SpeakerTalk

[henryc]

Hello all
I just thought you would like to see these unusual active driven by Quad built amps (According to KEF)
UPDATE: after research from Colin on the forum Now dicovered to be built by HH)

The LS5/1AC's are bi-amped speakers designed in the 1970's and updated by KEF in 1988 .
I bought these off a retired BBC engineer,who was very sad to let them go.

KEF /BBC DESCRIPTION FOR LS 5/1AC:
BBC MONITOR SPEAKERS
The monitor loudspeaker used by the BBC to judge the quality of its radio and television programmes has been developed by a highly experienced team of research workers and studio operations engineers over many years. Infact the model in current use is the result of thirty years continuous experience in operational broadcasting in which period the best brains and ears in Britain have contributed to its evolution. The basic conception of the BBC Monitor loudspeaker is quite different from most domestic high fidelity speakers in that the latter are usually only intended to give pleasing reproduction of a diversity of available programme material. To this end reasonable variations in tonal balance and colouration are considered admissible or even desirable if they produce aesthetic pleasure and it is not generally possible to control these effects sufficiently to ensure consistent performance between samples.

On the other hand a monitor speaker must produce the programme signal with the greatest possible accuracy without modification of tonal quality and devoid of colouration. Broadcasting and recording organisations use high quality speakers to asses the aesthetic and technical merits of their programmes and guide them in obtaining suitable microphone placing and studio conditions. It has been found by experienced engineers around the world that monitoring on mediocre loudspeakers can result in poorly balanced programmes due to faulty microphone technique as well as to allow technical faults to go unnoticed.
The BBC monitor Loudspeaker represents the best possible compromise between all the various requirements in the present state of the art added to which it has been tested at every stage of its development by subjective comparison between the reproduced sound and the original. In this regard this loudspeaker is virtually unique since no other organisation has access to such splendid facilities for both subjective assessment and technical analysis.

DESCRIPTION
The speaker system employs three units, which together cover the audio range available from present day records and broadcast programmes. The enclosure is constructed from high-grade veneered chipboard of a quality, which has been found by experiment to possess the smallest degree of self resonance of all suitable construction materials. The structure is reinforced by internal partitions which restrict the vibration of the back and sides and also by a metal strut between the front and the back panels. Mudocel damping is applied to the top panel.
The internal volume of the enclosure is 4.7 cubic feet and a small vent resonating with the volume of about 50c/s is used to give a slight boost to the low frequency output. Standing wave effects are damped by internal pads of absorbent material.

LF UNITS
The lowest frequency unit is a 15 inch paper cone type with a 3 inch diameter voice coil and PVC roll surround. Its axial frequency response extends smoothly up to about 2 Kc/s and it is relatively free of colouration effects. The sound output for this unit radiates through a vertical slot 7 and a half inches wide to ensure good horizontal dispersion up to the crossover point with the high frequency unit s at 1750 c/s.
HF UNITS
Two identical HF units are fitted each having a phenolic impregnated diaphragm. The frequency response extends smoothly up until 13kc/s above, which it dies away in a regular manner. Tests have shown that the output to be free from prominent low damped resonances. The two HF units are mounted in a vertical line above the LF unit in such a way that the separation between high and low frequency sound sources is not noticeable to listeners located over four feet away. Both units are operated in parallel at middle frequencies to increase power-handling capacity in the region close to the crossover point.
Above 3Kc/s the input to the upper HF unit is reduced to induce dispersion in the vertical plane and the phase is retarded as to direct the combined radiation pattern upwards

CROSSOVER NETWORK
A thirteen element printed circuit crossover network incorporates facilities for equalising minor irregularities in the LF unit as well as providing for adjustment for relative levels of LF and HF units in 1 DB steps. An additional network corrects the diffraction effect at low frequencies due to the finite size of the cabinet, as well as the low frequency loss due to the motional impedance.

