The Audio Amplifier looks to its Laurels

Radio Record, Volume III, Issue 46, 30 May 1930, Page 27

The Audio Amplifier looks to

its Laurels

New Direct Coupled System cuts Cost and increases Power

T is not often that anything» revolutionary happens in radio, but it seems that the Loftin-White system of direct coupled amplification will cause us to modify seriously our ideas of audio amplifica--tion, In short, the system makes old fashioned such conceptions of quality ‘as push-pull, resistance, or transformer coupling, hum balancers, and smoothing chokes. It employs only two valves beside the rectifier-the 224 and the 250 or 245, one smoothing choke in the rectifier system, no transformers other than the power, and but few condensers, and these are not expenive, It rectifies without a special detector alve and is at the same time a per-

. fect gramophone amplifier-there are , no tricks in its construction, it is cheap to build and remarkably efficient. The overall amplification is over 300, thus delivering more power than ample for general use from the ordinary pick- , up. | "The frequency range of amplifica_tlon is astonishingly large when the ‘ gereen-grid vaive is used. It covers a ' range Gc a few cycles to a point , where amplification ceases at about , three million cycles. A good amplifier " based on modern conceptions cuts off I at 10,000, or at the most 12,000 cycles. , The high frequency end is extended to |this astonishing limit by the sereen‘grid valve, which does not allow the 'eapacitatively reactive plate circuit to jeause degeneration through the valve ‘eapacity. ) "When it is used as a detector ampli‘fier system it automatically alters from jan extremely sensitive condition’ when "po carrier wave, or a very weak one, is ‘Ympressed thereon to a_ heavily ‘biased power handling condition for [ism signals. biasing is automatically reguated by, and in conformity with the strength of the carrier wave itself. In

detector operation the shield grid valve prevents reverse feed back of the radio frequency from the plate circuit, so that its load on the tuned input circuit is very small. This permits a much higher resonant voltage rise than with the normal detector and a constant inerease in sensitivity and selectivity, as viewed from the plate circuit of the preceeding radio-frequency valve or antenna circuit. The system, comprising but two valves and operated whether by an antenna tuner or preceding radiofrequency valves, has the same sensitivity as the grid leak detector and a two-stage transformer-coupled three valve system, without the detector overloading characteristics of the former, "The amplifier and the characteristies that fit nicely into the present practice of so-called power detection to aid in adapting the detector to sensitive handling of weak signals and powerfully handling strong signals." It combines the qualities rs power and grid-leak detection without their attendant limitations. These are the terms by which the originators of the circuit, B. H. Loftin and S. Young White, described their new circuit in "Radio News," where first the circuit constants were published. The series of articles running in this publication have aroused considerable interest throughout: America. ‘It is contemplated that they will bring about a great change in radio reception. It will not mean scrapping our

present outfits, but if the system proves itself as it is promising, it will mean a revision and rearranging. The compactness of the system can be seen from the illustration at the top of the page. . As is usual, New Zealand is not far behind new developments. Already Loftin-White amplifiers have been built up, and we know of several that are operating with remarkable suc-

cess. One Wellington firm has put a few in operation, and are preparing kit-sets, so we announce the good news that in probably the next issue of the "Radio Record" we will be publishing full constructional details. Our expert, "Megohm," whose constructional articles have been a feature of the "Record" since its inception

nearly four years ago, is preparing details so that any constructor may convert his present amplifier or make this new one. He will explain fully how these new principles are attained by the directly coupled Loftin-White system. ‘

Coming!-The fall construe‘tional details of the new LoftinWhite amplifier system. PHOCHESRGERCOTSELSGORE

A new system coupling the grid and plate directly together, abolishing transformers and coupling condensers. CLORECHULEERCERELESLELES

Amplification from a few eycles to three million, with ability to handle tremendous volume, all at a cost below the usual for audio amplifiers, are two of the outstanding features of this remarkable system.