thread Nimo_jon. What progress have you made with the Seas in the 104aB?

Very interested in the circuit you come up with as I also use T27s in my transmission lines and if you could develop an upgrade for the T27 you will be doing the classic KEF community a great service. Once completed, any chance of running the circuit added to the P.Atkinson SOTA crossover through the SPICE program to see if it is appropriate there also?


Good evening Sir!

The SEAS 22TAF is a modern and efficient tweeter. The crossover circuit took references from KEF as well as several DIY designs, which takes good care of tweeter resonance at 1.1khz, stability of crossover frequency, and impedance matching. These all add up to a very sweet, detailed and smooth high frequency. The result is better than using the Hiquphone. For the skeptics all I can say is this is what it is and please do have a listen. And the good thing is the 22TAF is darn cheap for what it delivers.

In contrast the T27 is a tweeter from the 70s. It is very different from the 22TAF, and definitely not as smooth. I would say the T27 sounds aggressive at times when played slightly loud, and using the ALCAP helps... to smooth things out.

SPICE program helped in determining the accuracy of the crossover point, the attenuation and also taming of resonance peaks. That is all. In actual implementation, hearing tests and tweaking come into play. In this case, I was very lucky as the attenuation was almost spot on.

The SOTA crossover can be seen here:
http://www.diyaudio.com/forums/multi-way/145632-atkinson-transmission-line-2.html

The Coles is fed thru a series of capacitors and the last one is just a 1uf
The T27 is fed thru a combo of 1.5/1.5u =3uf and attenuated by a VR
Two Celestion HF1300 in parallel again attenuated by a VR

This is a pretty complex and advanced design!

If the T27 is dead, and no replacement found yet, then I suppose you can try out the circuit topology here in this article. I have attenuated the 22TAF by 5db, and perhaps 4db could be enough to match the T27. Also, take away the impedance matching, etc. removing the zobel and the 1.1khz suppression to mimic the T27 as is in the SOTA implementation. I don't know. I don't own the SOTA and cannot provide an effective suggestion.

I started off with nothing for the Carlton, why not you experiment on the TL as well? I am sure you will find the 22TAF a competent performer. Maybe just an adjustment of the VR, it will be a good match? Cheers!
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mulane
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PostPosted: Tue Aug 30, 2011 4:33 am    Post subject: Reply with quote

Good evening Nimo_jon,

thanks for your reply. I should have told you that I use the T27 on its own and in the ETI crossover it is fed by 6uF in series and 0.4mH in parallel, together with a 5ohm/12ohm L-pad:

http://i134.photobucket.com/albums/q89/metako42/hi%20fi/KEFTLcrossover-3way2.jpg?t=1314673413

My T27s are not dead (yet) it's just just that I am rebuilding the crossovers with Jantzen Crosscaps/Z-Superior caps, and while I am at it I might as well upgade the tweeters and get the appropriate capacitors, especially at the low price of the Seas.

My talent with crossovers is limited but I might try the Seas and if they don't work out they could always go into the car Very Happy

Also, I am not used to seeing a crossover schematic like the one you provided above-so just to confirm, are R73 and R66 resistors components to be added, or are they a part of the electrical properties of the tweeter?
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Nimo_jon
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PostPosted: Tue Aug 30, 2011 11:34 am    Post subject: Reply with quote

Hi there

R73 and R66 are the sensitivity compensation network to bring the 22TAF down from 93dB to 88dB (as the original tweeter).

There are two types of attenuation: series resistance and voltage divider. Series resistance is simple but less desired as it may cause cross over F change dynamically. L pad compensation network is better, and R73 and R66 in this case were chosen from an attenuation table or calculator such that the total impedance of the driver remains the same as the standalone driver:

So for a 8 ohms tweeter, here is the table (google tweeter attenuation calculator)
-DB R1 R2
-1dB 0.9 63.6
-2dB 1.6 30.9
-3dB 2.3 19.4
-4dB 3.0 13.7
-5dB 3.5 10

If the driver is other than 8 ohms, say 6 ohms, to get the corresponding R : divide by 8 and then multiply by 6 to get the actual values.

