The Correct Method of Controlling Volume

Radio Record, Volume II, Issue 13, 12 October 1928, Page 10

The Correct Method of Controlling

Volume

Listeners troubled with the problem of controlling volume adequately will find this article of special value. It should be kept for reference purposes.-Ed.

see Lt is perhaps quite safe to i} say that even in these days of enlightenment 90 per cent. of the broadcast receivers in use are not sn # equipped with suitable volume controls. For some reason or other there is the general belief that if the volume is too loud the mere turning down of the filaments will bring the signal to a comfortable degree of loudness and still retain the necessary quality. This is quite an erroneous impression and listeners who continue to use this antiquated method of volume control are denying themselves

the best reproduction their sets are sapable of giving. IS order to ascertain exactly what is the effect of filament control of volume, the writer made a series of measurements on a good quality audio amplifier. The amplifier was of three stages impedance coupled, and _ the characteristic curve A shows that its frequency response is substantially flat from 60 to over 8000 cycles and with only slight attenuation beyond these limits. For all practical purposes

therefore the reproduction from this amplifier is perfect. During the test, filaments, plate and bias voltages were set to the correct values. The filament voltages were then reduced 10 per cent. and a fresh curve taken. This is shown at B. ‘ It will be seen that the overall amplification has not only been reduced, but that frequencies beyond

about 150 become greatly attenuated and below 80 cycles they are entirely lost. Moreover. the same thing happens above 1000 cycles, though somewhat less suddenly. Music reproduced on such an amplifier would certainly not sound natural even supposing a perfect detector and a perfect loud-speaker were used-and yet probably hundreds of listeners to-day get worse frequency characteristics from their receiver than this, and then turn round and blame

the poor old broadcaster for poor quality. The purpose of these few notes, however, is not to discourse upon all the possible sources of distortion in a receiver. Many receiving sets, particularly those located in close proximity to a transmitting station, suffer from overloading. To overcome this, their owners "Turn down the filaments." The quality still seems poor, so they sit down and write to the Broadcasting Company or whoever it may be and say something to the effect that "even when they turned down the volume (filaments though they may not say it) the quality was still not good." The correct place to control volume is before the signal enters the receiver at all, and even then the only permissible point of entry is at the antenna terminal. (THERE should really be no pick up at all when the antenna and earth are removed if the set is to function properly. Perhaps the simplest form of volume control is a variable

resistance connected between the antenna and ground terminals as at A or B, or a perfect repeater tube can ve used as at C. This latter type of volume control is incorporated as a feature in at least one-good type of factory built receiver. Then again a yariable coupling can be used to the antenna coil as at D. Any of these methods .can be used and all will operate satisfactorily to control the volume in the proper place. Another form of volume control which is frequently used is a rheostat on the filament of the radio frequency tubes only. [DISTORTION of the radio frequency signal is not so very serious. The previous remarks apply to the audio cunplifier-but where variation of the radio tube voltages is made, the plate and filament voltages of the audio tubes also vary somewhat from the normal if they are fed from a common A battery and eliminator. F Then again we frequently find volume controls in the audio amplifier. It is very nice to have one there, but if strong signals are being received there is a chance that distortion will be occurring in the detector tube and no amount of volume controlling in the audio will eliminate it. To conclude, always remember to control vour volume . before it enters

the receiver at all.

J.M.

B.