Radio Broadcast (May 1928-Apr 1929)

Hoid Much Output Power is Needed the IN ATTEMPTING to state our point of view on the moving -coil loud speaker in November Radio Broadcast, we neglected one point of interest — it is brought to our attention by Zeh Bouck, who says. "There is one point in favor of the dynamic speaker which you failed to bring out, namely, its capability in the way of handling power. "If volume in excess of that generally considered adequate for home reception is desired, a dynamic speaker is practically a necessity. It has been my experience that the best of cones suffer noticeably with even moderate usage and, after a few months of service, overload on relatively low volume — a fault that probably will never characterise a good moving-coil speaker." Mr. Bouck is correct, of course. The ability to stand a lot of punishment in the form of power is one of the chief advantages of this type of loud speaker, of which the Jensen, Magnavox, Peerless, Rola, etc., are good examples. When using a i7iA-type tube to provide the volume considered as necessary in most homes, the dynamic gives the impression of being able to handle this power without pin rattle or pole-pieces chattering on low notes. We are glad to note that Mr. Bouck recognizes there is a certain output of power beyond which it is not necessary to go for home reception. There are many people who refuse to recognize such a level; even if they use a 250-type power tube with full output, they want more. In the smallest apartments and in the midst of the most intimate conversation the radio is geared up to the limit. Many listeners seem to revel in a vast amount of sound, as others do in a riot of color. It is the amount of sound that attracts them, not the form or the sequence of the mixture of sounds. However, it is true that the complete benefit of a loud speaker which reproduces low notes is not secured unless the volume is rather high because it is only then that the low notes attain full perfection. But one cannot realise the full beauty of his automobile motor — if it is a good one — unless he goes about 75 miles per hour — but few of us find it necessary to extract the absolute limit of pleasure out of anything. Many readers have asked what kind of amplifier we use. It is a single 171-type tube with something less than 180 volts on the plate and the reproducer is a standard moving-coil loud speaker in a three-foot baffleboard — and you never saw a more awkward object. We listen to symphony concerts, which we believe to be a good test of the volume range of present-day broadcasting, and manage to enjoy them without bothering the neighbors. We understand Eveready (National Carbon Company) engineers made a series of tests a year or so ago in which many listeners, some of them musicians, voted on the volume level they desired while a receiver behind a screen was adjusted in output. The majority indicated a volume corresponding to the output of a 171 a tube without knowing what the actual volume level was. Laboratory There is also a feeling, we have heard, at the General Electric laboratories that a 171 a is all that is needed for home reception. Mr. Bouck states that cone speakers do not stand up under hard usage. We have never run This Month the Following Subjects are Discussed in "Strays" 1. HOW MUCH POWER IS NEEDED 2. WHAT IS A DYNAMIC SPEAKER 3. OBTAINING C BIAS 4. THE TASK OF EDITING 5. EMPIRICAL RULES AND FORMULAS into this difficulty, nor have we seen the similar trouble that some moving-coil speakers are said to have, namely, the disintegration of the paper cone or the leather or rubber circle that attaches it to the iron frame. Has any reader had an experience of this nature? What Is a Dynamic Speaker? AFTER great deliberation the Aural Devices Committee of the Engineering Division of the R.M.A., of which Paul G. Andres is chairman, approves the following definition of a dynamic loud speaker. "A Dynamic Speaker is one in which the ~able I portion of the conductor carrying the alternating signal current is a part of the moving system, the force producing the motion being due to the location of this conductor in a magnetic field." This, in our estimation, is swell. And now the N.E.M.A. has a definition, and if the I.R.E. gets up a definition, and the Bureau of Standards gets up a definition, and they can all agree upon one which the National Better Business Bureau will not object to, we may be able to learn what a dynamic speaker is. By that time another craze in loud speakers will probably be on the horizon, and we can repeat the performance ad libitum. Some Interesting Formulas ANY one wanting a pleasant hour or two of calculation and speculation may substitute values into the expression below for the turns ratio between secondary (tuned) and primary in the radiofrequency transformer coupling two tubes in a conventional r.f. amplifier. The values IB of turns ratio and maximum gain for "if the tube and its transformer are functions of the inductance and resistance of the secondary, and the plate resistance of the tube. Some constants of German tubes, measured in the Laboratory at the request of the manufacturer, are given in the accompanying table, and some coil data will be found on page 260 of September Radio Broadcast. (Turns ratio)2 = N2= — X tt=" N = R \/RRT Rd RRp and when this turns ratio is used, the maximum possible voltage gain is i>-L<» K = iN(J.= Vrrp It is particularly interesting to substitute into these equations the constants of the German Seibt tube W-404 which has an amplification factor of 28.5 and a plate resistance of 41,000 ohms. Obtaining C Bias for a.f. amplifiers 5 1 Type 4A15 4N08 4H08 4L15 H407 U410 L418 L419 W404 If Ip C Rp Gin Ep Eg .13 2.6 14.6 10,000 1,480 90 — 3.0 .068 3.7 10.0 9.000 1,100 90 — 1.5 .068 2.3 13.0 13,000 1,000 90 1.5 .125 4.6 6.0 6,600 1.000 135 —12.0 .16 4.0 17.0 18,000 900 135 — 1.5 .095 2.4 11.0 13,000 810 90 — 4.5 .12 11.0 6.0 5,600 1,070 135 — 9.0 .32 8.0 6.0 7,300 1,100 135 —13.5 .16 1.4 28.5 . 41,000 690 157.5 — 1.5 AUDIO amplifiers continue to absorb the attention of Howard Rhodes, Technical Editor of Radio Broadcast. In December, we described some data taken on a two-stage transformer-coupled amplifier made up from Sangamo type-A transformers which proved that a well-designed amplifier does not necessarily give a better response to low frequencies when operated out of a low impedance. Now let us consider the business of getting a C bias for an amplifier tube, not the last stage, by using the voltage drop across a resistor. What is the effect on the frequency characteristic of the amplifier of bypassing this resistor? The results of Mr. Rhodes' measurements are shown in the following table. Clearly there is a definite gain in bypassing the resistor — and this gain comes at the 160