Output Transformers

Author: N.H. Crowhurst

The most common form of audio transformer is used in the output of an amplifier to match the actual load impedance (usually that of a loudspeaker) to that required by the output tubes. The transformer here serves the additional purpose of avoiding both supply and audio losses because the winding resistances are low compared to their respective impedances. The way impedance reflects in a push-pull transformer dejpends to some extent on the way tubes are operated. In class A, both tubes are delivering part of the power throughout the cycle, so the load is shared between them. If the ratio make 16 ohms actual impedance equivalent to 6400 ohms at the primary, each tube has a load of 3200 ohms average.

But in class B, only one-half of the primary works at a time. The other is inactive for that half-cycle because its tube is cut off. Consequently the impedance transformation is based on the ratio to each half-winding. If the whole ratio is 20:1, this is 10:1 each half. So 16 ohms connected to the secondary makes a load of 1600 ohms for each tube, but the tube takes the load for only half a cycle.

A further advantage of push-pull operation in the transformer is that the magnetizing effect due to steady plate current cancels, whether the tubes are operated class A or class B. This means that no air gap is necessary to prevent saturation. In turn, this allows a much smaller core to be used for providing an adequate primary inductance with the available turns in the primary winding.

Of particular importance in output transformers is the leakage inductance between windings. Due to the load current in both windings, the leakage flux induces a voltage difference between the ideal ratio and what you might actually measure. Being an inductance, this voltage difference becomes larger with higher frequency. Its effect is to "uncouple" the load from the tubes at these higher frequencies, resulting in high-frequency loss. This uncoupling effect of the leakage inductance is like connecting an inductance in series with the load impedance.

Output transformer matches high-impedance vacuum tube output to low-impedance load