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Distortion Graph

3rd and 5th harmonic distortion is displayed in this graph. Why not 2nd and 4th harmonics? Push pull amplifiers notch out even harmonics, while freely passing odd harmonics. Why do commercial push pull amplifier exhibit even order harmonic distortion products? Even harmonics are usually the result of distortion from the input, phase splitter, and driver stages or from mismatched tubes and poorly wound transformers or from power supply interacting with the signal.

There are no input, phase splitter, or driver stages in TCJ Push-Pull Calculator, only the output stage. Why just the output stage? Two reasons: the output stage is the star of the amplifier, with the other stage playing only supporting roles (of course one bad player can ruin a show just as one bad part can ruin an amplifier, but without a star, you don't have much of a show: the audience didn't pay to hear only the backup band an Elvis Las Vegas show); and the program would take half a year more in time to finish and would have to cost ten times more to justify the outlay of time. Besides, my experience with many tube enthusiasts is that too many variables can only cause grief.

For example, imagine an actual amplifier in use with one major problem: one of the resistor used in setting the negative bias has opened, turning the class-AB amplifier into a class-B amplifier. It sounds bad. Most of your audiophile friends recommend a different brand of coupling capacitors or a different type of resistors. Others insist that RCA tubes should never be used as input tubes, while others claim that replacing the coupling capacitor with an interstage transformer will solve all problems. If the amplifier consisted of only the output stage, at least we would cut the amount of audio-advice noise down by tenfold.

This time a little more seriously, when a tube (or solid-state) power amplifier, getting the output stage straight is primary (and difficult enough in itself), but once you do have it set straight, the next step is record just what the output stage needs to function well, then find a way to give it what it needs. This means the next stage to be set straight is the driver stage, then the phase splitter, finally, the input stage. Each step we move away from the output stage becomes easier to design, as less is demanded from it.

Linearity Graph

3rd and 5th harmonic distortion result from the amplifier's departure from perfect linearity. This graph is useful for spotting trouble spots, such as crossover notch distortion, gm doubling, and high-order distortion. A perfect output stage would yield a line as straight as the blue plot in the graph. In this example the bias is close to right on, as the red plot line is fairly straight when it passes through the zero-input signal point in the center.

Gain & Zo & DF

The output stage's gain, output impedance and damping factor are displayed in this graph. In this example, the gain is set against the load impedance. As we would expect, the gain falls with the load impedance, but the output impedance remains constant, while the damping factor DF would fall with a decreasing load impedance. Only one plot can be displayed at a time in this graph, as none of the plots share a common left axis (if they did share same axis, the gain plot would look perfectly flat).