I find D90 sounds harsh or bright everytime I turn on the unit (accumulated 6-hour usage), I need to tune down my integrated amplifier volume, after 5 to 10 minutes, the harsh sound will go away. Is that normal? What phenomena is this ?
How observant of you
Yes, that is known as the
"warming up effect", and actually that warming-up can be a permanent requirement whereby you will want to "warm-up" the device each time before you start listening.
Typically, you'll find that after 100-150 hours that harshness will be gone without warm-up being required. Others swear their set up sounds better left powered on all of the time. Not only AMP's / Pre-amps, but DAC's too.
One of the places you hear about "burn-in" most is at small boutique audio specialty houses. Why is that? Because they are audiophile enthusiasts themselves that have the luxury of having access to brand new units as well as "burned in" units, and newly opened boxes that customers return without allowing burn-in time to pass.
It's hard to convince someone to keep a device past their return period to allow "burn-in"... really they aren't trying to rip you off - as the purchaser you simply need to accelerate your "burn-in" efforts during your available return time period so you have enough time to accomplish "burn-in" and decide if you are keeping the device, or not.
It's as simple as listening to a unit that has gone through 100-150 hours of burn-in, and compare the sound against a brand new unit right out of the box.
The harshness is clear when that seemingly obvious test is performed. It's mind bogglingly obvious, but no one hardly ever realizes this and tries it out for themselves. Go do your favorite audio boutique - when Covid-19 is finally over - and ask them to let you compare your brand new unit against a demo unit with 100+ hours of run time on it. Then you'll know what to expect.
When I hear people say, "It's all in the mind", I have to laugh when I think "it's all in their mind", as in my mind I know exactly what I've seen and experienced first hand hundreds of times - that's what's in my mind.
I was fortunate, having an interest in audio from a pre-teen time through getting a job as a youth and being able to afford "electronics" of various kinds - including audiophile equipment. I made friends with the guys selling the equipment and they enjoyed showing me the "ropes" of the business, including letting me A/B brand new out of the box equipment vs demo equipment.
Over the years I've continued this pursuit through many set up's over many decades and knowing early on what I learned as a youth I was able to experience the same comradery with many others in the business and confirmed many times the value of giving burn-in time to all sorts of equipment.
Then there's the engineering, manufacturing, quality assurance and long term support of digital and audio equipement before I switched to software in the 80's. Then that experience helped me find and troubleshoot "burned in" equipment - that was working and stopped working en masse - root cause debugging in hardware and software. It's all related.
Please let us know when you think the turn on sound is good to listen to, and how long it was to "break-in".