Projection engineering (Jan 1932-Mar 1933)

Page 8 PROJECTION ENGINEERING were designed for these microphone outlets such as relays operated by patching up the microphone termination to the mixer and thereby putting power on the lines connected to the outlet. After the audio-frequency signal, which has been picked by the condenser microphone, goes through the various amplifiers, it is fed to six distribution panels. Each distribution panel is capable of feeding and controlling the signal to eight large dynamic loudspeaker units of the W. E. 581 or similar type. Here again, special circuits were designed so that both magnetizing current and speech current are fed to the public room loudspeakers through relays which are operated when the correct jacks are patched up from the distribution panel to the loudspeaker termination. The speech current relay is operated by a key which is located on the loudspeaker key panel near the operator's desk. A novel feature incorporated in the system is a power panel that remotely controls two 240 volt d-c. motors driving 15 kw., 115 volt, single phase alternators which are located in the generator room on the seventh floor. Associated with this panel is a duplicate set of ammeters, voltmeters, and frequency meters to give the operator an indication of the a-c. load supplied to the audio amplifiers, radio receivers, and reproducer sets. The alternators are equipped with exciters and arranged with automatic starters for each set. In addition to rheostats which are manually operated, there is automatic speed and voltage regulating equipment mounted on the main control panel in the generator room. In the remote control panel, Fig. 2, which is a part of the main board of the radio room, can be seen a set of pilot lamps to indicate which set is in operation. The alternator switching is so arranged as to allow only one machine to be connected to the load at a time. The battery room contains two 280 ampere-hour, 12 volt Exide batteries supplying filament current to the condenser microphones and the seven vacuum tube voltmeters or volume indicator panels. A charging panel * i ~__ ~r'> ' <\ V, :ss"-!«s:»j;js«t^ \[ usi^sr,: sessskh S^\ \ 1 -M 1 Fig. 6. Typical condenser transmitter circuit. having the necessary switches for the transfer of the one battery to load and the other to the charging position provides resistances for controlling the current to the 12 volt batteries from a 110 volt d-c. supply. A necessary piece of apparatus should be associated with all battery panels and that is an ampere hour meter. Between the 20th and 42nd floors of the hotel are located 140 radiofrequency amplifiers for the use of guests in the residential suites who wish l" J'— I Hill llll 1 > 'i—A """"," iii Fig. 3. Fidelity characteristics of No. 10-A radio receiver. Fig. 4. Automatic gain control characteristics of No. 10-A radio receiver. to operate their own receivers. The centralized antenna system consists of a radio antenna located between the two towers some 600 feet above the street level. Three aerials are provided, one for the use of the six radio receivers located in the radio room on the sixth floor, and two for the radio frequency amplifiers — seventy to each aerial. To insure a strong signal, free from noise, to the r-f. amplifiers as well as the control room radio sets, the aerial is connected to a low impedance transmission line which is loaded at proper intervals thus keeping down to a minimum, interference between the various sets. The r-f. amplifiers which are inserted between the antenna and the apartment radio jacks, make up for transmission loss and thereby insure a good quality of reception, removing the necessity of erecting unsightly aerials. The Western Electric 10A radio receiver used in the Waldorf-Astoria for program pickup has been designed primarily for the purpose of high qual ity reproduction that is not possible to obtain from any commercial receiver on the market today. This receiver has been designed for rack mounting in keeping with the appearance and general layout of the other equipment. In Fig. 1 it will be noticed that the six receivers are mounted on the first two racks. Practically all the wiring is on the front of the panel. This arrangement of equipment not only permits high electrical efficiency and accessibility of apparatus, but improves the stability of the radiofrequency system. Electrical Characteristics The circuit of this receiver consists of a three-stage tuned radio-frequency amplifier aperiodically coupled to a three-element biased grid detector and to a three-element gain control tube. The amplifier coupling elements consist of three sets of two tuned coupled circuits, employing a six gang variable tuning condenser. The variable band-pass filters are coupled to the amplifier plate circuits by a combination of two primaries, one of which resonates below the broadcast spectrum and one above. The coupling between the tuned circuits of each band-pass filter consists of a condenser and reversed mutual inductance. The input circuits are so designed to permit the coupling of several of these receivers in multiple to the same aerial or the same antenna transmission system in such a manner that the operation of any one receiver will be independent of the adjustment of the others. The selectivity of the preselector is sufficient to prevent appreciable modulation or cross-talk in the first tube. The sensitivity of the receiver is such that a standard output of 50 milliwatts will be obtained from the receiver used in conjunction with an amplifier having a gain of 47 decibels with an input voltage at the antenna of approximately 10 microvolts (2.5