ALTERNATIVE CABINETS
There are two versions, one a floor standing cabinet for studio and control room work and a suspended model for use in television control rooms.
The floor standing speaker model LS5/ 1A is intended to be operated on a 15 inch plinth above the floor which positions the HF unit at optimum listening height.
The plinth is designed to accommodate the equalised power amplifier conveniently beneath the speaker enclosure.
The suspended model is equipped with metal suspension gear, which enables the entire speaker to be tilted downwards at the required angle The LS5/2A requires additional equalisation at low frequencies to compensate for the loss of floor reflections. A suitable equaliser is available which can be incorporated with the power amplifier if required.
Both cabinets are available in a choice of oiled teak veneer or hammer grey lacquer.

SPECIFICATION

SIZE: LS5/1AC 33 X 19x 17 inches
Weight: 82 Lbs
Nominal Impedance 25 Ohms
Maximum input: 35 watts R.M.S
Frequency response : 40 –13 K/cs +_ 5db
Directivity index: Better than 54 db up to 3 Kc/s reaching 7db at 10 Kc/s
Calibration Accuracy: The axial frequency response checked against a BBC approved standard sample is adjusted so that the curve of the loudspeaker under test does not differ from that of the reference loudspeaker by more than the following amounts:
50 c/s - 200 c/s +_ 1.5 db
200 c/s – 400 c/s 0db
400 c/s – 13000 c/s +_ 1.5 db
13000 c/s- 15000 c/s +_ 2 db - infinity
The reproduction of the Loudspeaker under test is also compared with that from the reference speaker using high quality programme sources and white noise.

LF Unit: 15 inch Heavy paper cone with plasticised P.V.C. Roll surround.
Fundamental resonance: 25 c/s
Flux density: 9000 oersted on a 3 inch dia centre pole
Total Flux : 16000 maxwells

HF Unit: 1.5 inch diameter direct radiator with Phenotic impregnated - fabric diaphram
Flux density : 10,000 oersted
Total Flux : 25,500 maxwells

Crossover Network 13 Element printed circuit network employing Radiometal - - cored inductors and closed terminal metallised foil capacitors. - Completely enclosed and shielded in a steel case.

Input Connections : Canon XLR3 . Other connectors can be fitted to suit req.

Plinth: PLS5/ 1A Constricted from heavy plated steel finished in - - satin chrome with amplifier section brackets and heat shield.

Equalised Power Amplifier: Type MA40/LS5/1a . Fixed bias push pull amplifier delivering - 35 watts output over the entire audio range . The input
circuit is arranged for bridging 600 0hms balanced line at
- zero level and incorporates equalisation for the HF loss due - to roll off in the upper high frequency above 3 kc/s An input - attenuator is fitted giving infinitely variable control of - sensitivity. Output impedance of 25 ohms.

Input Connections P.O . Jack No. 500B6 in parallel with Painton 4 pin socket
Type 310035

Output Connection : Terminals

AC Mains Supply 110-140V or 220-250 V 50 c/s adjustable


Speaker units
LF unit : 15 inch (Unidentified)
HF Unit: Very powerful KEF T52. Same as fitted to the KEF KM1 studio monitors. Info :
http://www.hifiloudspeakers.info/Anatomy/SpeakerSystems/AnatomySpeakerSystems.html
&
http://www.kef.com/history/1980/km1.asp

KEF ARCHIVE DOCUMENT: http://www.hifiloudspeakers.info/Anatomy/KEFDocuments/LS5.1AC/WebPages/KEF5.1ACPaperp1.html

How do they sound?
Incredibly powerful at all volumes.
Very efficient wall of sould with amazing width and detail
Phenomenal Bass from the huge bass units.
Sweet detailed HF at all levels .
I 'd like to know whether I can bring the amps up to date
I suppose one could replace the caps with more modern equivalents,
they are the first things to start to fade with old age..Any Suggestions
as to who could expertly do the work ?
I'd like to insert some pictures intio this mail but am unsure how to,maybe someone could enlighten me?
Does anyone out there have any further info on these speakers, I would welcome any comms on that front...

Kindest regrds
Henry

[speakerguru]

Henry,

I'm a bit confused by this post. The first sentence implies Quad amplifiers and then you never mention Quad again.

