Improving the Audio Frequency Amplification of Receivers

Radio Record, Volume II, Issue 11, 28 September 1928, Page 28

Improving the Audio Frequency Amplification of Receivers

ae [HE average radio enthusiast who has had his set long enough to have passed through the period of midnight and early morning searching for distant stations, and to have settled down to the enjoyment of programmes from nearby stations, will sooner or later become interested in tone quality. Tone quality depends almost entirely on the audio frequency end of the receiving set. Such advances have been made during the last year or eighteen months that uniformity of amplification with transformers is now possible and music from any of the older commercial or amateur made sets sounds thin and distorted in comparison with that from sets using the more modern and efficient audio frequency transformers, such as the FerBanti type. Many dissatisfied owners will purehase the more efficient audio frequency transformers and _ substitute them for the original small transformers in their sets. In some cases other changes for improved selectivity, volume, etc., will be made simultaneously with the change in audio frequency transformers. After the change has been completed the tone quality may not seem to have been improved. There

may even de whisties and bpurbies that were not heard with the old transformers. The fault should not be charged to the new transformers, but to the set as a whole, and some minor adjustments will usually -be necessary to obtain full advantage of the improved amplification. "THE modern transformer, due to its high efficiency, requires more care in the arrangement of the wiring of the set. Few people, other than radio engineers who have made a study of it, realise the amount .: feedback or regeneration that may"exist in the audio stages. With the old type of transformer the regeneration was kept down by the relatively large losses, or if Os-

ciliation did occur the frequency was well above the audible range. With the more modern type of transformer the losses are small, and if the regeneration is sufficient to cause oscillation, the frequency is sure to be well within the audible range. Even if oscillation is not present regenerztion may cause distortion by increasing or decreasing the volume of one or more of the notes. Feedback 2an decrease as well as increase the siznals. To prevent feedback, make the wiring as straight and as simple as possible. Start at the antenna end aud go straight through the set keeping plate and grid wires away from each other and as short as possible. Never run 4 grid wire parallel to a plate wire. The use of 1 mfd. or 2 mfd. condensers across the B batteries will frequently improve results. They should be placed inside the set and as close to the transformers as possible. These condensers by-pass signals out of the B batteries which as a result of their resistance may cause resistance coupling from one stage to the other. It is sometimes necessary to pull out the wiring of the audio end of the set aad replace it, following the above instructions.

A radio frequency by-pass condenser of the correct capacity is built into the Ferranti audio frequency transformer. Therefore it requires no other by-pass condenser across either winding in any stage. HE valves may also be a source of trouble when more efficient transformers are used. The general increase in amplification and the greatly increased volume of the low notes may overload the last valve, resulting in distortion. This can be overcome by using a power valve in the last stage. In some cases the last valve will carry the volume if the B and C voltages are increased.

= [N making your comparisons after the set has been changed over, the following points must be kept in mind. The chief difference between the old and new transformers is the increased amplification ot the lower notes. If the music that you are iistening to has very few low notes, the ditference will be small. Organ recitals, orchestral music, and male quartets will show up the real quality of the transformers. This does not mean that only this class of music will be improved. Reception of all kinds will be improved, but the improvement will be most marked in the above classes of reception. It is very misleading to try to compare transformers by temporary wiring with switches to throw over from one transformer to the other. This is due to the strong tendency to regeneration that will exist in such wiring. The volume may be increased or deereased by regeneration as much as 50 per cent. When such an arrangement is used, the amplification of each stage and of the combined = stages should be measured by accurate methods. The combined amplification should approximately equal the product of the individual stages. Although care is necessary in installing Ferranti transformers in existing sets, there are few cases where they will not greatly improve the quality of reception. The results to be obtained are well worth the trouble because the tone quality is so nearly perfect that radio music sounds real and satisfying.