In your case, the 5 and 12 ohms appear to be attenuating at 4 or more dB off the tweeter output, and will make it look like a lower impedance load. I can run a simple simulation for your circuit and shall post it later of the day. That is based on KEF's published data on the T27 and assuming its sensitivity remains as factory spec.

For the B110, which model is it? It is used for the mid range. The bass is taken care by the active B139. Is this a transmission line design? I can only do electrical impedance V mag V phase simulation.

Cheers.
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mulane
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PostPosted: Tue Aug 30, 2011 2:10 pm    Post subject: Reply with quote

Thanks for the information regarding the resistors.

The B139 is a SP1044, the B110 is a SP1003 and the T27 is a SP1032. Yes it is a transmission line. I would very much appreciate it if you could do a simulation - thank you! Smile

I have never been totally happy with the midrange as it seems to shout a bit and is a bit 'cuppy' but this could be the cheap Elcap capacitors I have been using there. I have been forced to tone it down with a 15ohm series resistor which I know is not ideal. I wonder if the simulation will turn anything up there.
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Nimo_jon
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PostPosted: Wed Aug 31, 2011 5:17 pm    Post subject: Reply with quote

Hello Mulane

I looked at the circuit you provided and compared with the Atkinson design. There are some discrepancies. In the original design, there are a total of 4 tweeters : 2 Celestions , 1 Kef and 1 Coles. Attenuation required for the KEF and Celestion tweeters to give a balanced HF; otherwise it would be too overpowering. (I have wiped out the connections to the Celestion and the trim pot for the KEF)



I assume you are using only the KEF T27a as tweeter? It would be like removing the super tweeters and the Celestion. The 5/12 ohms divider will be too much! The simulation of the circuit you provided shows a lack of HF... which is not surprising... B110 blue trace, T27 green trace



I propose a crossover circuit which is closely related to the original design. Do watch out for the values of components. The 4.4uf is actually 2X 2.2uf in parallel. The attenuation of the T27 is done using a 3ohms or 2.7ohms resistor (taken from a CS1 I built before). This should bring the output of the T27 to match the B110. I am assuming the T27 is aged and its output dB has dropped somewhat, so the resistor can be anything 2R to 3R. You have to test it yourself. If 3R sounds ok to start with, then let it stay.

I am proposing to use the 22T/AF to replace the Coles super tweeter (simulated by a simple 4.9R with 0.05mH). Note that the Super tweeter polarity is in phase, and you can vary the cap 1uf from anything 0.47uf to 3.3uf to get a designed output thru listening.



Now examine the output, the T27a output is matched to the B110, and you have the 22TAF acting as the super tweeter, as in the original design by Atkinson. If you are game enough, we can add in the two Celestion tweeters, but the T27 will have be attenuated more. Please don't ask me to do that simulation because it will take a lot of work and I don't have the actual speaker as base line.

Blue trace B110 Red trace T27 Green trace 22TAF super tweeter


Feed the High pass crossover directly from the amplifier. There is no need to go pass the large 60uF.
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mulane
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PostPosted: Thu Sep 01, 2011 1:31 am    Post subject: Reply with quote

Hi Nimo_jon,
thank you very.very much for doing that simulation for me! Don't worry I wont ask you to do another. I am determined to learn how to use the Spice program myself and have downloaded the tutorials etc.

I have a few questions (as usual):

1. the first simulation you did which showed a miss-match between the B110/T27 - was this done with the ETI crossover with 6uF/0.4mH/5ohm/12ohm?

2. does the T27 not go as high as the Seas 22TAF and is this is why you recommend using it as a supertweeter? There is not alot of room on the baffle for another driver-how close to the T27 and B110 does the supertweeter need to be?

3. the level of the 22TAF in the 2nd simulation seems alot lower than the other drivers - is this not a problem?

4. the B110 should be connected out of phase?

5. will your 4.4uF/0.35mH/3.3uF section for the T27 preserve the 3.5khz crossover point of my current 6uf/0.4mH ETI crossover?

