HomeThe Difference Between Phase and Polarity

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It's a widespread misnomer that the output of an inverter, such as a common-cathode tube stage, is "180 degrees out of phase" with its input. This is loose and misleading use of language. "Phase" specifically refers to timing relationships of waveforms, such as frequency-dependent delays in filters. An ideal inverter simply changes the signal's polarity (makes the output a "mirror image" of the input), independent of frequency. (Of course, like any amplifier, all real inverters also have a low-pass filter characteristic, which does cause increasing phase shift and signal attenuation as frequency rises above a certain value.) The input and output signals of an inverter are in phase, but have opposite polarity. That's why it bothers me that the first stage of a push-pull tube power amp (as in most Fender amps) is commonly called the "phase inverter" or "phase splitter." This stage's two outputs have the same phase but opposite polarity; if the outputs were actually 180 degrees out of phase, the amp wouldn't work at all. That's why I call the stage a "polarity splitter."

The phase-polarity distinction is also discussed elsewhere on this web site. For your convenience and additional perspective, here are excerpts:

Confusing Polarity with Phase: Commonly, signals of opposite polarity are wrongly called “out of phase” or "180 degrees out of phase" because they can cancel each other out (the irony is they cancel only when they are in phase). Some preamps have a polarity reversal switch (as a feedback countermeasure) miss-labeled as a “phase” switch. Phase properly refers only to time relationships of signals. For example, a mic mixes sounds coming directly from an instrument with sounds coming from the speakers or reflected from walls, floor and ceiling. These sounds are delayed by different time intervals depending on path length. At the mic, they may be “in phase” (reinforcing each other), “out of phase” (cancelling out), or anywhere in between (shifting the timing of phases). With the speed of sound as a constant, the exact result depends on frequency and path length. The point is: there are infinite possible phase relationships between signals, but only two polarities. Polarity only refers to the sign of a signal; two in-phase signals may have either the same or opposite polarity. Do not interchange the terms polarity and phase. [From Part 4 of "Notes on Amplified Fiddles"]

Defining terms: Unfortunately, audio engineers, who should know better, frequently conflate the terms “polarity” and “phase.” It doesn’t help when polarity switches on many mic preamps and mixing consoles are inaccurately labeled “phase.” Phase refers to timing differences between signals; the possible number of these is infinite and the consequences are frequency-dependent. In contrast, there are only two possible polarities (“+” and “-“), and these are independent of time and frequency. Reversing the wires hooked to one monitor, for example, changes its polarity relative to the other; it does not put the monitors “out-of-phase” as it is too-often called. However, depending on frequency, sound reflected from a wall in your control room may arrive at the ear in-phase or out-of-phase with a monitor’s direct sound, due to the time delay caused by increased path length. [From Quick-Start Guide for Modified Altec 1567A PDF].

It's easy to find external links to the polarity-versus-phase issue in a web search. Here are three examples:

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