Radio broadcast .. (1922-30)

Some Remarks on Audio Amplification If You Aim at High Quality in Radio Reception, Here Are Some Suggestions on Improvement Through the Use of High Plate Voltage and a Special Use of By-Pass Condensers— A Discussion of What Occurs in the Audio Circuit BY GEORGE C. CROM, JR. ^TT rj^HE search for high quality in the audio circuit of the radio receiver is growing more ^| -* general and more popular every month. In the August , 1925, RADIO BROADCAST I/ an article by John B. Brennan appeared describing the construction of a two-stage audio amplifier of very high quality. This article by Mr. Crom, while in no sense a construction article, contains some interesting ideas on methods for securing better quality. These suggestions, as far as we know, have not been formulated by any other writer. The Crom amplifier demands a high plate voltage, which is best supplied from alternating current and Mr. James Millen will describe an amplifier unit, practically embodying Mr. Crom s suggestions, in an early number of this magazine. — THE EDITOR OME of the most common of all radio devices is the two-stage amplifier, which, common though it may be, is often not constructed or operated in the most satisfactory manner. Insufficient thought is given to each of the components and their relation to each other and this results in the production of sound in the loud speaker utterly different from that imposed on the input circuit of the audio amplifying arrangement. The faults of design are made very evident when the audio amplifier output circuit is fed to a good loud speaker of the cone type. In some instances, poor reproduction has been blamed on the speaker instead of on the audio amplifier, where it actually belongs. THE PROBLEM OF GOOD AMPLIFIER DESIGN AN AUDIO amplifier must be capable of raising a weak audio signal of rapidly varying frequency to the required strength without materially changing the relative value of each frequency. It should, if it is to be an ideal amplifier, be easy to build and operate and should be low in first, and upkeep cost. The latter requirements, if the others are not to be sacrificed, are very difficult to realize. In order to visualize the problem more completely, let us consider the functions of each part in the amplifier circuit, assuming that the tone quality being received on the detector is sufficiently good for most purposes. The small currents in the detector circuit must be fed into the primary of the trans former and converted by the transformer into a voltage variation on the grid of the first amplifying tube. Although, perhaps, this process does not seem to be particularly difficult, it is well to remember that these small currents are composed of three entirely distinct forms. We have a direct current which is supplied by the detector B battery, a radio frequency current derived from the carrier wave of the broadcasting station, and the audio frequency current resulting from the rectifying action of the detector tube. It is this last current which we wish to amplify. Fig. i shows the paths of the various currents in the detector circuit. The radio frequency or carrier current is by-passed directly to ground by the condenser placed between the plate and the negative filament terminal. The usefulness of this path is directly proportional to the size of the condenser, and if this part of the circuit were considered alone, a very large condenser would give best results. But a practical difficulty arises in that a large by-pass condenser here will also by-pass some of the audio frequencies which we wish to amplify, that is, the upper audio frequencies, approaching the lower radio frequencies. So in practice, the size of this condenser is limited by the necessity of conserving the audio frequency voltages present in the plate circuit, and sizes between .001 and .006 mfd. are generally used. The exact sizes that give best results for a particular layout are found by experiment.