To set the record as straight as I can, I offer the following. The 5/1AC was the KEF active version of the BBC 5/1A. The 1AC used HH power amps with Peter Baxandall designed front end / protection circuits and KEF active filters. The 1A was a passive XO design which came with Radford MA40 valve amplifiers. I don't remember anything about the Radford amps eq, although I guess it was just a bit of bass boost to keep the cabinet size down.

As far as I know the 5/1AC was never used by the BBC but is was widely used by the UK commercial radio stations.

[henryc]

Hello SG
And thank you very much for your reply
I wrote and spoke to KEF archivists. and they reckon that Quad built the amps.
The updated notes came from a document sent to me by KEF verbatim.
I am really as confused as you are and want to get the true history of these speakers.
The original docs state the BBC origins .
I can send you some recent pics of the amps external and internal views.
I am in the hands of the experts views here .
I would love to know who made them and perhaps you could identify them from the pics
I would also like to put some pics on the site so the forum members can see them
but unsure where and how to do it in the forum.
Best regards
Henry
_________________
Kef Nut Music Lover with a strong respect for quality vintage HIFI

[proffski]

Stirling Stuff!

Any chance of some pictures at all? Interesting debate, I knever knew about the HH amplifiers for a start...

Almost as controversial as NAIMs Electrostatic LS which never made production.

Another KEF Nutter!

[henryc]

Hello Proffski
Thaks for the input.
I am just trying to start a debate as a newcomer..
I am very happy to post pictures but unsure how
I could email them to you or SG or Terry and one could add them with the description already on the site:
http://www.hifiloudspeakers.info/Anatomy/KEFDocuments/LS5.1AC/WebPages/KEF5.1ACPaperp1.html
Perhaps Terry could help
Hello Terry hope all is well
Best regards
Henry
_________________
Kef Nut Music Lover with a strong respect for quality vintage HIFI

[proffski]

Ok, I missed that, startingly interesting info!
Have you any more regarding this topic?

[henryc]

Hi Proffski
First off as You can tell I'm no expert on speaker technology just a lover of music and the wonderful KEF's.
I own several types much to my wife' and family's dismay but I sadly can't resist protecting our hifi heritage from the unit strippers!
I bought these beauties out of reverential curiosity and rarity value.
I had no idea how good they were until I had opened them up and have now moved them to the den out of earshot of the boss and the neighbours The grand Wagner staging and othe rbig classical works just fills the room with wonderful almost tear jerking stuff produced.

I have stopped watching telly and spend hrs just wallowing in the grandness deail and scale of the lovely music they produce.
My aim if possible is to, update the older electronics in the amps if necessary and possibly the internal wiring,I wait for advise and expertise on that front.
A bit of background:
The Pair I have now were built in the mid seventies and are the updated version which was carried out by KEF in 1988 at the same time as the introduction of the KM1's. the labels on the T52? shows 1988.

KM1's http://www.hifiloudspeakers.info/Anatomy/KEFDocuments/KEFKM1/KEFKM1SG/Gallery/pages/01KM1pamphP1.htm

The front original configuration as shown in the KEF document
had two HF units in a vertical line above the LF unit.
I have pics that show the original speakers from a Japanese website and their is little else available online about these beast except this rather esoteric japanese sites that worship our old gear.

http://www.sound-pit.com/vintage/kef-ls5-1ac-2.jpg
http://www.sound-pit.com/vintage/kef-ls5-1ac-3.jpg
http://www.sound-pit.com/vintage/kef-ls5-1ac-1.jpg

http://images.google.com/imgres?imgurl=http://www.sound-pit.com/vintage/kef-ls5-1ac-1.jpg&imgrefurl=http://www.sound-pit.com/vintage/kissa4.htm&h=305&w=350&sz=29&tbnid=FnEt_UswTMs_dM:&tbnh=101&tbnw=116&hl=en&start=3&prev=/images%3Fq%3D%2Bsite:www.sound-pit.com%2Bkef%2B5%26svnum%3D10%26hl%3Den%26lr%3D%26client%3Dsafari%26rls%3Den

http://66.249.93.104/translate_c?hl=en&u=http://more.main.jp/ana2.html&prev=/search%3Fq%3Dkef%2BLS5/1a%26hl%3Den%26lr%3D%26client%3Dsafari%26rls%3Den%26sa%3DG

