AUDIO-FREQUENCY AMPLIFICATION

Radio Record, Volume I, Issue 11, 30 September 1927, Page 15

AUDIO-FREQUENCY AMPLIFICATION

_COMPARISON OF THREE "METHODS In continuation of Mr. Preston, B. Billing’s weekly talks on radio matters from 2YA, the following was delivered at Wellington on Monday, September 19 :- In the case of low frequency transformer coupling there are two windings on an iron core. These two windings are insulated from each other; and they are connected to the vacuum valves as shown. When aun electric impulse of a changing nature (which ‘may be designated as a changing voltage) is applied between the grid and filament terminals (these are marked G and F) of the valve, a changing electric current results in the primary ‘winding of the transformer. The pas. sage of a changing current in the prim‘ary winding sets up a magnetic field within the iron core, which the primary and secondary windings surround, This ‘changing magnetic field will then, in ‘its turn, induce an increased electric pressure in the secondary winding of the transformer, the increase depending ou the turns ratio of- the transformer in use. If the transformer were pcr-

fect the voltage woull be the image of the voltage applied to G and F of the first valve. In a_ well-designed transformer it is possible to multiply voltages simply by constructing the device. in such a way that there are more turns of wire in the secondary than in the primary winding, in somewhat the same way that a lever may be used to multiply forces. It is this characteristic of a transformer which makes it very suitable as applied to a low frequency amplifier. It is not only possible to obtaif the (voltage) amplification of the valve itself in this way, but it is also possible to obtain even more voltage amplification by having a turnsratic on the transformer greater than

one. Vor example: A_ transformer marked having a turns-ratio of 2 to 1 is one with twice as many turns on the sec: ondary as on the primary. ‘The impedance method for coupling two vactum valves does not have the same propertv of multiplying voltagea nossessed by the transformers. With the same valves it is impossible then to obtain 4s much amplification per stage by this ‘method in comparison with the trans. former coupled combination, and for that matter the resistance coupled com‘bination is in the same position, perhans worse. Using the so-called high "Mn" tabes, however, this comparison ig much more optimistic. Resistance coupled and, in some cases, choke coil amplification, is most effectively accomplished by the use of tubes having a high amplification factor or "Mu." With such tubes the amplification per stage approaches the value oltainable in transformer coupled amplification. Personally, Io would place the relative methods for securing inter-stage low frequenegy coupling in the following order as regards their all-aronnd qualifications. Transformer coupling ‘first, with this method jn use if the transformers are of good design and construction, and the valves are suited to them or to the loads which they are to handle, the B and C battery yalues heing carefully selected, this perticnwr method leaves nothing to he desired from any point of view. The impedance coupled methel is next, if the chokes are of good desizn and the correct valne of condenser is inserted between the grid of the next succeeding ralyve, also a grid leak should be pro vided for between the erid of the next succeeding valye and filament ‘This method gives very good reproduction in Inv opinion equal jn quality to the transformer counled combination, but ‘t is not so powerful, stage for stage, using the same vyalyes and battery voltages. With regard to the resistance counled method, vou will find that this method is considerably weaker — than either of the two previous methods, in fect, it will take three very good ctawec of resistance coupled amplification to even equal two of cither the forecoing methods. The tone from the resistance coupling is auite all rieht, but in me oninion at its best it is not the ecual of the transformer oneratine under the conditions which I have already cunnicrated. ven asenming it to be the equal there is still the weakness of it per stare, and in addition this method js rather wasteful, as it reauires consider"bly more B battery pressure to seeare he Lest results from it. Taken all revw], then, the transformer method ic still ean) to anv, and a geod dent ~tronger than cither of the other two methods, but do not ree too hich a ration of {nrns with fhe traneformes method if the best results are desired The prime idea showid Le quality. of tone, he Oa, ane) noe WP eto a? ee