Laboratory Jottings

Radio Record, Volume III, Issue 46, 30 May 1930, Page 40

Laboratory Jottings

Blue Spot 66Rhighly popular type of unit is the "Blue Spot," which is now well known in radio circles. Improvements are continually being made in these units, and the writer feels certain that great interest will be aroused amongst home constructors by 2 further addition to the "Blue Spot" family. This unit, the 66R (a useful addition to our laboratory equipment), has now been running on test for a month on broadcast sessions, and gives wonder reproduction. The high notes stand out clear and brilliant, and the bass is there, too, right down to the low notes broadcast or in gramophone records. Fed with the output of a power amplifier with two push-pull stages, 210's in the last stage, there is plenty of undistorted volume available, and the amount the speaker will carry without rattle is tremendous. ‘The clarity of reproduction leaves little to be desired, and the quality is practically equal to average moving-coil quality. Designed to work with the unit is.a 13-inch cone mounted with a non-resili-ent "surround" in a metal chassis. The unit bolts to the back of the chassis, when the spindle automatically lines up to the centre of the cone, to which it is bolted. ‘The whole is there ready to attach to the back of a baffle-board, in which is cut a circular aperture 13 inches in diameter. The outside diameter of the chassis is 14 inches. The superiority of this unit is obtained by complete re-designing on improved lines. In order to carry heavy

ane en volume without rattle, the armature has — been made quite broad, and the gap increased. 'To compensate for the {ncreased gap, the pole-pieces are built of numerous laminations to equal the width of the armature. This gives a powerful, yet more sensitive and eflicient movement. The coils also are huge in comparison with those on the average unit, and contribute to the efficiency. There is an adjusting knob working through a wide range, but once set this needs no further attention. When it is stated that the total height of the new unit is six inches its size in comparison with earlier types will at once be apparent. The chassis and cone complete solves an awkward problem for those who do not feel like tackling the construction of the actual cone. Tests were carried out first in a 2ft. silk diaphragm speaker, and then with the chassis and cone in a 29 x 24in. baffle of three-ply with Gin. sides. If a baffle 40in. square is used, sides may be dispensed with. If sides are used, the back must be kept well away from the wall-6 inches or more. Further test was made on a box baffle 2ft. square, with grille cut in three-ply, and 8in. sides of 7-Sin. rimu. This proves to be the ideal where bulk is to be kept within certain limits. A demonstration of this speaker quickly proves its exceptional qualities. and it is certain that many units will be replaced by the new one, which would also give a new lease of life to an indifferent factory-built speaker. The unit has also been tested upon small amplifiers, where sensitivity counts for even more than with a powerful one. A surprising increase of volume with the same output was the result. ‘