93db 4ohm tweeters required, but a specific size needed?

Hello again,

Does anyone know of any 93db 4ohm tweeters of good reputation which have either 95mm square face plates, or removable face plates with the face plate fixing holes for the drive unit with a 60.5mm diameter?

I am after something to replace the T33B 4ohm 93db tweeters in my KEF R107. Though my tweeters work, they probably need a service and are the weekest part of the speakers performance (to my ears).

As most may know, the 107 head asembly is a moulded affair which can only accept a specific size drive unit, 95mm square with a 105-110mm? fitting diameter.

As the T33 face plate can be removed, and it has a 60-60.5mm fitting bolt diameter. So it may be possible to retro fit another treble unit to the T33 plate?

Edited to add, it is a 25mm dome, and cloth/soft would be preferable, though I would quite happily fit a Ribbon if it can be fitted with no modification? Though I do not mind widening the drive unit cut out.

Thank you,
Steve

 

have you checked out the scanspeak

r9204/700000
Ring Radiator Treble

Diameter - 104mm
Sensitivity (2.83v/1m) - 94dB
Impedance - 4 ohm
Nominal power handling - 150w
Free air resonance - 520Hz
Frequency range - 2-40kHz

though they have round plates these can be removed* and its not to difficult to build a square plate

I had a 2900 relavator that the front plate was removeable

 

Thankyou Zanash,

I will give these some thought, I guess I would also have to do the increase attenuation with resistors in series-parallel thing?

 

I would never recommend swapping components on factory built loudspeakers like this.

If you knew the effort and research that went into blending the drive units and designing the optimum crossover prior to release then you would probably agree. Drive unit substitution is a last resort in the event of a failure - not IME and upgrade opportunity.

Just to start with, do you know the impedence of the T33 at the crossover point?

 

Mike is right, there is more to it than that. You need to know the actual impedance plot of the original driver and also the frequency response and sensitivity. For example, if everything was correct except that the original extended down to 1.5KHz before rolling off and the new one only extended to 2.5KHz then the crossover would be fudged up.

Find something either designed as a replacement for that unit, or find a 'real' replacement.

Having said that, it sounds like some of the Morel tweeters might work, have a careful read of this - http://www.diyaudio.com/forums/showthread.php?threadid=83650

 

Hi Guys,

Yes, a lot to consider! I am still looking into the info and hoping to hear from some other sources before taking action.

I am unsure of the impedance at the crossover point, which is 2.5khz. I assume this is more complicated than the sensitivity and impedance of the drive unit being a nominal 93db @ 4ohms? within a 24db/octave crossover? From what I can gather looking at the Modulus of impedance curve, it looks 4ohm at the 2500hz mark with a slight rise on a gentle slope as it reaches 20000hz, and a strong peak between 1000hz and 2000hz? Is this the correct graph?

All I have is the T33B specification leaflet from Kef, which has some details regarding the unit.
Frequency responce is 3 - 20 khz. 93db sensitivity. Nominal impedance 4ohms.

I understand a lot of effort goes into speakers such as these, but as has been pointed out in a few reviews the T33 was a little poor compared to the competition back when this was released, it does seem to hold back performance. It does seem a pretty basic 25mm soft dome unit.

An example of how messed up it can be getting replacements, Wilmslow Audio recommend an 8ohm 90db sensitivity fostex. Some people have bought these for R-107 apparently?
The very first few 107 seem to have came with a different spec HF driver according to a couple of sources, as such a few pairs of 107 on eBay have been sold with a T33A 8ohm 89db sensitivity HF unit, but the HF specified by Kef in the R-107 is a T33B 4ohm 93db model.
So I guess Wilmslow are wrong?

The Morel unit has been discused in depth on the Speaker Talk site, and I believe it has not been as good as anticipated? Some seem not to have been happy with it.

My own guess was that a well designed drive unit as close as specification to the T33B would be the best place to start investigating.
The Raal 70-10D I have been told may just require basic attenuation with two resistors in series-parallel to get me started with this better HF unit, though I will get superior results if I have measuring equipment and crossover scematics to work from?
The Raal 70-10D can be custom made with regards to the face plate and 4ohm impedance, though sensitivity is 92db? (I think I initially thought 94?)
Frequency responce is 2khz to 100khz, recommended filter is 4th order L-R @ 2800hz for the 8ohm version. Its impedance looks pretty benign at the crossover point, but seems to rise slightly as the frequency extends?

The guy at Raal seems quite happy to discuss the crossover circuit of the 107 once I trace it or have the schematic.

I could have the original units re-built, I also have used spares in the loft, but would prefer a smoother more detailed unit if possible?

Of course, as you can see I am not too clued up on this, so the help and advice is appreciated. I cannot help but feel the Tweeters performance is holding these back though, and it is also mentioned in the Stereophile reviews of the Kef R-107?