6. what effect will the new 80uF on the B139 in place of the old ETI 55uf have?
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Nimo_jon
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PostPosted: Thu Sep 01, 2011 1:44 am    Post subject: Reply with quote

Hi there,

1. the first simulation you did which showed a miss-match between the B110/T27 - was this done with the ETI crossover with 6uF/0.4mH/5ohm/12ohm?

Yes, that was done using the circuit diagram you supplied.

2. does the T27 not go as high as the Seas 22TAF and is this is why you recommend using it as a supertweeter? There is not alot of room on the baffle for another driver-how close to the T27 and B110 does the supertweeter need to be?

The T27, according the spec sheet, shows a gradual roll off at about 15khz.
The T27 can go all the way up to 30khz.
SEAS 22TAF is very smooth, so I would recommend you use it as super tweeter to compliment the T27.
I like the character of the T27, removing it will make the speaker sound different.
No worries about the baffle, you can make a L shaped bracket for the super tweeter.
The super tweeter can sit on top of the speaker. This is what a lot of people do. You get to play around with the positioning and toe in of the super tweeter!

3. the level of the 22TAF in the 2nd simulation seems alot lower than the other drivers - is this not a problem?

The T22AF is helping the T27 here, not to take center stage. You can increase the coupling cap to 3.3uf or even 4.7uf for higher output, but that will defeat its purpose as a super tweeter.
Using a 1uf should be good enough.


4. the B110 should be connected out of phase?

(correction) Connect the B110 out of phase.
This circuit is somewhat similar to the KEF Concerto crossover.
http://www.hifiloudspeakers.info/Anatomy/SpeakerSystems/Concerto/ConcertoCrossover.html

5. will your 4.4uF/0.35mH/3.3uF section for the T27 preserve the 3.5khz crossover point of my current 6uf/0.4mH ETI crossover?

Yes, its around there, this follows the original Atkinson design.
The formula is Fo = 1/ (pi SQRT(L C) )
where pi=3.14159 or 3.142
The simulations show similar crossover point.

6. what effect will the new 80uF on the B139 in place of the old ETI 55uf have?

The 80uf interacts with the 8mH to give the desired low pass frequency. You can use a combo cap here (a 20uf poly cap, 10uf poly in parallel with a 47uf non polar electrolytic to save cost).
The value of 80uf is what the original design stated.
Using a 55uf will push the cross over F higher.
If possible, I would add a 10R - 10uf in parallel with the B139 as a Zobel impedance network. Its up to you. Wink

Finally, when you are done with your KEF TL, do share the pictures and tell us how it sounds! Smile
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mulane
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PostPosted: Thu Sep 01, 2011 12:02 pm    Post subject: Reply with quote

Thank you very much for those answers Nimo_jon. And yes I will definitely post some pics when it is all done. Very Happy
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mulane
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PostPosted: Thu Sep 01, 2011 11:20 pm    Post subject: Reply with quote

mmm...I have thought of another question: your 104aB crossover here
http://www.hifiloudspeakers.info/speakertalk/viewtopic.php?t=1230
uses a 0.6uF cap as a 'snubber' circuit to damp the 1.1kHz resonance of the T27. Can I add a similar cap in the circuit you have suggested?
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Nimo_jon
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PostPosted: Fri Sep 02, 2011 1:18 am    Post subject: Reply with quote

Yes, the cap will tame the T27.
However, 0,6u is not the value, you should use 0.33uf.
0.33uf interacts with the L, C, values here to give the desired effect.

Sometimes KEF adds a 220ohms in series with the 0.33uf to cater for parts tolerance spread. You can omit the 220ohms resistor here.
In that way, the 0.33uf will serve as a 'bypass' for the super tweeter, and anything above 10khz will have a slight 'lift'.

Great suggestion there!
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mulane
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PostPosted: Fri Sep 02, 2011 1:22 pm    Post subject: Reply with quote

Great! I'll use the .33uF cap across the 4.4uF and 3.3uF caps. Thanks again. Very Happy
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