What is phase and what does a phase switch do?

We have a number of products that feature something called a “phase switch” (like the Pre-Phase Mini Preamp). Phase switches are usually marketed as a feedback-controlling device, but they accomplish more than just that. Phase determines at what point in time a sound wave has its peak or trough.

This picture shows two (basically identical) sound waves (sine waves, to be exact), but the wave on the right (2) is inverted in respect to the wave on the left (1). Interestingly, if these two waves were played back simultaneously from the same speaker with the same volume, they would completely cancel each other out and no sound would be heard. There would be silence.

Out of phase guitar sound reproduction also suffers, to a degree, from phase cancellations. Amplification systems, effect units, or any electronic audio device may or may not invert the phase in of the signal coming in. In most cases the manufacturers do not specify.

Every acoustic instrument projects sound waves in a certain phase when it is played unplugged. The phase that is projected from the amplified speaker source needs to be “in phase” with the acoustic instrument’s own sound waves or the result will not be optimal.

If the instrument’s natural and amplified sound waves are out of phase, the result is early feedback and tinny, unbalanced sound, which may be tolerable in some tone ranges (positions on the fret board) and quite noticeable in others, even resulting in sound cancellations on some notes.

The phase switch allows for instant correction of this problem. When you toggle a phase switch back and forth, you will notice fuller, warmer and nicely balanced tone with less feedback in one setting. Test especially the low E sting up the B on the 7th fret. Phase problems are easier noticeable in the low register. Whichever setting sounds warmer and fuller is the correct setting for the phase switch. Keep in mind that it may be different with different amplification systems.


Dieter's Sound Bite


I have feedback problems, phase issues etc.: This podcast describes why acoustic instruments may experience feedback and how to avoid it. I also talk about phase issues and what characteristics you can expect from the Pure pickups. Right-click to download: Feedback and Phase Issues Podcast

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