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Why use sine waves for testing?
Hi guys

The sine wave is extremely common in the natural world and is a very useful waveform to use for testing audio equipment. The sine shape is a continuously changing, smooth, curved wave and it is easy to see if it is distorted or relatively pure. It is also symmetrical, so we can see if our circuit is introducing distortion or error if the wave loses its symmetry.

If we feed a signal into a preamplifier, we can use an oscilloscope to probe various circuit points to check the gain of the circuit and see the headroom for each gin stage. As TUT5 (The Ultimate Tone vol.5) demonstrates, we can see if the tube grids are being clipped and can fix those issues. If the preamp has EQ controls, we set these for the most correct sine wave initially, just to see characteristics of the circuit even though this may not be an EQ setting we would use for guitar. In hifi, it would be a setting most used for music play back.

Similarly, when testing a power amp we want to see that the output is symmetrical at the rated power. The amp might clip first on one half of the wave or the other depending on the design, although in general clipping should be symmetric. if clipping is far from symmetrical there might be a problem in the amplifier, either by design or by component defect, or by a mistake in wiring of a new build. 

For hifi equipment a 1kHz sine wave is used, where guitar amps are tested with 400Hz. The difference stems from the fact that a guitar has a lower range of frequencies in its output than does full-bandwidth audio. As TUTs state, the highest note on a guitar is around 1,500Hz, with harmonics that are higher.

The sine wave amplitude when connected to a guitar input is 70mV RMS, which is 100mV peak, or 200mV peak-to-peak. Some guitars produce more voltage than this but it is usually the case the the preamp or integrate guitar amp will have "excess" gain to assure that even the lowest output pickups will drive the amp to full output, if not to clipping.

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