Regards,
Steve

 

Wilmslow have a good reputation .....and are normally right but everyone can make mistakes

but have you contacted kef ? they'd be my first port of call.

 

Once you know the crossover components used inside the speaker, as well as having a frequency response and impedance measurement of the current driver you can start to get somewhere.

 

93db 4 ohm tweeter

You might find the Kef enthusiasts forum helpful. www.hifiloudspeakers.info/speakertalk
I managed to unearth a cct for my 103.3's Kefcube.
Would be wary of changing tweeters without the equipment to measure the result. The crossover will most probably require adjustment.
I'm not sure of the age of your Kefs but if they have been built using crossover capacitors of the non polarising electrolitic type it would be worthwhile changing them for modern non electrolitic types before changing tweeter units. I am about to embark on this excercise with my 103.3's ( built 1985 ish)
Ianmac
Stewarton
Ayrshire

 

Hi Ian,

Yes I know the site well (Parts), My Kef R-107 crossovers were replaced with new ones around the late 90's, I have new bass drivers and mids.

I can have the tweeters rebuilt, but may be better off with a better drive unit.

I am short of time for the tweeter replacement at the moment, and supposed to be fixing up my 105.4 but been too busy with work and the kids.

I have the spec sheet for my T33B, the Raal is pretty close but not quite a perfect match, still need the crossover schematic from my Kef, but will have to wait until I have more time unless someone else can offer this.

 

Hi,

I've just spotted this thread whilst bored internet searching..

I have some Kef 107s that I am still yet to refoam the bass units on but I was also keen to try replacing the T33B as people say it is inferior to the rest of the design like you have said also (the wiring is pretty cheap-mains-cable like too, right down to the brown/blue/green-yellow colours of the head unit connection lead! Have you noticed?).

Did you ever get round to trying any tweeters out? I did speak to Wilmslow Audio about a year ago about it (that's how long these 107s have been a 'project'), also Kef spares and both spoke of satisfied customers using the Fostex tweeter, one customer having two sets of 107s, comparing upgrade with original and talking of greater air etc etc. Wilmslow judge these things by ear, trial and error and for me that is what counts..

Still, the Fostex look awful so I took the plunge and bought 3 ScanSpeak 95000 on ebay for the price of 1 new. I discovered that the fixings to the face plate are exactly the same as the T33 (it was an educated guess) but does require about 1 mm of widening of the plate's central hole - basically that means you can go back to the T33 without any trouble.

I've yet to try them out but will be doing so in the next few weeks. I'd be interested to hear what you have discovered also.

One question to everyone though: Why, when posing these questions on forums, do people always say not to touch speakers or change/upgrade them in any way? That attitude implies that we should scrap older speakers as soon as anything goes slightly wrong with them! These Kefs are over 20 years old, the idea that any of the capacitors in the crossovers are still in spec now seems rediculous, let alone the speaker as whole displaying the same frequency response or even the two speakers being matched any more. I bet even the Q of the B110 surrounds are different now (not that that ever worries ls3/5a fans!). Every commercial speaker is compromised by repeatable manufacturing processes and cost so DIYing is the only real way to realise a design's full potential in my book. The attitude of 'don't touch, you might ruin it' surely just freezes us, makes us powerless, only able to sup on the 'breast milk' of speaker designer gods as it is fed to us (horrible imagery..) and paying through the nose - if you've ever seen a baby do that after feeding, you'll know what I mean.

If you do see this Steve, would you mind pointing me to somewhere I can get a copy of the T33B specs/plots please as I'd like to compare it to the 95000 if pos. It is an 8ohm tweeter but with 90dB sensitivity so it might work out similar to 4ohm/93dB but know nothing about working this out. I'll play it by ear instead. At least resonant frequency is the same.

Cheers, Nat

 

Hello Nat,

I almost went the fostex route, until I found out it was an 8ohm unit and the Kef has a 4ohm treble unit.

My Kef have been re-wired with Chord cable, though I believe Van Damme would also be a good choice and a lot more flexible.

I would recommend trying to get the drivers re-conned by someone like Wembly Loudspeaker repairs? I believe it worthwhile getting the T33 rebuilt/serviced anyway.
I myself still want to try the Raal units when funds permit.

http://www.wembleyloudspeaker.com/

Do you have any pics of the fitting of those Scanspeak drivers? I take it those have circular faceplates?

Here is a pic of the T33B unit specs.

 

I had forgotten about this thread... I've been bogged down with other things but want to try to concentrate on these now, hifi wise ..

I actually have 97000s it turns out, I thought they were 95000s (they were lightly used by a DIY-er and got three for £80 on ebay!).

Physically they fit very well - remove the circular faceplate and the screwholes match up exactly to the square Kef ones. As I said before, the centre hole on the Kef faceplate is 1 or 2 mm too small so these need to be widened. A tip here is that the Kef C60s and C80s have the same faceplate (with a T33 tweeter, the one from the 104/2, SP1191) although a slightly lighter colour so if you find some beaten up ones, you can use that faceplate instead. I fortunately have some scrappy C60s with a blown woofer/crossover so will keep the originals intact.

