Also, Amir used that to simulate real music playback. What else can I use to illustrate that when bass is loud, high distortion amp likely to contribute to extra db in the highs?
You can't because it doesn't happen unless the amp is clipping or broken.
And why do you keep saying masking issue? I am not saying all freq are at the same volume. I am saying typically bass are louder than treble in songs. Our hearing sensitivity in the highs are much better than the bass, right? Even bass is at 90db, we can hear 1200hz at 65db just fine. If the noise at 1200hz is at 70db, then I am sure I can sense 5+dB increase in 1200hz even when bass are in 90db.
Of course you can hear 1200 Hz at 65dB when bass is at 90dB as there is only 25dB level difference. You can even hear that when 1200 Hz is at 30dB. I am sure you can even hear a 2dB increase in 1200Hz at that point as well.
Here's the thing. The 1200Hz at these levels will be distortion free and close to noise levels. The 90dB bass is still easy to hear.
That bass will have its own harmonics. The speaker will generate its own harmonics as well. The amp will generate harmonics from the bass fundamental note itself + generate harmonics from the harmonics present in the music.
The bass being played has many harmonics from itself. The lower harmonics will be the highest, the higher the harmonic order the more those generated harmonics drop. At the 5th harmonic the contribution from the largest source (in your case the deep bass note) will not be there anymore it is in the noise floor below audibility levels. The amp does not contribute.
The speaker will have the largest distortion by far as shown by the calculations I made earlier the amp does not contribute to that (well 0.0something dB).
That bass note is only present very shortly at a high level. At that point it has a shitload of harmonics. The harmonics from that bass note are much higher in amplitude than the generated harmonics even by the ACA.
As I demonstrated the 'extra energy' from amp generated harmonics in the lower mids is just 0.026dB or so. You can't hear that.
I know what you want to prove, or think I know. When you play loud bass you hear the treble turn 'nasty'. This is not caused by the amp but is acoustic behavior of the speaker, headphone or room itself or an amp clipping or when you have a really nasty amp with tons of IMD you may be hearing that.
Maybe you are listening to a filterless DAC and the speaker or amp really doesn't like the produce >24kHz signals.. There can be plenty of reasons.
In any case... and amp with 0.1% distortion at max. power levels or even measured at 1W won't have 10% distortion in the treble.
The energy in the mids is lower by itself which could generate 2nd, 3rd, and 4th harmonics into the treble range. The bass cannot. Some upper bass harmonics maybe could but these are already down very low and the amp will add virtually no harmonics at lower levels.
IMD from the bass note (the fundamentals) will generate small amplitude poles right next to say a 1.2kHz note. Small and masked. You can't hear those. At least not the ones generated by the amp from the bass.
In the end an amp with 0.1% distortion at full power or at a certain level will not generate 10% at higher frequencies.
What you do see in Amirs plots is that distortion seems to rise in percentage when the signal level gets lower.
Here's the thing.... that rise is the noise floor. The smaller the signal the closer it is to the noise floor. That is what it shows.
Distortion does not increase at smaller levels but actually decreases.
Now we are talking about amps that do not have a crappy or poorly adjusted AB idle current. Those amps will have increased distortion when the level gets smaller. This too can be spotted in plots though and is not the case with modern amps and certainly not with the ACA.
The elephant in the room is the speaker plus the room. It's not the amp unless it is broken or clipping. it certainly isn't the DAC as distortion is already far below audible levels. Only filterless DACs playing 44.1 or 48 without upsampling could, in certain conditions, throw some IM dirt in the music.
In the end blame the speakers not half decent amps. Just read lyric interviews on subjective forums by golden-eared folks with high end equipment. They only hear a wonderful sounding amp. Not a harsh one. And remember they can hear a gnat fart on Mars which we mortals with our cheap Topping gear and LS305 can not.
