VOLUME AND QUALITY

Radio Record, Volume I, Issue 47, 8 June 1928, Page 3

VOLUME AND QUALITY

VIRICE OF POWER VALVES. In a case where quality, rather than quality plus tremendous volume is the primary consideration, there is nothing to be gained by the use of very high "B" battery power. ‘To be enjoyable, the device of very high voltage would of necessity have to be operated at a point considerably under maximum ontput. If a proper means of controlling this volume were used, the quality would not suffer; but it would not be better than the maximum output of an amplifier more in accordance with the tastes of the average set owner and the acoustical capacity of his home. Detuning Condemned, Of course, there is no denying the fact that power and fidelity in reproduction are, up to certasn points, closely associated. The set owner should remember, however, that a moderatelypowered amplifier puts every bit as much energy into the loudspeaker as a very-high-voltage device operated at a fraction of its volume, In the latter

case, the energy, as expressed in the A.C. variations of the plate current, remains in reserve, or is dissipated by means of the volume control. When control is attempted by means of reduc-) ing the filament voliage, or through’ slightly detuning the set, the results, particularly in the case of the highvoltage amplifier, are far from satisfactory. In the case of sharply-tuned receivers, serious distortion results from the suppression of the side bands when the set is detuned; while ex. traneous noises, static, line disturb. ances, etc., are amplified out of ell proportion to the signal. Volume of Quality. In the last analysis, quality in the re. produced signal is not so much a matter of volume as of freedom from overloading. Were it purely a matter of amplification, valves of 201A type, or the "high mu" valves, would be far superior to those of the 171 and 210 types. The superiority of the power valve lies, fiot in its ability to amplify, but in its ability to handle great yolume Without overloading. When, for example, a valve of the 201A type is used in the last stage of an amplifier with, let us say, 90 volts on the plate, the sounds issuing front the loudspeaker are apt to be thin and with the tones in the lower end of the musical scale missing. Now, as we increase the plate voltage, the quality of reproduction improves nntil a point is reached where, on loud signals, seri« ous distortion occurs, owing to the fact that valves of this type have a comparatively high amplification factor, so that signals of even‘ moderate intensity, drive the grid positive. This overloads ing, in the case of the 201A type, occurs leng before the volume has reached @ peint commensurate with the requirements of the listener, and before sufiicient current is flowing in the plate circuit to reproduce accurately the bass notes. In a broadcast station, for example, only signals of fittle more that telephone intensity are expected from valves of the 201A type; whereas, in the set of the average radio enthusiast, they are expected to furnish unlimited volume.