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Softening the T27
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BobL
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Joined: 27 Jan 2004
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Location: West Midlands, UK

PostPosted: Mon Mar 31, 2008 8:14 am    Post subject: Softening the T27 Reply with quote

Hi,

I have KEF R105s MK1 for the front speakers and R101 for rear speakers on my surround system. It is very noticeable that the R101s are much more aggressive in the treble than the R105s. Its hard to describe but the T27 do bite compared to the T52 in the R105.

I have replaced all the capacitors in both speakers and the R105 was reborn afterwards - fabulous but the R101 remained quite hard IMHO.

Has anyone ever attempted to calm the T27 down a little? Maybe some series resistance or inductance or of course parallel capacitance? I do appreciate this can cause frequency response errors and in the case of anything parallel it can give amplifier load problems. Just thought I would ask if anyone else has been down this road before.

Lastly, is there a better non KEF replacement? I really don't like the T27 as it sounds in my R101s?

Cheers,

Bob.
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ColinR
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PostPosted: Wed Apr 02, 2008 12:05 am    Post subject: Reply with quote

Quote:
thick strips of heavy felt surround the dome - presumably to soften it slightly and make it more directional


There should already be a grey acoustic foam "rectangle" in the grill frame to do this.

It's missing from most secondhand and all ePrey offerings though.
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BobL
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Location: West Midlands, UK

PostPosted: Wed Apr 02, 2008 1:39 pm    Post subject: Reply with quote

Guys I think you may have hit the nail right on the head. Although I got the R105s new on sept 1st 1978, I got the R101s a couple of years back off of ePrey as I thought they would be a good rear to match the R105 on the front.

When I got them they were in a bit of a state as some foam inside the grill had disintigrated into dust. I cleaned it up and thought no more about it. My R105s had foam on top of the bass cabinet. Was this functional or just cosmetic?

I need as much info as possible about replacing this foam. Any info gratefully received like:


Does this foam have a particular name/type, where from, thickness, what shape, fix to grill or tweeter, etc.

TIA for any help.


Cheers,

Bob.
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ColinR
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PostPosted: Wed Apr 02, 2008 3:24 pm    Post subject: Reply with quote

.
Quote:
I need as much info as possible about replacing this foam. Any info gratefully received like:
Does this foam have a particular name/type, where from, thickness, what shape, fix to grill


You might be able to just see the bottom witness mark left by T27's foam as the whole piece was held in place on the grill fabric with double sided tape.

I've measured from my new in 1981 Christmas present to myself pair Smile .

Make a 0.375" * 4" deep curved foam "rectangle" to fit in the top of the grill frame.

Cut a central hole parallel to the grill edges leaving a 0.75" wide frame of material, job done Very Happy .


The type of soft pliable grey foam used is usually found in computer hard drive and motherboard packaging.
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proffski
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PostPosted: Wed Apr 02, 2008 8:56 pm    Post subject: Reply with quote

Almost all the projects I was involved with when using T27s utilised ever increasing diameter circular felt rings so that it looked as if the tweeter was sitting in a shallow horn.

Imaging was laser like and holographic, the huge penalty was that the 'tweak' produced a strict and very defined sweet spot for the listener and nobody else.

I'm not sure if I've sent in any pictures of this for the forum or not...
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BobL
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Joined: 27 Jan 2004
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Location: West Midlands, UK

PostPosted: Thu Apr 03, 2008 3:33 pm    Post subject: Reply with quote

Thanks again guys. All I have to do now is find some foam. Do you think the principle reason for this foam was for imaging or to tame the T27s bite?

Cheers,

Bob.
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centaurus
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Joined: 21 Apr 2006
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Location: Okland, CA

PostPosted: Mon Apr 07, 2008 11:24 pm    Post subject: Reply with quote

i dig the T-27 with tubes - mcintosh MC60 on KEF Calinda (T27, B200, passive BD139).

lively, but never harsh. more extended than the beryllium domes in my Yamaha NS-1000M.

the things that will make the tweeter really nasty are:

1. not having them at ear level - real spitty when firing off-axis.

2. if you use tubes - not having them on the 8 ohm tap. i tried the Calinda on the 4-ohm tap - BLECH!
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speakerguru
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PostPosted: Mon Apr 14, 2008 2:26 pm    Post subject: Reply with quote

The foam around the T27 on Model 101 was to make the on-axis frequency response curve smoother when measured with the grille on and therefore more acceptable for publication. The measured response with the grille off was always very good. Much better than the LS3/5A because the 101 cabinet did not have a diffracting picture frame on the front face. I'll draw a veil over the BBC perforated metel tweeter cover and thick felt blocks, although I'm sure the metal cover was essential for shall we say "robust" pro use.

Again, I doubt that anyone would be able to hear the differences between grille/foam/felt on/off in a true double blind listening test.
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proffski
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PostPosted: Tue Apr 15, 2008 12:57 am    Post subject: Reply with quote

Interesting comment and I bow to your intellect.
Yet my good friend M.M. ripped off the Proffski felt cone around the T27 as did A.C.R. bless them.
They never sat at the sweet spot, I suppose that a stepped layer of felt rings may not exactly match the argument debate here, yet they felt strongly enough to remove the 'tweak'.
I hope that the site has a photo of the rascal!
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Parts
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PostPosted: Tue Apr 15, 2008 1:47 am    Post subject: Reply with quote

Regadring the heavy felt found under the heads around the top of the bass cab on early Kef R105.
Did this not serve some purposeful diffraction and dampening for the first reflections from the midrange on the upper bass cab assembly?

