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Thread: Info: BBC monitors 1960's

  1. #11
    Join Date: Oct 2010

    Location: Anglesey, UK

    Posts: 7
    I'm Mark.

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    No well I have tried but not too hard as I don't want to force them! I know they were refinished in the late 90's and had new cloth put on the grils and painted a horible brown.

  2. #12
    Join Date: Aug 2008

    Location: London

    Posts: 2,411
    I'm Nat-andthat'swhyIdrink.

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    Goodness, look at those cross-overs..

    My tip for removing grills like that is to put a thin bit of plastic (like a flexible library card or ID card) down the side of the grill (to protect the wood) and pry it slowly and carefully in different places all round with a builder's type pallet knife thing (the thing used for filling holes with plaster).

  3. #13
    Join Date: Dec 2008

    Location: Yorks

    Posts: 16,643
    I'm Nobody.

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    Looks like some false info floating about.

    They are not Plessy 15'' unit they are Goodmans..those twin tweeters should be Celestion 'HF1300' the model is actually a very clever design..I'll leave you wondering but it involves the twin Tweeters in a way you wouldnt dream of.
    Last edited by Rare Bird; 20-10-2010 at 14:46.

  4. #14
    Join Date: Dec 2008

    Location: Yorks

    Posts: 16,643
    I'm Nobody.

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    Quote Originally Posted by bocswn View Post
    No well I have tried but not too hard as I don't want to force them!
    Dont worry it's only strong velco all way around.

  5. #15
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,859
    I'm openingabottleofwine.

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    Quote Originally Posted by Andre View Post
    Looks like some false info floating about.

    They are not Plessy 15'' unit they are Goodmans..those twin tweeters should be Celestion 'HF1300' the model is actually a very clever design..I'll leave you wondering but it involves the twin Tweeters in a way you wouldnt dream of.
    Oo - you little tease you!

    Regards
    Barry

  6. #16
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,859
    I'm openingabottleofwine.

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    Gentlemen

    Clearly none of you have looked at the links I gave in my post (no.5). For the LS5/1 and LS5/1A the following information is given in the BBC document ENG INF No. 43, from which I quote:

    "LS5/1, and LS5/1A
    The studio versions of the LS3/1 and A, designed to replace the LSU/10. The cabinets were larger than the LS3/1 and the tweeters were mounted above the woofer, rather than in front of it.
    The following units were used:
    Plessey (LS5/1) or Goodmans C129/15pr (LS5/1A), 15-inch paper cone woofer
    2 x Rola-Celestion HF1300 tweeters. "

    Looking at the photos of your speakers, Mark, they are unquestionably the BBC LS5/1A.

    The following, taken from the audio forum 'Speaker Talk' might be of interest:

    "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/Ana...erSystems.html
    &
    http://www.kef.com/history/1980/km1.asp

    KEF ARCHIVE DOCUMENT: http://www.hifiloudspeakers.info/Ana...ACPaperp1.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"



    Regards
    Last edited by Barry; 21-10-2010 at 10:22. Reason: Additional information
    Barry

  7. #17
    Join Date: Dec 2008

    Location: Yorks

    Posts: 16,643
    I'm Nobody.

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    Quote Originally Posted by Barry View Post
    Gentlemen
    [B]
    Clearly none of you have looked at the links I gave in my post (no.5).
    I've stated from the offset what they are

  8. #18
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,859
    I'm openingabottleofwine.

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    Quote Originally Posted by Andre View Post
    I've stated from the offset what they are
    Touché, though you still requested photos for assured confirmation.

    Regards
    Barry

  9. #19
    Join Date: Oct 2010

    Location: Anglesey, UK

    Posts: 7
    I'm Mark.

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    Thank you all so much for the Info I have been struggling for a while now to get any info. One last question where should I put the units up for sale and how much are they worth??

    Thanks
    Mark

  10. #20
    Join Date: Dec 2008

    Location: Yorks

    Posts: 16,643
    I'm Nobody.

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    I'm suprised Marco aint intrested in these tbh..

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