Radio Broadcast (May 1928-Apr 1929)

"Z. RADIO BROADCAST JS7 On the Byrd Antarctic Expedition Only DURHAM S are Used! another tribute to the DURHAM Metallized principle ! — another tribute to the extreme care with which DURHAM Resistors, Powerohms and Suppressors are made! — another tribute to DURHAM accuracy and utter dependability!— read the above letter from Chief Radio Engineer Malcolm P. Hanson of the Ryrd Antarctic Expedition. In effect he says, "We are using DURHAMS exclusively because past experience has taught us that they can be relied upon for perfect performance under even the most adverse conditions." DURHAM Resistances are available for every practical resistance purpose in radio and television work from 250 ohms to 100 Megohms and in ra tings for all limited power purposes. Used in leading radio laboratories, endorsed by leading engineers and sold by leading jobbers and dealers. Descriptive literature on the entire line of DURHAM products will be gladly sent upon request. JUCUJ jgj METALLIZED RESISTOR S & POWEROHMS INTERNATIONAL RESISTANCE CO. 2006 Chestnut Street, Philadelphia, Pa. The Radio Broadcast LABORATORY INFORMATION SHEETS 'T'HE aim of the Radio Broadcast Laboratory Information Sheets is to present, in a convenient form, concise and accurate information in the field of radio and closely allied sciences. It is not the purpose of the Sheets to include only new information, but to present practical data, whether new or old, that may be of value to the experimenter, engineer or serviceman. In order to make the Sheets easier to refer to, they are arranged so that they may be cut from the magazine and preserved, either in a blank book or on 4" x 6" filing cards. The cards should be arranged in numerical order. Since they began, in June, 1926, the popularity of the Information Sheets has increased so greatly that it has been decided to reprint the first one hundred and ninety of them (June, 1926-May, 1928) in a single substantially bound volume. This volume, " Radio Broadcast's Data Sheets", may now be bought on the newsstands, or from the Circulation Department, Doubleday, Doran & Company, Inc., Garden City, New York, for $1.00. Inside each volume is a credit coupon which is worth $1.00 toward the subscription price of this magazine. In other words, a year's subscription to Radio Broadcast, accompanied by this $1.00 credit coupon, gives you Radio Broadcast for one year for $3.00, instead of the usual subscription price of $4.00. — The Editor. No. 265 Radio Broadcast Laboratory Information Sheet Electrifying Battery-Operated Sets March, 1929 TT IS much easier and generally more satisfactory to change a battery-operated receiver over to complete a.c. operation by the use of an A-power unit than by rewiring the set for a.c. tubes; in both cases a source of B and C voltages is, of course, necessary. The use of an A-power unit to permit light-socket operation may be accomplished without rewiring the set, and, if the unit is a good one, one can be sure that the operation of the set from the A-power unit will be essentially the same as when it was run from the storage battery. An A-power unit is somewhat similar to a Bpower unit, both of them consisting of a transformer, rectifier, and filter system. The A unit differs from the B unit simply in the rectifier and filter system which must be capable of supplying two or three amperes instead of a few milliamperes. The circuit of a typical A-power unit is given on this sheet. The transformer, T, supplies a.c. voltage to the rectifier, R, which feeds pulsating d.c. to the filter system where the ripple is removed so that the current leaving the output terminals of the filter system is practically pure d.c. The arrangement of the chokes and condensers in the filter system varies in different units. In some cases both the chokes are placed in the same side of the line and three condensers are frequently used inst ead of two. The transformer, T, is generally provided with taps on the secondary, as we have indicated, so that the output of the system may be corrected for different current drains. The greater the output current required from the unit, the higher must be the voltage impressed across the rectifier. 110V A.C. No. 266 Radio Broadcast Laboratory Information Sheet Effect of Room Acoustics March, 1929 TV/TR. IRVING WOLFF, of the Technical and Test Department of R. C. A., remarks in an article on loud-speaker measurements (Proc. I. R. E., December, 1928) that, "We are sometimes annoyed after having conducted listening tests on a loud speaker, and having reached the conclusion that it is pretty good, to find it unsatisfactory when moved to a different room or even a different position in the same room. It is, therefore, very important when taking loud-speaker curves to consider the question of room acoustics and loud-speaker position. '"Some of the factors which may be expected to have a pretty big effect are: Room absorption characteristics Room resonances Position of loud speaker in room Position of listener with respect to loud speaker. High frequencies are radiated in a beam. If high response is wanted the speaker should, therefore, be pointed and placed so as to cover as large a portion of the audience as possible. Placing the loud speaker in a corner or in any kind of a cavity will usually have a big effect on the response. The space between the back of the loud speaker and wall or other obstruction will act as a resonant chamber whose vibrations will be excited by the vibrations of the rear side of the loud speaker diaphragm. It is impossible to say whether this effect will be pleasing or otherwise. It will depend on the unadulterated response characteristic and whether the resonance is of such frequency as to supply a region which is lacking. "Under present broadcasting conditions where the range of frequencies transmitted is cut off prett y sharply at 5000 cycles or below, tube overloading on a loud speaker which reproduces real high frequencies show up as a roughness, rasp, and ver^' often as a sound which resembles a paper rattle. This is caused by the generation of harmonics and combination tones. These added notes show up particularly badly when they are produced at the higher frequencies, as there is no true transmitted sound of the same frequency to act as a mask." Note: The serial number of Lab. Sheet No. 256, iiPower Output*9 in the January issue was duplicated accidentally in the February issue by a Lab. Sheet, ''Three Types of Graphs,** bearing the same number. In order to correct their records, readers may assign the number 264 to the sheet entitled ''Three Types of Graphs." « march, 1929 . . . page 346 •