Cause of distortion by harmonics

[notadave]

Is there a way to tell the cause of distortion from the harmonics?
I looked at a bad (just crap straight out of the factory) Chinese preamp and found that it had little 2. and 3. harmonics but real bad 4.
Without having had opened the case, I was baffled. I had seen 3. and other odd harmonics but never 4.
That raised the question whether there are some wise rules from experience or theory as to what to look for as the cause.
Usually you can not see anything wrong on the scope so you have to go by the spectrum alone.
I am looking for something like:

strongest harmonic:	likely cause:
First	relax this is what you wanted
Second	saturation towards one polarity
Third	symmetric limiting
Forth	asymmetric supply
Fifth	same as 3.
Even	asymmetry
Odd	symmetric limiting

There should be a list of things to check/look for.

[PKTKS]

If your audio is using these "new" things with SMPS or class-D gizmos.

Forget it.  They are just noisy oscillators producing vast amount of harmonics.

Any audio forum will confirm that TUBES and real LINEAR  good PSUs
will do your audio the way it should be.

But these devices are long gone considered vintage...

go figure if high-fi devices are vintage now...
Paul

[notadave]

Chinese preamp and found that it had little 2. and 3. harmonics but real bad 4.
Without having had opened the case, I was baffled. I had seen 3. and other odd harmonics but never 4.
That raised the question whether there are some wise rules from experience or theory as to what to look for as the cause.

Turns out, that in this case the input was a NPN with the load at the collector (Common emitter), that had a very small base current and low resistor at the base. A tiny signal was enough to turn the BJT off.
The supply was not the cause.
The simplest fix was not to raise the OP current but to increase the input resistance.