I think the 93/95/97000s look a pretty good match behaviour-wise . The only thing is I think they may be about 5dB down over their whole response.. Some say at higher frequencies the ear is less able to distinquish level differences of a few dB but that's for another discussion.

Here is the impedance/response graph for the 97000:



The 107 crosses over at 3k I saw somewhere, just after the impedance hump so that can be ignored. It looks like the impedance varies over frequency in the same way but with the ScanSpeak an ohm or so higher. The Scanspeak is easier to drive but slightly less sensitive. I imagine the Scanspeak to be subjectively smoother, faster and with greater extension though.

Anyway, it should always be in the listening and my T33s that I'll be directly comparing to will no doubt be duller with age anyway - comparison to a new 107 back in '86 might have shown up the difference more but here we are in 2008.

I'll take some photos of the fit soon.

Nat

 

Hmmm. So the Scanspeak rolls off at 500 Hz and the KEF rolls off at 1500 Hz. Even if they had similar sensitivity, the Scanspeak would output more in the 500 - 1500 Hz region and the crossover would need to be redesigned to account for this.

 

Hi,

I can change the frequency response of any speaker just by changing how I sit, I'll lean more forward for these..

I would hope that they weren't being asked to output anything at 500Hz or 1500Hz anyway, I'd leave that to the mid-range. The crossover point is at 3Khz and is meant to be pretty steep too - I'm no expert though so there may be some spillage into that resonant area but I'd hope they designed around it at KEF, at least for these 107s.

The problem with that thought process though if you follow it through is that you are left with only one option, to get (buy or build) completely new speakers!

At KEF when these were made, they measured and catagorised all their components by measured values and then built and tweaked each crossover and Kube to match the individual drivers by selecting the appropriate component values to give the correct frequency response and flat 4 ohm impedance - sold in matched pairs with matching serial numbered Kube.

Moving forward 22 years, the ferrofluid will have dried, the B110s will have losened up, coils changed slightly in impedance through heating/cooling effects, capacitor values randomly drifted, the foam surrounds have rotten (my matching Kube has blown chips, the reason I had aquired them). So yes, to get them back to their original, new sound you would have to rebuild the crossovers from a large selection of measured components using the same methods and equipment they used. While you're at it you may as well tweak the design to encorporate a better tweeter. However, I'm at home with a set of ScanSpeak tweeters, some old dried up T33Bs, a soldering iron and a pair of ears..

To be honest I do see your point and agree on some level but I just feel that thinking that way leads to inaction, indecision and ultimately audio paranoia that 'it's not as it should be' with only two directions to head in, buy new or DIY from scratch. I'd rather play with these great speakers, get them sounding great (if not exactly as they were designed) and try to ignore the honking untreated room sounds that have more effect on my listening anyway - I can get a whole new frequency response just by leaning forward and backwards on my sofa!

 

Hello,

Pair matching of the Kef was mainly for the MF/HF drivers, to .5db, hence when ordering replacement drivers in early days you gave a serial number and ordered a pair of similar value. Yet these replacements were never marked left or right, and nor were the replacement crossovers. (so I am sure they were only pair matched at driver level not with adjusted crossover or Kube?)

I believe the crossovers and drivers themselves were pair matched to 0.5db as pairs, not the complete individual speaker and Kube, if you get my meaning?

The Kube itself I am not sure was serial number specific to each pair and taht pair only, I think it was a universal part of the crossover to boost LF content and reduce treble somewhat, basically to balance out the speaker to +-2db 20hz - 20khz.

I myself have a blown Kube and a replacement Kube, plus schematic blue print for my Kube, I'm sure if I check, Kubes will have the same values?

Like I said Wembly speaker repair could re-build the T33 drivers to spec, which I feel would be ideal if you cannot get an identical or damn close specification drop in replacement.

I myself am still interested in opinion regarding the Raal drivers I am interested in as drop in replacements?

 

I just came across this thread - a bit late, as usual. I used to have a pair of 107s and totally agree that the treble is a weakness, to put it mildly. There's no doubt that the Scanspeak tweeters are very good, and if they can be physically transplanted I'd be very tempted by that route. The problem is that the 107 crossover expects to see a given tweeter impedance, and the Scanspeak's is different, so it won't roll off at the intended point. Also you'd need to take several dB of attentuation out of the tweeter crossover to get the right kind of sensitivity. There's no harm in the 'suck it and see' approach, except you might waste the cost of the Scanspeaks if they don't integrate properly - and they're not cheap. Really you'd need to completely redesign the crossover, which is not impossible if you have a measuring mic and suitable software package (LSPcad, Soundeasy etc.).

Ian

 

hello,

can anybody me says the technical data of kef kube 107?
i am lost my powersupply and need one new. 18V and mA?
Thanks
MK