Push-Pull Amplification

Radio Record, Volume I, Issue 26, 13 January 1928, Page 12

Push-Pull Amplification

THE SYSTEM EXPLAINED

The advent of super-power valves and high-grade audio transformers has_ for a time rather caused the push-pull amplifier to be neglected, but in the continual search after volume combiued with quality, this system is quite likely to once more spring into fayour. . The feature of this amplifier is that, following an ordinary transformer- | coupled stage, two transformers with centre-tapped secondary windings are suitably coupled to two similar power--valyes, so that the whole forms one. stage of amplification, giving full-wave rectification of signals, instead of the usual half-wave rectification in ordinary stages. This is a non-technical idea of the general principle. Now an attempt will be made to set forth the action of the push-pull amplifier in as simple a manner as possible. It may be stated here that the object of the amplifier is, not so much amplification as the elimination of distortion, but as this elimination allows fireater volume to be taken advantage of, the greater amplification is thus obtained, even if in an indirect way.

Referring to the diagram, the input transformer, ‘I'l, is seen on the left. Both transformers are similar to the usual audio transformer, with the exception that the secondary winding of. one aud the primary of the other is, provided .with a centre tap, Needless to say, quality in these transform-, ers is a highly essential feature, otherwise quality might be Jost rather than gained. The transformers .are. retail-. ed in pairs, one alone being of no use. The output of the previous valve is fed into the primary of Tl. Each end of the secondary winding is connected to the grid of its, respective amplifier valve, and the ‘centre-tap to both valve filaments, a grid-hias battery, D, being inserted in this lead as shown. ‘The output of these two valves is then passed through the cen-tre-tapped primary of T2. The secoudary of this transformer is then connected to the loudspeaker, acting in the same way as an crdinary output transformer, so that no direct current passes through the speaker windings. Tt is in the output circnit that the so-called ‘push-pull’ action takes place. The special input transformer is required merely for the purpose of dividing the input between the two ralves, Suppose the current flowing from the apping-point B to the point A is slightIv decreased, this causes a slight decrease in the magnetic fiel@ produced by the plate-current flowing into tlhe valve If, so that less opposition will be offered to the flux produced by the current flowing from B to C. In other words, a slight decrease of current from B to A will give a-slight increase from B to C, so that a decrease on one side of the primary adds to the effect of the eurrent on the other side. Put in another way, when the terminal of coil G is positive, its valve

grid is positive, but the grid end of coil H and its valve grid are at the same instant negative. So while the grid of valve F is positive, this valve is delivering heavy current through coil A, and while the grid of valve I is negative, it cannot leliver any current through its plate coil C. ‘The current in A then induces a proportional current in the secondary winding. When there is no voltage induced in coils G and H, the plate currents neutralise each other, and f1o output current is induced in the secondary of T2. With the opposite direction of the return half of the .oscillation, the reverse action takes place, the grid and coil of valve FE now becoine negative, and the grid and coil of valve I positive, inducing by means of coil C a current in the secondary of ‘T2. GRID SWING DOUBLED. In the case of vonuccting two valves in parallel, and a.c. resistance is halved, but the amplification factor remains the saine, and although extra amplification may appear to be gained, it can only

be very slight, as the grid swing is not increased. The eftect of push-pull is to double the grid swing which would be obtainalle with the use of one yalve alone in the ordinary maunuer, ‘lake separately, the oscillations in the coils A and C would produce characteristic curves showing distortion, but when the two oscillations are added together as they are in the secondary of the output transformer, the resulting curve is symimetrical anid free from distortion, tle elemeuts contributing to distortion acting in opposite directions, cancelling one another. Altheugh ordinary small power valves may be used in the push-pull amplifier, it is now being constructed with two power valves of the 171 type, and with 180 yolts on the plates, these will deliver to the speaker as much energv as one 210 type valve with 350 volts on the plate. Whether 120 tvpe drvsell or other tubes are used, the amplifier will always give good and proportional results.