The design and development of the Model 70

The DM Model 70 was intended to be the first totally in house design produced by B&W. Planning and conception first started in 1968 with the following design guidelines.

1) New thinking on the distribution pattern, especially within the important mid frequency region of 400 Hz to 5 kHz, aiming towards exploring the advantages of a distribution pattern approximating to spherical.
2) A mid frequency unit capable of handling input powers in the region of 25 watts r.m.s. with distortion factors of 0.5% or lower.
3) A really low distortion bass radiator capable of truly complementing aspects 1) and 2) above without occupying an unduly large enclosure volume.
4) Exploration of the listening advantages by increasing reverberant to direct sound.
5) all units designed and built within our own factory to ensure maximum quality control and retention of the exclusive original design.
6) Styling and general appearance to be acceptable in a wide range of domestic furnishings.

Model 70 Specifications (click to enlarge)Various designs were considered in order to meet the above considerations. Firstly a full range Electrostatic was considered but would have meant producing a pair of panels at least 7 feet high and 3 feet wide each to deliver 25 watts r.m.s. with less than 0.5% distortion across the Fq range. This was deemed to be impractical in relation to point 6).

As a result it was decided at an early stage to split the Fq loading between two units, moving coil for low Fq and a choice of either Moving Coil, Electrostatic or Ionic transducers for Hf frequency handling. Ionic units had already been used in the B&W P2 Monitior and some modified units were produced to try to meet the brief. Very quickly the Ionic tranducer was abandoned, one prototype had a horn length of two feet and an R.F. oscillator of 70 watts. With many other technical and production issues it was deemed unmanageable.

Moving Coil HF units were quickly eliminated because of poor dispersion characteristics in comparison to the electrostatic units which also had low distortion and have a better transient response.

The ELS HF panel

A whole series of prototype HF electrostatic units were produced, based on the principle that....

Phase coherence with curves (click to enlarge)1) The bottom four octaves should be dealt with by a specially designed Moving Coil unit.
2) That an electrostatic transducer be developed to handle freqencies above 400 Hz and that a super tweeter be employed should it be deeemed necessary in order to meet the required distribution pattern.

As a result of the experimentation it was discovered that serious phasing problems resulted from the true pistonic motion of large flat ELS panels. It was decided to create a curved panel which was large enough to produce the required output but provided smooth horizontal distribution. Vertical dimenisons were fairly small relative to length, this was deemed the best ratio in order to approximate a live source. Many variations of panel curvature were tested until the final design was decided.

First prototypes of the ELS panel were mounted vertically but the physical dimensions of the complete loudspeaker system were considered too large (a breach of point 6 in the original brief). Eventually the curved ES panel was mounted horizontally in a slightly raised position above the main enclosure. Great polar response was achieved at various high frequencies as a result of the final design and a super tweeter was not employed.

The Crossover and Filter Network

Special consideration was made to the design, construction and positioning of the crossover network and its individual components within the DM Model 70 system. Firstly the crossover was split into two sections with the high pass filter forming part of the PSU unit for the 701 ES panel. The intended crossover frequency was to be between 400 - 600 Hz and after some testing 500 Hz provided the best results. The creation of the low pass filter to included the use of large series inductor capable of dealing with high current delivery whilst distortion was kept low. In addition high spec custom wound capacitors were employed. The cost of these alone would have paid for a conventional cheap loudpeaker system of the day.

The Bass Unit

Drive unit specifications (click to enlarge)The bass unit was designed with the intention that it should handle the lowest four octaves of the Fq range with inputs of 25 watts and less than 1% distortion. In 1969 this was considered impossible by most, however the designers set about the task deciding on the use of a 12" unit (resonance below 40Hz with free air resonance in the region of 18 - 22 Hz). Designers began experimenting with new materials for the cone. The original driver cones were formed using expanded polystyrene, this material was considered an obvious choice as there were no mid/high frequencies involved. However testing showed that the material was coloured in the 200 - 400 Hz region. Energy was being lost at the apex of the cone due to kinetic conversion within the polystyrene, the polystyrene was partially breaking up and providing this unwanted colouration.

The final cone assembly is a laminated (I assume paper) construction in order to provide the stiffness required. It is further braced and mass adjusted by small rectangular synthethic rubber sections affixed the the front face of the cone at critical positions (the speakers featured in this page have no such rubber sections. I can only assume that advances were made to the drive unit that negated the need for the extra bracing at time of production September 1975).

Loudspeaker Measurements

Various response plots (click to enlarge)Exstensive measurements were made of the completed system with particular attention paid to polar response charts at all key frequencies. Needless to say the speaker measured very well, however some unusual measurements were noted whereby the axial plot beacame less smooth as dispersion increased. This was not attributed to design problems with the loudspeaker itself, it was found that the anechoic chamber used for the testing was not truely anecoic and also there were issues with the calibration microphone which was being used. As a result a whole new range of calibration microphones and testing equipment was purchased. How often is it that the speakers on test show the testing equipment to be flawed?

Presentation

The Standard version of the DM 70 Monitor (click to enlarge)It was decided that two different versions of the DM70 be produced. One was intended for the home market and another for a broader market with "Continental" styling. both cabinet styles are very substantial units with hefty 24 mm thick chipboard panels covered with real wood veneers. Each speaker weighs in at around 34 Kg's and are mounted on either wooden feet (domestic market) or a custom metal stand (continental version) The domestic version was available in American oiled teak or walnut, with the Continental available in satin white or walnut. It was intended to introduce a rosewood finish as an option but I guess that it never happened (I have never seen any), the DM70 was a very expensive unit and I am not sure that it was ever a commercial success.

Continental specs (click to enlarge) Continental options  (click to enlarge)

 

 

 

 

 


More specs and info

I hope to have produced a page which answers questions about this enigmatic and fine example of British audio history. However I fear that the page still falls short. Now that the page is advanced I may contact B&W and try to find out more about the DM 70 story. They deserve more recoqnition. If I find out more I will let you all know.

Matt O'D 2006