The new tweeterfitted is the same as the one developed for the KM1 and designed to cope with extreeme peaks of sound like feedback and high pitched loud noises such as tape rewinds etc, obviously it had to be tough to cope with recording or radio studio conditions.
The speakers are built like tanks to withstand been wheeled about they have handles on the sides and castors on the bottom.
Like the KM1's They were designed for the professional market to very high build and accuracy standards often used to monitor live broadcasts in TV and Radio stations.
The sound volumes achievable,not that I ever drive them that hard!,without any distortion is quite astonishing.
The Bass response is bone shattering deep and tight ,when I first got them I went through the regular favourite recordings and was amazed what I had been missing on the bass front,even at very low levels the bass is extended and accurate.
The amplifiers according to KEF were built by QUAD possibly even Meridian according to the archivist. as were the KM1's built in Amps Quad 405 units. however they may well have been specified by KEF and built
by HH according to Speakerguru and I won't argue with him ! javascript:emoticon('')
Since SG's suggestion I have spoken to Quad and II have sent them some pics to to see if they can recognise their work, and I'll report back with any views from them.
The output is 75 watts per amp with separate amplification for both HF & LF units.
I would like to bring the amps up to date if necessary and perhaps if someone can help in that respect I would be very grateful as I would love to have them singing at their best .
Hope this is of interest
Kindest regards
Henry

[/b]
_________________
Kef Nut Music Lover with a strong respect for quality vintage HIFI

[ColinR]

When working on "histories" of BBC speakers you're always going to hit problems where O.B. kit or equipment that may have been used by the O.B. people. Your never going to be sure of "what's in the tin" until you look inside, because keeping it running was/is far more important than specifications and when you have audio and video desks that can turn sows ears into silk purses, why worry.

Regarding HH amplification, I've been informed by an ex-sound/video editor/broadcast equipment designer, that the total amount of HH kit used in the 80's by the Beeb was equal to all other marques, as Quad and other makers kit was too expensive to use as a matter of routine. Although you'll find various brands of amplifier modules were used "to keep it running" however, this rule also involved a degree of cost and if you cannot get it today or from RS or Farnell, don't use it. The Radfords were liked but the frames used to split because the spot welds weren't up to the job!

[henryc]

Hello Colin
I can send you the pics of the insides of the amps and speakers.
The amps do not look like they have been touched for a while
and who knows who made them?
Sound incredible anyway
I am happy to send them to you ,& other forum members to see if the manufacturer can be recognised.
All the best
Henry
_________________
Kef Nut Music Lover with a strong respect for quality vintage HIFI

[ColinR]

I'll have a gander, but you've missed a trick as the NVCF was yesterday, so, if nobody can work them out by Christmas get some hard copies from Boots to be hawked around the Tonbridge Audiojumble in February .

[proffski]

Are the twin tweeters Celestion HF1300s?

[henryc]

Hi
I think so
Replaced by the KM1 T52's
Can send you pics
if you send me your email address
ATB
Henry
_________________
Kef Nut Music Lover with a strong respect for quality vintage HIFI

[ColinR]

"One of the old school of Radio 3 announcers used to eat a full breakfast on the announcers desk while playing discs into the early morning show. If you turned up the LS5/1A (the main mono monitoring loudspeaker) you could hear him cutting the bacon coming over the turntable! You couldn't hear it when monitoring in stereo because we used Goodmans Maxims - very small speakers with no bass response." - Russell Hedges

[henryc]

It's funny you say that I can hear noises from recordings that I have never heard before mainly from older vinyl and live radio broadcsats.
Very atmospheric and it all adds to the experience
Can hear instument percussive knocks as well , it sounds very intimate with violins dbl basses etc and drums are particularily precise in that respect, a very tactile sound.
Sounds like a party going on R4 today programme J.Humphries coffee mug on the table ,he's a noisy sod on the bass!javascript:emoticon('')
ATB
Henry
_________________
Kef Nut Music Lover with a strong respect for quality vintage HIFI

[terry]