The Transformer-Type Phase Splitter

Author: N.H. Crowhurst

We have talked about using tubes in push-pull and said that to do this we need to have an audio input that makes one grid fluctuate positively while the other one fluctuates negatively. The question now comes, "How can we get this kind of drive or audio voltage for push-pull output tubes?" There are a number of these so-called phase-splitting circuits.

A center-tapped transformer can be used as a phase splitter

The simplest arrangement uses a variety of audio transformer similar to the output transformer. The secondary, however, has more turns than the primary and is center-tapped, like the primary winding of the push-pull output transformer. The center tap is connected to the negative grid bias voltage. When no audio is passing, both ends of the secondary winding have the negative grid bias voltage for their respective grids. When an audio voltage is presented to the primary of the transformer, one end of the winding goes positive from the bias voltage while the other one goes negative, and vice versa.

This system has some very useful features. It is simple, and it gives good balance in voltages supplied to each grid, if it is well designed. It has, however, a disadvantage when used in a feedback amplifier, as will be explained later, and for this reason various phase-splitting circuits that do not employ transformers or chokes are preferred in modern amplifiers.