WIRELESS NEWS.

Press, Volume LX, Issue 18129, 19 July 1924, Page 11

WIRELESS NEWS.

LOUD TALKER CON9TRUCTION. NOTES ON REFLEX CIRCUITS. (ntcuui waiwaa roa "ran rasas.*) (By "Electra.") Loud-talkers, which are frequently | nothing more than a specially-devised j telephone" receiver fitted with a horn ■ in some fashion or other, have become i a very important topic of conversation among radio enthusiasts. Loud-talkers and so-called amplifying chambers for them, such as used in our phonographs, have become of such importance that a few words in description of what actually occurs 1 in such horns may not be amiss. As pointed out by Mr O, B. Blackwell, a woll-known telephone engineer of New York, and also by Prof. Dayton P. Miller, of the Case School of Applied Science, therm is actually no amplification of sound produced in any horn That is to say, there > is no. amplification of the energy which is put into the telephone receiver or phonograph reproducer diaphragm, the horn simply acting as an acoustic or. sound relay; and its function is to connect the Vibrating diaphragm of the receiver or reproducer, with the oir in the room. There are many erroneous ideas entertained regardinjr, the. horns used on such instruments, and one of the greatest fallacies is that tho horn is never nny good if it is straight, but that it should be curved or coiled into several artistic knots and loops, as are the big brass baso horns and similar instruments playod by tho jatz fiends. The reason why such band instruments have a tubular chamber wound around in a coil several, times .'is not because the sound is any stronger or better because it has, perchance, to follow | around the innumerable corners and twists; but is dtie to the simple fact, as pointed out by Prof. Dayton C. Miller, that, in order to permit a resonant acoustic or sound chamber to function at maximum efficiency for a certain tone we must make this chamber of fair length; The average tone register, let us say, is a baritone, Which is between the high and tho low notes usually sung or played, and the proper length of horn, for an averago baritone note is four and one-half feet H which corresponds to the length of this wave or vibration in air. .If,the'horn.is a.great deal shorter than this wave length, it will vibrate at but a partial tone, or, in other words,, our acoustic relay, as it. were, will have to set up this note by vibrating a volume of air equivalent to one of its harmonics, or a partial wavo length. ■ . "" . , To build a first-lass horn, make it good and solid, of some hardwood like mahogany, oak, or. maple; polish ,the'inside as smooth, as possible, with: sand-paper, and give it ft cbar. or two of good varnish or shellac. "The horn must be tight at every point," said l»rof. Miller, '<and if we will remember that We are simply vibrating a; column which is contained within our horn, we.:Bhall have; a clear conception of just how we must build this chamber." In other words, the walls should be sufficiently heavy and solid to ensure that only the column of air is vibrating, and not the horp itself. Of course, this resnlt is,.never achieved with a metal or tin horn; unless it is made of drawn 'or. pressed steel at least one-sixteenth or-an inch thick. Tho material forming the horn should not: sing or. vibrate, and -the. best music Prof. Ikliller ever heard, from a.photu> iy-aph, be : wasthis-: -produced from a condrgtei njirri over • .even y, feet 'long,- with walls four, indies thick In other ..words, wood" one-quarter to onehalf inch thick- is better for building such horns from .thou thin wood; and many of the better-class phonographs are now being fitted, it is; botjceable, with very substantial sbtiiid chambers' havirig wpod at least on&helf' sri inch thick.' . ; .--;- ■'',.. '' 0/ course a loudttalker is. of.. littio or no use id those who are'using simply a crystal detector or one valve, but whero two or moro > valves' are employed, then the loud-talkSr comes in demand. ', . ,• i - There is quite a lot to be said about the placing of a loud-talker in a room v In a small room especially; always place the loud-talker in a corner .and near. the ceilint; if possible, as, this serves to distribute the sound to the beet' advantage about the room. 'One peculiar fact is that the radiophone talk :ilw«vs sounds-much clearer. »t. a ittir distance from .the ldud-talker, 'this heinit due in part to ihe fact that when one sits dose to the horn, he usually j hears a slight .frying noire or. hißsing, due to the action of the Valves. Sometimes another station is inclined "to interfere/ and if this station -is rather weak, .this interference will -be , lost when the desired ir!tati#U,B and clearly, in. prbyidinjt you do not sit too close to the loud-talker.

