Radio Broadcast (May 1927-Apr 1928)

Radio Broadcast Photograph MAKING FINAL ADJUSTMENTS ON THE PUSH-PULL AMPLIFIER DESCRIBED IN THIS ARTICLE Measurements of the grid bias voltage are being made. Note the electro-dynamic Magnavox loud speaker in the background. A circular baffleboard has been attached to it in the laboratory PUSH-PULL AMPLIFICATION— WHY? By HOWARD E. RHODES THE essential prerequisites for faithful reproduction from a radio set are, first, a properly designed receiver capable of giving reasonably distortionless amplification and, secondly, a good loud speaker fed with power from a source able to supply the necessary energy without overloading. Much of the distortion in receivers is due to tube overloading, which usually occurs to the greatest extent in the last audio tube. The cure for this condition, obviously, is to use a tube, or combination of tubes, in the output circuit that has a high enough power rating so that overloading will not take place. As will be brought out in the following discussion, this requires that "power" tubes be used in the output circuit of the receiver, and at the end of the article some constructional details will be given regarding a pushpull amplifier employing 210 type tubes. Such an amplifier will deliver a large amount of power to a loud speaker without overloading. Let us first determine approximately what requirements are necessary in the output of a receiver to prevent serious overloading. By the term "overloading," in this discussion, we mean that the input voltage on the grid of the tube is so great as to cause the grid to become positive at times so that current begins to flow in the grid circuit. In the operation of any ordinary amplifier, care must be taken that the signal input voltage is never great enough to cause grid current to flow for, when this does occur, the input signal will be badly distorted. In determining the characteristics of an amplifier to prevent overloading, we must assume certain values, with the result that the final answer will not be exact, but should nevertheless give a good idea of what conditions must be met. Suppose, to take an average case, that an orchestra is broadcasting and that the ratio of power between the fortissimo and pianissimo passages as played by the orchestra, is a million to one, corresponding to a power ratio of 60 TU. Because of the characteristics of the amplifier Radio Broadcast Photograph A CLOSEUP Showing the plug which provides for variations in line voltage used to pick up this music, it is necessary to cut down this power ratio somewhat so as to keep the pianissimo passages above the noise level and to prevent the fortissimo passages from overloading the amplifier. In practice, this ratio is cut down in the control room at the broadcasting station by an operator in charge of the gain control. The power ratio is, after being cut down, generally about 40 TU into the amplifier system. This corresponds to a ratio of ten thousand to one. Let us assume that this ratio is maintained throughout the entire broadcasting and receiving system, a condition which will be true if there is no overloading at any point. Suppose that the energy in the pianissimo passages as they are reproduced by the loud speaker is 3 microwatts (0.000003 watts). To get an idea of what this amount of energy represents, it may be compared to the average speech power delivered by a person speaking, which is about 10 microwatts. The energy associated with the fortissimo passages will be 10,000 times as great, or 0.03 watt. It is now necessary to assume a figure for the average efficiency of the loud speaker, but because the efficiency of a loud speaker varies considerably over the range of audio frequencies, it is hardly accurate to assume an average efficiency and have it mean very much. We will do so in this case, however, merely to get some idea of how much power is required. The efficiency of a loud speaker is very low, we will assume 202