Adding an Audio Stage

Radio Record, Volume III, Issue 9, 13 September 1929, Page 26

Adding an Audio Stage

To Increase Volume and Quality

Quvrat correspondents. haye asked how an additional.audio stage may be connected with their receiver, and the following remarks, probably some what unnecessary for the experimenter, should, nevertheless, clear up the problem for these people.

An extra audio stage can be added only to small receivers where the previous audio valves do not exceed one. Three audio’ stages introduce very many troubles, and the extra amplification secured thereby is spoiled. Small sets comprising detector alone, detector and audio, and radio, may have an audio stage added in the method described. Diagram 1 depicts a crystal set with a one-valve amplifier, and this will be taken as a typical example from which to add another stage. It will be seen that the plate of the valve is connected with one of the speaker terminals, the other terminal being connected to the, B battery. The first step is to remove these terminals and replace them by an audio no

frequency transformer. Obtain a good make, which will cost from 47s. 6d. upwards.. Connect the plate lead to "P" of the transformer, and "B-+-" (of the transformer) to about 674 volts, unless this is the detector, in which cise the applied voltage will be considerably

lower. Connect "G" of the transformer by the shortest lead possible to "G" of a valve, socket, and take the plate of this to the speaker terminal. By connecting the highest "B" voltage available (about 90 volts) to another terminal, the speaker may be connected. "GB" or "C" of the transformer just added is connected to a negative tapping of a grid bias battery, and the positive of this is connected to "A-." The "A" supply for this last valve may be derived from taking a lead from each side of the preceding valve or from the two wires from this valve at a point nearest the new socket, If the

voltage is higher than that stipulated. on the valve cartoon, put a resistance in one of the leads. This resistance may be a variable rheostat or a fixed resistance of the amperite type. So much for the actual connections. A very important part of this is the layout. The transformer should, if possible, be at right angles to one another, and the grid and the plate wires short, and. free from one another, Keep these above the baseboard, which should be either of # non-conducting substance such as fibre or well shellaced timber. For best results, the whole audio stage should be remodelled, and to facilitate the layout several diagrams with explanatory notes have been appended, and these should make quite clear any difficulties. Grid bias on the first nilo yalve is essentiial ouly where very high voltages are to be used.