How an Output Transformer Causes Distortion Part 2
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How an Output Transformer Causes Distortion
In Two Parts - Part 2
Audio, March, 1957, Vol. 41, No. 3 (Successor to RADIO, Est. 1917).
Norman H. Crowhurst
The operation of audio transformers has long been surrounded with an aura of mystery. This article distinguishes the different forms of distortion an output transformer can produce, and gives some simple measurement methods.
As this distortion due to reactive loading is quite similar to the varietes that a transformer causes at high frequencies we will consider both together. (A) in Fig. 8 shows the practical circuit of an output transformer while (B), Fig 8 shows the load seen be the output tubes.
Fig. 8. Practical and equivalent circuit of output transformer for high frequency response: (A) actual circuit: (B) equivalent plate load for output tubes.
Directly shunting from plate to plate is the primary capacitance of the transformer. The load resistance gets stepped up by the ratio N2 but, due to leakage flux that gets between the primary and secondary windongs, there is an effective inductance between ths load and the tubes, shown in the equivalent circuit of (B), Fig 8 as leakage inductance.
The winding capacitance has the same properties as any other capacitance in a circuit. A leakage inductance is precisely similar to any air-cored inductance: it cannot introduce distortion of itself.
However, if leakage inductance is the dominant reactance at the high-frequency end, then the load resistance, referred back to the primary will look like a resistance with an inductance in series. If the output tubes cause distortion with series reactance added to the load resistance, then this kind of transformer will appear to cause distortion.
In other amplifiers, distortion may appear more rapidly when a reactance is added in parallel with the load resistance. In this case a transformer, in which the winding capacitance is the dominant reactance at the high frequency end, will show distortion more rapidly.
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