Adapting Audio Transformers for the "PushPull"

Radio Record, Volume II, Issue 51, 5 July 1929, Page 31

Adapting Audio Transformers for the "PushPull"

A CORRESPONDENT to "Question and Answers" has asked how the ordinary audio transformers can be adapted to function in a push-pull arrangement. As 2 rule. transformers having a split secondary are more expensive than the ordinary variety in which the secondary is not centre tapped, and an amateur desiring to try a push-pull arrangement for the last stage will welcome a few tips showing how it can be done, using ordinary transformers. Strictly speaking, the

following arrangement does not function in the same way as would a transformer having a centre tapping, but the output derived froni,this circuit can be considered as equal to twice that of each valve singly. By studying the diagram it will be seen that across the secondary of the input transformer are connected two resistances joined in series, the centre join being used as the mid point of the secondary to which the grid bias is applied. The value of these resistances is not critical so long as they are of sufficient value to prevent any bypass of the higher frequencies. ‘Two half-meg-ohm resisters are quite suitable. Having provided. the input arrangement, there still rémains the output to’

be considered. In lieu of a centre tapped output transformer or choke, seyeral methods avail themselves. Perhaps the simplest is the one shown in which two separate audio chokes are used, the centre connection being taken to B+ terminal. The speaker in this case is connected across these two chokes. It is as well to mention here that the impedance across the entire output of a push-pull amplifier is doubled, and that unless a speaker having a fairly high impedance is used, a certain amount of energy loss will be entailed. A solution which suggests itself to the writer would be the adoption of a 2-1 ratio centre tapped output transformer for use with the avernge speaker. A more practical idea would be the use of two speakers connected in series, the centre connection taken to the centre connection of the chokes, in order to obtain: better stability. Whére no output chokés or transformer is available, a good method can be adopted by connecting the two leads from a horn-type speaker, one to either plate terminal.on the amplifier, and to use the connection between the two bobins on the speaker magnets, inside the unit, as the actual B-- connection. Thus one-half of the speaker is in the plate lead of one valve, the other half heing ih the plate lead of the other valve. This experiment should be tried only when a pair of small valves is used, as all the direct current is going through the fine wire of the speaker.