Radio age (Jan 1927-Jan 1928)

10 RADIO AGE for October, 1927 line giving even amplification throughout the entire audible range. This range should take in at least eight thousand cycles, for in many instances true amplification of frequencies of seven to eight thousand cycles are required. Should they be left out entirely, as is frequently the case, the result is powerful base note amplification, fair amplification of the middle register, but a suppression of the high notes with a corresponding disastrous loss in shadings and overtones of the voice. Overtone amplification is obtained in but one manner. The ability of the Audio Transformers to produce an output energy which does not over accentuate nor suppress any frequencies. This allows full amplification of both high or low notes together with any given intermediate frequencies and the overtones are recreated. Only in this way is it possible to obtain perfect amplification accurately following the true shadings of the artist's voice. When an Audio amplifying system over amplifies some frequencies and suppresses others, the same effect will appear in the overtones where the rich deep shadings and life of music are produced. If the amplifying system does not faithfully reproduce the weak delicate shadings of the overtones they will become entirely unlike the original, for even the faintest trace of under or over amplification on a very delicate overtone gives a false reproduction of the most important element of music. The system of amplification used in this receiver has been worked out according to the above theory. The overtones that distinguish the work of an amateur between that of our old masters have received the greatest consideration throughout the entire receiver and the result of this consideration is truly a revelation in the faithful reproduction of music that is thrilling both to the music lover and D. X. hound alike. Precision Selectivity The problem of selectivity alone is somewhat easily overcome, but to obtain selectivity, tonal quality and distance combined is a decidedly different problem. To obtain selectivity and still pass a wide enough frequency requires an extremely careful balancing of the component parts of the receiver. It is necessary, in order to retain amplification over the entire audible range that we have a band of at least ten kilocycles in which to work. If we increase the selectivity beyond this point, we immediately enter the field of reproduction and begin to cut side bands of the music and loss of high audio frequencies is the result. The problem, therefore, is to get as close as possible to the point where we do not cut side bands. When this is done we have reached what might be called the peak of selectivity. We can go no further. This point can be only obtained by precision peaking of the filter transformer to cut off on either side of the ten kilocycle band, five kilocycles each side of the peak. We continually see glowing reports of hair splitting selectivity, knife edge selectivity, and so forth, but to the seasoned engineer and I hope eventually to the fan and set builder all of these superlative adjectives mean but one thing — "hokum" — for the very simple reason that any Super can be designed so selective that a station cannot even get through. Yes! Knife edge and hair splitting selectivity, but is it of any value to the fellow who wants to get stations with a smooth comfortable ease without interference and maintain the finest tonal quality? This can only be done, not by freakish transformers nor freakish methods of filtering, but by precision methods of calibrating. Sensitivity Sensitivity is rather difficult to define. It might be likened to the acceleration of a motor car. Its chief requirement is that every part in the receiver operates at maximum efficiency. Coupled with this, of course, is the number of R. F. stages used. The number of stages, or the number of tubes as one might wish to look at it, is, however, rather poor guide as to the sensitivity of a receiver. As an illustration of this, a set may be built using ten or twelve tubes, each tube operating perhaps only SO per cent efficient. The over all result in sensitiveness then, we will say, would be little better than a five tube receiver. For this reason, it is The handsome simplicity of the completed receiver as enclosed in cabinet is shown above far more important that the efficiency of each stage of amplification be taken into consideration more so than the number of tubes used in the receiver. The machine described used only seven tubes but the efficiency of every tube is as high as it is possible to attain. Constructional Details The first step in the construction of the receiver is a careful study of the different diagrams. On page 8 is the schematic from which a bird's-eye view of the receiver and circuit may be obtained. On page 8 is the pictorial wiring diagram. ^ A study of this diagram before beginning the wiring is advised. All parts have been marked on this drawing to conform as closely as possible with the general layout plan and no trouble should be experienced in wiring the receiver from this drawing. On page 9 is the layout and drilling plan. Again every instrument has been marked to indicate its location in the receiver. From, these drawings the builder may if he wishes build the set on a wooden baseboard, for the arrangement of parts allows sufficient room to place all parts above the sub-panel. While the use of a wooden baseboard will decrease the cost somewhat the slight extra cost for a bakelite sub-panel is well worth while from a standpoint of appearance. The sub-panel is not mounted directly against the front panel but should be one and one-half inches from the front to allow room for the rheostat. Looking at the diagram on page 9 The left hand condenser is the oscillator and the right hand one the antenna or loop. Volume and Oscillation Control There is only one rheostat in the entire receiver ; this controls the volume and also any tendency of the R. F. tubes to oscillate. This rheostat being used in the R. F. tubes alone does not effect the tonal quality of the receiver when the volume is decreased and at the same time allows control of volume from a whisper to maximum with smooth variation. For the sake of simplicity binding posts have been used throughout the entire receiver. A Yaxley cable could very nicely be worked in, however, at the rear of the panel and the leads all brought direct to the cable. Antenna or Loop Operation Either an antenna or a loop may be used. The loop will give somewhat better selectivity on local stations but of course does not have quite the range of the aerial and ground. When a straight wire antenna is used the additional coupling coil is required and should be connected as shown in the schematic diagram of page 8, however, if the set is to be operated on a loop antenna the Ellis antenna coupler may be left off the sub-panel. The pictorial wiring does not show the connection of the antenna coupler. When this is used it should be mounted directly behind the first detector tube socket. The few simple connections required will be seen on the schematic diagram. The G. A.