Regarding the metal grill found on the T27, I am sure this was a BBC modification for protection and to alter the response slightly by increasing the HF output slightly? I think the principle and metal grill itself may have been taken from the Celestion HF2000?

The metal grilled T27 and Celestion HF2000 were commonly used by Rogers, and Celestion, maybe Spendor & Harbeth too?



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speakerguru
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PostPosted: Tue Apr 15, 2008 8:07 am    Post subject: Reply with quote

As far as I can remember, KEF never used felt. There was a block of foam on the top of the 105 bass cabinet and, yes, it did somewhat reduce the diffraction and reflection caused by the front top edge of the bass cab on the output from the upper mid/hf box.
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ColinR
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PostPosted: Tue Apr 15, 2008 7:43 pm    Post subject: Reply with quote

According to my ex BBC friends who soldered together the prototype R&D versions of many of the LS designs including the LS3/5a and confirmed later by a couple of BBC Oxford Road OB engineers when I was at UMIST.

The “hammer protector” came from the Celestion HF2000 that has a lobed polar response without it, as the dome is made from thinner Mylar c.f. the T27.

The primary use was to limit "manual handling" dome damage on OB jobs.

The reflective qualities of this modification were measured later!

It's pure coincidence that this bit of brass came from the HF2000 as there were several other perforated washers in the drawer.


Now two little teasers Cool.

From where did the BBC engineers pinch the original design which became the LS3/5a?

Can you name the building?
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eardrum
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Joined: 05 Dec 2008
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Location: Los Angeles,CA.

PostPosted: Wed Dec 10, 2008 4:03 am    Post subject: Kef 105 MK1 Reply with quote

Greetings, BobL! Could not help attempting to contact you after you mentioned experiencing NARVANA after re-capping 105.1's. As I am desiring to do the same, my question is; "where did you purchase your caps"? Also,
what brand were they, were they Alcap's? As I live in Los Angeles, CA. USA
and not in the UK, I notice the dollar is becoming somewhat stronger and I may just be able to afford these parts from overseas. Also, did you find the part values / count, the same as ColinR's 105.2 "1st edition schematic" with the (4) paralleled 120uf caps at the input? Sincerely, eardrum..
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HightechRedneck
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PostPosted: Sun Sep 11, 2011 6:24 am    Post subject: "It is very noticeable that the R101s are more aggressi Reply with quote

I have read this train of comments before and reasoned the foam missing from my pair of KEF R101 speakers was probably of little consequence. Still I noticed a very significant shift in the response with the grills on or off the speaker. I wondered if the foam like the picture below provided any real performance advantage. It looks so benign and simple.



I also wondered why KEF and Rogers both chose to use a material around the T-27 tweeter. The LS3/5a uses a thick strip of felt. They wouldn’t do it just because it looks cool. So there must be something about the T-27 that needs this. It was apparently there to cut some sort of reflection from the tweeter.

This afternoon I remembered that I had some foam weather stripping out in the garage. I checked to see if it would work as a substitute for the foam inside the grills, which came OEM on the KEF. The material I have is not quite the same as the OEM, but the thickness and width are about right. It’s open cell foam, but has a skin on each side and adhesive on one of the skins.

I cut some strips and used the self adhesive to apply them to the inside of the grill cloth. It made for an acceptable job for test purposes.



Actually I prefer Rogers approach to placing it on the cabinet, but I stayed with the KEF method of placing it behind the fabric. I completed one and put on some music while I went to work on the other grill. The sound was so much better that I hurried to finish the second grill and press it into service.

All three sets of KEF 101’s I’ve heard recently lack the foam inside the grill. Each one seems to have a little glare and roughness, whenever loud passages or strong notes are sustained. It comes off as a lack of refinement or hardness in the speaker, that I’ve always attributed to their age, or the available capacitors from the time period they were built. I even thought it was bad source material causing it. All of these may still have their signature on the KEF, but this foam works wonders!! I don’t know what it will do for any other KEF 101, but the foam is a must have on my set. This has made an incredible difference and I only wish I had tried it sooner. There is far more refinement and smoothness from the speaker and it seems like it has increased detail and sounds more musical. Perhaps it’s only because I can listen into the signal more without the glare and this is perceived as more detail and sweetness.

It clearly takes my speakers up in performance. Perhaps using a different grade of foam or the Rogers Felt strips would work even better. Who knows? All I can say is the KEF R101 sound rough and nasty without the grills and foam installed. The foam was clearly a part of the design as intended by KEF, or for that matter, Rogers and the BBC.

My advice to anyone missing the foam, is to Replace It. Nuff Said.
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BobL
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PostPosted: Sun Sep 11, 2011 12:36 pm    Post subject: Re: "It is very noticeable that the R101s are more aggr Reply with quote

HightechRedneck wrote:

This afternoon I remembered that I had some foam weather stripping out in the garage. I checked to see if it would work as a substitute for the foam inside the grills, which came OEM on the KEF. The material I have is not quite the same as the OEM, but the thickness and width are about right. It’s open cell foam, but has a skin on each side and adhesive on one of the skins.


Thank you for the post I would like to try this myself.

Can you tell us anything more about the foam you used? Thickness and width, maybe material ie. rubber, pvc, urethane and if we're very lucky manufacturer?

Thanks again.

Bob.
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