AN D RADIO REVIEW (16/fr Year of Publication) No. 458. WEDNESDAY, JUNE. 6TH, I928. VOL. XXII. No. 23. production, as did also the introduction of valves Editor: HUGH S. POCOCK. specially designed for low- frequency amplification. Assistant Editor: F. H. HAYNES. Editorial Offices: 1x6 -117, FLEET STREET, LONDON, E.C.4. Shortcomings of Loud -speakers. Editorial Telephone: City 9472 (5 lines). For quite a considerable time, however, the loud. Advertising and Publishing Offices: speaker remained a source of some disappointment to DORSET HOUSE, TUDOR STREET, LONDON, E.C.4. those who aimed at the ideal. Many of the early types Telephone: City 2847 (13 lines). 'telegrams: "Ethaworld, Fleet, London." of loud- speaker were, of course, excellent in their way, COVENTRY : Hertford Street. and were often superior in their performance 9btegrame- "Cyeliet, Coventry." Telephone to the sets 5210 Coventry. to which they were connected, but with BIRMINGHAM: Guildhall Buildings, Navigation Street. the improvement Teleprarne: "Autopress, Birmingham." Telephone: 2970 and 2971 Midland. in the receiver itself the shortcomings of the loud -speaker MANCHESTER : 260, Deansgate. became more apparent, and to -day it can be said that Te.egrante: "Ilife, Manchester." Telephone 8970 City (4 lines). it is the loud- speaker, and in particular the moving coi.' Subscription Rates : Home, 17s. 4d. ; Canada, 17s. 4d.; type, which has served to introduce a new standard in other countries abroad, 199. 6d. per annum. broadcast reproduction. As )many of the circuits and apparatus described in these pages are covcr:d by patents, readers are advised, before making use of then, to satisfy themselves With a well- designed receiver operating a modern coil - that they would not be infringing patents. driven loud-speaker, we arrive so near to perfection al CONTENTS OF THIS ISSUE. the receiving end that the least error in the transmissions PAGE becomes obvious; whilst this fact shows up to a remark- EDITORIAL VIEWS .. .. .. .. .. 593 CONSTRUCTING AND 'OPERATING PROBLEMS OF THE MOVING COIL able extent the very high standard which the B.B.C. have LOUD -SPEAKER. BY F. H. HAYNES .. .. .. .. .. 594 achieved in their transmissions, it also serves to indicate Cot,, DRIVE IIORN LOITD- SPEAKERS ,. .. 599 THE DESIGN OF THE MOVING COIF.. BY L. E. T. BRANCH .. .. 601 that nothing short of the highest standard in transmissior CURRENT TOPICS .. .. .. .. .. .. .. .. 604 will MOVING COIL LOUD -SPEAKERS, COMMERCIAL TYPES .. .. 605 hereafter be tolerated by the public because of the PUSH-PULL PROBLEMS .. .. 614 inability of the modern receiver and loud- speaker to fail CABINETS FOR MovING COIL LOUD- SPEAKERS .. .. 617 A NEW LOUD -SPEAKER .. .. 617 to reproduce possible errors in transmission. Just as the BROADCAST BREVITIES .. .. 618 LETTERS TO THE EDITOR .. .. 619 moving coil loud- speaker is exacting in respect of the READERS' PROBLEMS .. .. .. .- .. 6211 transmissions, so the user is now realising that it is also necessary to pay particular attention to the operation of RAISING THE STANDARD OF the loud- speaker in the home, and because its perform- ance enables the listener to be more critical of reproduc- BROADCASTING. tion than hitherto, questions such as whether reproduc F we look back over the years which have intervened tion should correspond in volume to the original perform- since the initiation oß broadcasting, it becomes ap- ance hitherto ignored also begin to exercise the minds of i parent that progress towards perfecting reproduction critical listeners. ' has come about in a series of very definite stages, with periods of comparative stagnation in between, and not, Importance of a High Standard. as has often been suggested, by a gradual process of It is undoubtedly in the interests of broadcasting improvement of existing apparatus, with no outstanding generally that the listening public should be trained to - landmarks of major achievement. There have un- be critical and should appreciate the best in reproduction, doubtedly been definite steps leading up to the present and for this reason we welcome the popularity which the high quality of reproduction. We need only cite a few coil -drive loud- speaker has achieved, and feel that we instances. In the days when the transformer used for can take some credit to ourselves for our part in popular. coupling L.F. stages was a poor product, the aim being ising loud- speakers of this type and bringing their con- to achieve a high degree of amplification almost regard- struction within the scope of the modest workshop and sc less of equal amplification of the audio -frequency range, providing a new field of amateur interest. an important step forward in quality of reproduction The present issue is devoted mainly to the loud -speaker occurred with the introduction of resistance coupled and a review of the types of coil -driven instrument and 1 amplification in the L.F. stages. The later production