Sub harmonic distortion.

[Kokoriantz]

I have a PA amp using it till now to power a subwoofer. It has 0.01% THD at 100w@8ohms but doesn't sound good.
I checked with a modest scope FFT to see a 1/4 harmonic which gets higher by frequency increase. The picture bellow shows 10khz and 2.5khz.
I am doubting to be electromagnetic radiation from the rectified current rails, but I get on simulator 1/20 harmonic.
If someone can put me on the track where to look at.

[moffy]

What might provide more info is a scope trace of the loaded output and the input signal, just a sine wave.

[Vovk_Z]

TS spectrum looks more like a noise for me (it can be PS noise etc). I'm not sure 10-bit scope FFT is any good for an amplifier investigation.

[Kokoriantz]

It is my scope which is showing wrong. Sorry .

[TimFox]

This reminded me of student-days discussion between me (physics student) and my wife (music theory student), when I explained harmonics from a physics standpoint.  In musical acoustics, “overtones” are very important and are usually assumed to be harmonic (in the mathematical sense), but some instruments like kettledrums have overtones that are Bessel functions instead of integer harmonics since they are two-dimensional objects instead of one-dimensional violin strings or organ pipes.
Anyway, the library had very old books on music theory that discussed “undertones”.  The proof of these alleged frequency components was that if you lift the dampers on a piano and hit middle-C, the lower Cs (octaves below the note) are excited.  Spectral analysis can be subtle.

[mawyatt]

The proof of these alleged frequency components was that if you lift the dampers on a piano and hit middle-C, the lower Cs (octaves below the note) are excited.  Spectral analysis can be subtle.

This could be due the notes on the piano being highly underdamped "Resonate Structures" when undamped and lightly non-linear which can be sub-harmonic locked to the higher note when it's also undamped.

Think of this as Injection Locked Division, which we've shown with oscillators.
https://www.edn.com/injection-locking-acts-as-a-frequency-divider-and-improves-oscillator-performance/

Best

[floobydust]

Check the electrolytic capacitors, two green (low ESR type) bottom left in pic. They do not look happy.
Noise on rails can cause extra distortion.

[TimFox]

The proof of these alleged frequency components was that if you lift the dampers on a piano and hit middle-C, the lower Cs (octaves below the note) are excited.  Spectral analysis can be subtle.

This could be due the notes on the piano being highly underdamped "Resonate Structures" when undamped and lightly non-linear which can be sub-harmonic locked to the higher note when it's also undamped.

Think of this as Injection Locked Division, which we've shown with oscillators.
https://www.edn.com/injection-locking-acts-as-a-frequency-divider-and-improves-oscillator-performance/

Best

The strings in a piano notoriously form non-linear oscillators.  A normal note in a piano has three parallel strings, closely spaced and individually tuned to the same frequency (or thereabouts), hit by the hammer. 
A note (with its damper lifted by the key or along with the others by the pedal) will persist for a long time (underdamped). 
I saw a demonstration where as one string was tuned towards the frequency of the others, it would jump to lock when close enough, instead of continuously passing through "zero-beat". 
Good piano tuners will play with this non-linear effect to obtain desired attack and decay characteristics of the note sounded by the three strings.