Story below was supplied in a post on the SpeakerTalk Forum by Henry Coelho on 8 May 2006:
I just thought you would like to see these unusual active , Quad
(Acording to KEF) 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.
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.
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.
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.
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
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.
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.
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
Output Connection : Terminals
AC Mains Supply 110-140V or 220-250 V 50 c/s adjustable
LF unit : 15 inch (Unidentified)
HF Unit: Very powerful KEF T52. Same as fitted to the KEF KM1 studio monitors. Also KEF site and KEF ARCHIVE DOCUMENT
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...