parts which are now available on the market, whilst of low- frequency transformers, giving almost equal various contributions deal with the conditions under amplification over the whole audio -frequency range, which the coil -drive loud -speaker should be used in brought about a definite improvement in quality of re- order to ensure ideal reproduction. B 7 www.americanradiohistory.com WAQ@llsog 'Volta JUNE ht h, r92,Y. CONSTRUCTING AND OPERATING PROBLEMS OF THE MOVING COIL LOUDSPEAKER. -~4 dytIINee- Some Notes on the Practical Difficulties Encountered. By F. H. HAYNES. UALITY in broadcast reception, as dependent on loud -speakers sold has long been expressed by five Jreceiver and amplifier design, has been very care- figures. This rapid development has given rise to many i.\. fully threshed out during the past two years. queries based on difficulties encountered in setting up Problems arising out of the obscure networks existing in and working the loud-speaker, and the aim in this article low- frequency amplifiers have been carefully considered, is to answer briefly some of the typical questions asked. and the divergence of views formerly held on circuit component values have been replaced by a common Selectivity and Quality. unanimity and the setting up of " a standard practice." Incidentally, the difficulties met with are relative to Novelty of circuit system is fast disappearing, and there circuit practice rather than the loud- speaker itself, the is now only one right way of designing the home broad- associated receiving set being stringently put to the test cast receiver. Wireless World readers when operating a coil -driven loud- have been appreciative of the part speaker. First there is the ambi- played by this journal in pushing for- Much information has appeared in tious listener who asks for 'details of ward to apparent finality modern re- these pages concerning the construction a receiver that will bring in the Euro- ceiver design, including those step- and the underlying p-inciples of the pean stations, and arranged for ping stories, long-range receivers moving coil loud -speaker, and it has not operating the moving -coil loud- without reaction, the Everyman Four been the aim in this article to repeat speaker. To this question the reply receiver, battery eliminator design, information to which the reader already is that the loud-speaker will reveal to valves and values and voltage swings has access. The purpose is rather t2 the trained ear the imperfections of in L.F. amplifier coñstruction, as well deal with the many practical operating the long-range set and its inability to as the method of avoidance of in- difficulties that have arisen and to put maintain quality. Efficient high - cipient oscillation, and followed more fo. ward a summary of fads. frequency amplification demands recently by a realisation that to bring selectivity, while for quality it is es- the moving coil loud- speaker before sential that the wide spectrum of fre- the listening public would lead to an advancement in the quencies which it is hoped is set. up by the transmitter musical status of broadcasting. shall be present in the tuned input circuit to the detector, Following the series of lectures and demonstrations of the frequencies being represented in their correct relative the moving coil loud- speaker arranged in many centres amplitudes. A modulated wave occupies a frequency by The Wireless World during 1926, and the development band equal to twice the highest note -frequency involved; of the first constructional design for building and operat- thus if note frequencies as high as io,000 cycles, such as ing loud- speakers of this type, a band of enthusiasts has are emitted from the instruments in the studio, are to be sprung up specialising in this new branch almost on a carried by modulated waves to the receiver, then the fre- parallel to the transmitting amateurs of old, though in quency band is 20,000 cycles, so that at a wavelength of much greater number. To -day there are over thirty 300 metres there is a broadening of the response peak manufacturers catering for their needs, and the number to some 6 metres either side of 300. Bass notes are con- of sets of moving coil loud- speaker parts and complete veyed by wavelengths close to the value of 300, but the ss www.americanradiohistory.com JUNE 6th, r928. Wpdsz Wand The Moving Coil Loud- speaker.- arranged to be more free from distortion than the alter. very high note frequencies may be so far displaced on native systems of L.F. intervalve coupling, a multi -stage either side of the fundamental wavelength as not to be resistance amplifier is probably not the best arrangement. embraced in their correct relative amplitudes by a Two stages may not give sufficient amplification to occupy sharply tuned receiver circuit. the maximum grid voltage swing permissible at the out- With a single stage of high- frequency amplification in put valve.