WIRELESS NEWS.

Press, Volume LXII, Issue 18603, 30 January 1926, Page 8

WIRELESS NEWS.

AUDIO AMPLIFIERS. ARE WE GETTING THE BEST ? (speciali.? ~p.!T:r.r bcz 'TSL pszss.") (By '"Electra. :; Eadio seera3 to be passing out of the "tin horn" stage. We are beginning to realise the value of quality, but the majority of us are groping for it blindly. We do not know exactly what we are striving for. and, in my humble opinion, the manufacturers of radio equipment are doing very little to enlighten the public on tins subject. The writer does not mean that they are failing to improve their products, but lie is talking of the educational featuro of their proper use. A short timo ago a minute description of some new receiving set would close with the suggestion that the usual two-stage amplifier should be added for loud speaker work. The much abused word "distortion" was discussed, but persons ge'nerallly believed it applied only to muffling effects or the presence of extraneous sounds. They did not dream of distortion when the music came through clear and loud. That is why the advocates of resistance coupled amplifiers found it so hard to convince their public. However, they were able to demonstrate to most of us the presence of musical notes in the broadcast music which our transformer counlcd amplifiers were not producing; others we were producing very poorly, and still others we were producing with entirely too much volume. Improved Transformers. Once these facts were realised, the old amplifier no longer satisfied us. "We began to try to improve it. Some turned to the resistance type of amplifier, and in many cases were disappointed ; others demanded better transformers, which the manufacturers gladly supplied. There is no hesitation in saying that a great deal of time and money has been spent in developing this better type of transformer, and laboratory tests have shown them capable, theoretically, of amplifying evenly over the entire musical range of from below 100 cycles to above 6000 cycles, and if you should approach one of these manufacturers he would in all probability assume the attitude that the use of his transformers would ensure such results. Yet, in my opinion, very few even approach such a performance, probably through no fault of the transformer.

Let us look at some of the problems involved in using these transformers, problems to which the average person never gives a thought. In the first place, when we couple two tubes by a radio transformer, we introduce the primary into the plate circuit of one, and the secondary into the grid circuit of the other. These are actually two coils of wire, and that means three things in our circuits—inductance, capacity, and resistance. These are all taken into consideration in designing the transformer, as well as the core characteristics. However, we have other elements in our circuits. We have both resistance and capacity in our tubes, wiring, and batteries. These are bound to vary, and' will affect the operation of the circuits as a whole. we have -other variables which affect our results. For example: The potentials with respect to each other, under which the elements of our tubes are operated, the strength of the rectified signal which is passed to the amplifier, and the type of rectification used. It is evident that it is out of the question to expect the average broadcast listener to adjust all these things to conform to the conditions for which the transformer is designed. In many eases ideal conditions would demand a higher plate potential and negative grid potential than the user can conveniently employ, and even if we suppose that he should have a perfect adjustment, changing signal strength and run down batteries would soon throw him out of line. Whether the above conditions are responsible or not would be a speculation, but it is certain with nearly every transformer coupled amplifier used under normal conditions the notes in the middle of the scale have been brought out with much greater volume than either the low or high notes. Resistance Typ,e Amplifier.

To return to the resistance type of amplifier. We find here a far less complicated system. We entirely eliminate inductance and. the external resistance of our circuits as being so materially increases that a variation in the internal resistance of the tubes is of much less importance. Unfortunately, the only tubes on the market until very recently were not designed for resistance amplification. Their amplification factor was small, and unless aided by the voltage step up of a transformer (as was intended in their design) they did not give a large increase in signal strength per stage. This fact, coupled with the necessity of using a higher plate potential than usual, has retarded the popularity of this sytem. Furthermore, many tried this method without having complete information. For example: The resistances to be used, in the grid circuits depend upon the capacities of the tubes and the signal strength. The latter, of course, should be stronger in each succeeding stage. Better information on this subject, as %vell as on the coupling condensers to be used, is now available. Best of all. we now have tubes which are specially adapted to resistance amplification ir. the audio stages. The only two types that have arrived in New Zealand so far, are the . Clearton and Mullard. Thev are designed, for high-voltage amplification, not power amplification and are therefore easily overloaded. In consequence a power tube should be used in the last stage to feed the loud-speaker. These improvements have brought resistance coupling into its own weilmerited popularity, but you may see from the above that variables nre stili present and must be taken into consideration. Next week we will take the rase of loud-talkers and other units. with n few helpful suggestions. NOTES. The transmission last SumLiy liom 3AC of specially selected gramophone selections was first-class, and 1 consider that it could very well be repeated at an early date. I understand that on February 6th a special dance programme is to be transmitted from this station and also 1 hat very shortly new apparatus is to be added to the speech amplified that will help to make further improvements in the quality of transmissions. Mr W. J. Webb, late concert director. of 3AC station, and now -.iiiof announcer at V.L.8.N., paid short visit to Christchurch last week-end.