@Miska
First let me say I admire the desire to improve on things, whether they may become audible in some specific situations or not.
I appreciate the efforts and above all that you base on measurements.
The fact that you leave the perceptual part out of it is commendable and questionable at the same time.
The below is not 'bashing' on my side but simply asking questions that popped up in my small penguin brain as it were.
You were obviously compelled to start your search in this area .. what triggered it ?
I mean you must have 'heard' something and related it to something and investigated it.
Why not measure into the GHz range ?
In the many EMC tests I have done often there is a lot of life well above 5MHz.
For these frequencies to become audible, however, they need to be large enough to be rectified (as in AM detector) in general. -80dB signals do not qualify. These frequencies travel like a knife through butter across PCB's and cables.
Most amplifiers do start to roll-off above 100kHz though some do not. granted this is mostly 6dB or 12dB/octave as input filters are generally only 6dB/octave and the amp itself as well.
Do you have any practical shots/analyses of certain (preferably often used/well known) amplifiers loaded with actual speakers/headphones that clearly show problematic behavior of amplifiers when fed signals in the audible range and say around 1MHz combined which seem to show that when the 1MHz signal (preferably not a multiple of the test tone) is included or not that the output of the amp differs within the audible range ?
I assume your very extensive research has a basis and is clearly documented.
Preferably with the main frequency say -6dB and the higher frequency at -60dB or so.
Do you have any indication how big this issue is and how relevant it is to music reproduction ?
I mean one can measure well outside of any audible threshold. What one can measure does not need to be audible. Some claim vice versa as well but I am not one of those people.
Examples of amps that are known to exhibit this behavior ?
You mention
So any intermodulation products they may create in later stages (like power amp) are also fully correlated. So if you hear it sounds like artificial hardness.
Is this your experience ?
Can you clearly correlate this ?
What amplifier / transducers are you using to listen for this (so I can avoid that one) as you clearly state they MAY create it I assume you own one to test.
You say:
We cannot know what kind of amps there are after the DAC, so we need to account for all kinds of possibilities, such as analog class-D amps, etc. One dominating factor is how the amp's THD/IMD vs frequency profile looks like.
Agreed. What would be so poor THD/IMD that people could possible hear effects and what would constitute a good profile.
You seem to suggest that the often reported 'synergy' between DAC's and amps (almost infinite possibilities here) could (maybe) be related to this.
This should be very possible to capture with an RME after an amp when loaded with a real load.
Do you happen to know of any such recordings that show this is the case ?
You mention:
Looking at my own tests,..... Last detectable output from microphone for this instrument is at 65.7 kHz before harmonics disappear in increasing noise slope of the ADC noise shaper. But my measurement microphone is not claimed to have flat frequency response that high anyway... Too bad I don't have money for the
Sanken CO-100K...
Do you correlate listening tests with measurements ?
Are those listening tests 'blind'.
It's one thing to measure HF content but another thing to perceive it.
What's your current age (may I ask) and what is the max. frequency you can hear and what is the max. bandwidth you need to correlate recorded and actual sound quality ?
You state:
World is full of all kinds of amplifiers. These transition band (20 - 22.05 kHz) images are pretty much around -3 dB at 22.05 kHz. But you'll have much more problems with the images around 352.8 kHz.
Do you have plots/graphs/examples indicating this within the audible band (preferably using nulling) ?
As mentioned before... I appreciate the effort and feel that in principle any effort to objectively improve output quality is a good thing.[/quote]