SINGLE VALTE REFLEX. Latest developments in the radio ,fio!d have been - efforts to perfect methods whereby one-tube can be made v,o perform the work of two; The circuits employing thSs .*' principle [ are eaiied 're-Hex circuits. They are by no means new, sqme.extent. *4ring the Ifctd.w.ar,bat onl v recently have been perfected uiid"s:iujii;ne<l to the i>bint wiiere they oouiu le used in an easily-operated,. aflfcient -broadcast receiver.-; ". Many have hesitated about purchasing u radio-set, believing that great improvements in apparatus and accessories would make present • types obsolete. Many others nave been denied the lioasure of radio reception, because, satisfied with nothing less than loud speaker reproduction, the coat of a fOid installation, 'with its three valve*, storage batteries, etc., has; been pro- j hifcitive. Further, multi-tube sets have not always given the most desiraole results in tone quality and distortionless reproduction. For the last week " have had on test a set using this type of circuit and the results obtained were a plea-J sant surorise. This set, the Etherplan» Model ftX.I.., uses but a single tube. In it the tube doea .dcfaMo duty, first amplifying the incoming •signals at fradio ..frequency,T ? afMr which they are; rectified, or detected, by a sensitive crystal, and, then passed back through the same tube to be amplified at audio frequencies. In an instrument of this kind' nearly the entire surface of the crystal is responsive, and adjustment is merely-the operation of finding the positions" of maximum audibitiTy.' c "'" The combination of,the tube and crystal .then provides whatsis; pra#fcally equal to one stage' of radio frequency amplification, a detector, and one stage of audio frequency amplification—three tubes-ror, a cooxl regenerative detector -and one stage of audio ampUficaticn~tw£ lubes-: In tone qhsli. ties the refiexNs more perfect; than either of the above, for as has long been recognisecL ,'a crystal has the most perfect rectifying properties of all detectors. This was tried onion this set, a valv<* being connected la place of the crystal—and it wa* found that signal strength was no loader and not as clear as with the original detector, e i. One of the special features of . .the "Etherphone" receiver js a. modification of the nsuai radio frequency coupling elements nsed in W Plate circuit, whereby tuned circuit* permit

maximum amplification over too eatlff wavelength range of 1-0 to odO atstresv This is particularly detiiable sbee a* radio frequency transformer eta eeerate at maximum efficiency ever tie «s> tire range of wave-lengths aow in w by broadcasting stations. Another undesirable eaaraeteristis of transformer coupled refits; cireaits it the tendency to aet up aa e»>epKttiaf beat note on certain adjustments. _f_s tendency has beon overcomo in the "Etherphono" by providlag sorption circuit whleh will aet pewe?* tho Tiolent starting and tteppngVef oscillations. Oscillations do eeesr, however, bnt with the toted plate earouit, they are easily eoatfsueeV ever the entire wave-length range. Apparently, to obtaia the best results with a set of this kind, it la necessary to use a good aerial ass! earth. I first tried it out ea a smell single wire aerial 25ft high and about 30ft long—results not worth taHJaff about—but when it wat hitched to a, 50ft aerial, the phone began to rattle, The makers recommend an aerial a* least 90ft long and aa high as peasfete. On the transmissions from 3A.Q. and -Y.A., this set would work a leadspeaker and with another stage of audio frennenev added—maldng two valves—2B.L., Sydney, could bo heard all over the-room on a Aarplioa load speaker. 2Y.M. Oisberae aad Wet* lington broadcaster also come ia with good volume on the speaker. The makers also advise the ate of a 201 A valve. This certainly Menu to be the best as I tried about four different types but tho 201 A was by far tho best. I. have spoken to a number of OS* perimentcra who have at some tbao or another made up a reflex circuit aafaf one of the various hookups, bat have * not had a great deal of success. Es> porimenting on tho above art, I found that some of the important points to watch arc as follows: The crystal—Nothing but the soft and most sensitive crystals should bo used. The Valver-The best of amplifiers should bo picked out. The Valve Bocket—The Crosley porcelain type is recommended. If a moulded tvpc i» used, see that it Jo genuine material, and most "mod." The Aerial—Aa efficient aerial aad earth is most important. "B" battery voltage is critical and results depend a lot upon it. . For those who are contemptatfaHr investing in a single valve eireaJL too reflex should be of interest to than* as two or three valve results eaa bo obtained from a single valve aad crystal detector and at tho cost of« single valve set. Because of its errata! detector, fltte' instrument gives perfection of rspreV duction limited only by tho qnaßtjr of the transmission and the load speake* used. . \ . CORRESPONDENCE. J.1.J., Blenheim^—You do vet state 4 whether yon are using the tabojw 0 detector or aa amplifier, fhavelaiem these tubes to work as .detectors es If volts and efficiently at that. Year Ml plan is to try out various voltage* botfc below and above the 2tj volts. Oa ft , : set that I have worked with HUM tujsj * 45 volts was used.. ~.„..., -'$ PJ_:,Cfacford.—<l) The set in_ar__lt s«j j will giro the best results for mtf ia> itance work, while the one osJ-g two '- lof LJP. will be the best f«,tosaas% j [but would not be as sensitive «e>t** "\f former. (2) Certain mokes of StgSci ;<}'% \ valves will give results every aw as good. (3) Yes, regeneration wsuid tfe '< [Just'as good. . -' ' I _____ ! -* ■ ■ ' - ' , .«j Beadors, send your wireless 9aorlo*-to . this column.to be answered. -JUUamt £ 'all correspondence to: '*ineetra/'>ea*e > ; The Editor, "/The Press*'* No ehargo ~-\ is made for questions answered. .'-'X ———————f , <4