Originally Posted by kraut
Complex notes with sub-harmonics and harmonics galore
What gets amplified is not notes and harmonics, what gets amplified is the voltage (differences) of the signal. Check out an oscilloscope trace of a music signal. The curve is the signal, and that is all. That curve includes all your harmonics etc. There is nothing below that curve that an amp will respond to. A relative simple signal.
The parameters for testing is pretty straight forward but is done with single or double tones
I'l try to be nice and not return the harsh reply.
What amplifiers usually are tested with is not a simple sine frequency, what they get hammered with in testing is a square wave.
The testing to confirm most spec sheets can and is accomplished with single or double tones.
The classic means for testing amps are based on the following test signals:
Frequency response - chosen spot tones
THD - spot single tones
IM - double tones - SMPTE IM uses a bass tone and a treble tone, while CCIR IM uses two high frequency tones.
In modern times more different kinds of test tones can be used because we have the analysis software to isolate the various components of the complex results that they produce. Swept tones and multiple tones can be used, but they are still composed of pure sine waves.
The problem is one of interpretation. A square wave is just a collection of a the fundamental and a series of odd harmonics of a sine wave (see the equationsin the Wikipedia article for example), with the harmonics weighted in a specific way, namely each odd harmonic present is weighted proportional to the inverse of its order.
In the olden days we lacked the complex analyzers to decode this sort of thing. If you tried to run a THD test with a square wave you had the harmonics of the square wave right where you were hoping to find harmonics generated by the UUT. There were similar problems with IM testing. Frequency response testing had that problem, plus the fact that at high frequencies the harmonics were too close to each other. If you use say a 20 Hz square wave to run a frequency response test, you only test the amp at 20, 60, 100, Hz etc, when you really wanted to test the amp at 20, 25, 30, 40, 50, 80, 100..Hz. At the high frequencies the harmonics are dozens of dB down due to the 1/n weighting, and your harmonics are still odd multiples of 20 Hz which are 40 Hz apart so that around 20 KHz the components are way too close together to analyze.
It just wasn't done. Since waves at the right frequencies were just a crank of a knob away.
If a square wave was used it was use to get an idea of low frequency phase shift, high frequency ringing and slew rate. Note that none of those things show up on a typical amplifier spec sheet. Slew rate was only a serious issue in roughly the 1970s and 1980s and was found to be subsumed by other measurements namely high frequency nonlinear distortion, and more understandable that way. It fell out of fashion and is rarely mentioned these days. Another reason why square waves fell out of favor for high frequency testing is that due to the widespread use of digital media sampled in the 40 KHz range, almost all signals in acttual use are brutally band limited in the 20-25 KHz range.
In modern times we can extract a lot of knowledge using a wide variety of test signals including music - we can do a useful FR test of an amp just by playing music through it, no special test tones required.
I did my first test equipment test of a power amp in or around 1960 and have bench tested several 100 amplifiers over the years. In 1996 I started doing FFT -based equpment tests. I have my own personal AP test set operational in my home. So I literally grew up with this technology and kept on flogging it into my retirement.