An Optical inter-step has the big advantage of providing “Galvanic Isolation”, a break in the metallic interlinks that transfers electrical current.
This prevents electrically transmitted noise to spread along the shields of audio lines.

ADI-2 itself is totally immune against this type of noise, but devices like power amps and their connection scheme, specifically if unbalanced, might not.
Their audio quality can suffer from noise that origins e.g. from a computer.

”Galvanic Isolation” can be achieved by USB isolators too, but you have to pick the right one:
https://forum.rme-audio.de/viewtopic.ph … 03#p223103

Jitter is of no concern at all, as it is filtered out by ADI-2 in any configuration.

Quite often small differences in volume suggest there is a major sound difference.

As Ramses mentioned, RME Bit Test (manual page 101) reveals such within seconds.
This makes sure you’re not chasing a ghost.
You should get the “Bit Test … passed” in both configurations.

Please do the test and report back.

The blind test is the only way to exclude expectation bias.
Doing it several times in a row with random presentation of the configuration-under-test gives a reliable insight of true differences.

From ancient times our brain is configured to estimate the most out of as little information as usually was available- the click of a breaking leaf in the distance: is it the bear that wants to eat you, or is it the rabbit you want to catch and eat …
Receptions of faint sound, combined with visuals and smell, maybe even tactile vibrations was what kept people of the stone age alive.

This mechanism stands in the way of objective judgment, and blind tests can exclude it to avoid fooling yourself making expensive wrong decisions.

If the difference is significant the result will soon be obvious, but even tiny differences can be detected by making enough A/B runs until a statistically significant result is achieved.

In sighted A/Bs you never can be sure if it’s just the idea, the looks or whatever, that you prefer, and that’s true even if you believe you detected bigger differences.

The stories I could tell about self-delusion could fill a little book.
E.g. once I set an EQ (in the studio, producing music) and out of my experience how it should sound could hear every change I did to the knobs - to finally find out it wasn’t activated, didn’t do anything - stuff like that.

Based self delusion a whole “audiophile” industry lives from selling people snake oil of all kinds, perverting the goal of objectively improving music reproduction into an elitist money making machine.

ok, thank you I will tell you do I hear the difference or not with this topping.

About blind test and sight test in this particular scheme.
I have my doubts about blind tests because of that:

after I percept the difference in sound, I tried to connect everything through the Apogee (thinking that the reason of this difference in sound is a USB connection itself, I was 100% sure that optical cable somehow clean the sound or you could say prevent interference from the CPU to go further in audio chain).
And whether I connect it through USB or optical (through RME - backward scheme (in a sense what I wrote in my first post)) I hear no difference at all. But if in this case think that I was sure (100%) that this is the cable - I must hear the difference... But I didn't.

I understand that our mind can play with us. I also heard this story about 3 blind tests in a raw and when always first device/sound is ok, second is meah, and the third is awesome. Managers in audio strores use it to sell more staff.
I also can say that to really hear (it is only about my ears maybe) I need some time of listening session to make a conclusion. And in this case I heard both schemes more than 2 weeks sometime switching from one to another.

First of all:
You assumed the cable MUST make a difference, and were disappointed you couldn’t nail that.

I can tell you from my life-long experience in the studio:

Cables that are not broken and are suited for the purpose don‘t make a difference at all, neither analog nor digital.

This myth was pulled up from the few occasions where cable’s parameters are integral part of an audio transduce system.

In home-audio thats the MM vinyl pickup.
Here the cable‘s capacitance with the transducer’s coil’s inductance creates a strong resonance that typically is located in the audible range and shifts with different cables.
This of course is audible.
Without an MM vinyl pickup in the chain, e.g. on a line out, that resonance doesn‘t happen.

The MM-pickup experience started the cable discussion long ago.
As people don’t understand the circumstances, in the early ‘80s the idea was transferred to all kind of cable links, and clever marketing people jumped on that train to sell more and more rip-off priced cables to the believers.

Imagine, after buying a 2000 bucks worth of cable, who would not hear the “improvement” in a sighted (and often even positively commented) A/B?
Expectation bias at it’s best.

For digital cables it’s even more like nonsense - you did the bit test and all bits came in un-altered.
What could a “better” cable improve on that?
Anyway the bits are not transferred in the order they are handled in the DA-converter, so need to be sorted and re-clocked there.
ADI-2’s FS- (femto-second-) clock does that par excellence, much better than can be transferred even through a dedicated clock-cable at all.

Optical vs. electrical connection is another story - for the reason I explained above.

I’d say, concentrate on the points that really make a difference.
Using different DAs can, to some amount, but usually not night and day.

I think you miss a point a little bit. I said when I checked on second device the same thing, that I hear in RME, I didn't hear the difference. In that case I agree with you - the cables didn't sound different.

The second thing, I'm not in a studio, I'm using this system at home, and don't have an opportunity to create pureness of studio in a sense of sound quality. But I get it, that in a studio there are no difference in sound from different cables in digital or analog domain (maybe, I had never been in a BIG studio to realize that).

Can't say anything about vinyl, never heard it, it's like different dimension.

And again thank you for this open forum, that provide open discussion on every aspect of RME sound and everything in between.

PS. By the way. I always wondered RME ADI-2/4 PRO SE is a studio device or a home device? As I think there are different conditions for studio and home devices.

I'm confused and don't understand what are you propose me to do. I tried the same scheme with other device - doesn't it an attempt to exclude bias?

I would add that even blind test is not suffitient in many cases. Only double blind test gives 100% unbiased results.

Double blind test is done in a way that neither listener, nor device operator knows which device is on. It is known only to the third subject that is not in contact with listener and operator during tests.

Double blind test avoids leaking any information concerning device, even by unconsious way, from operator to listener. None of them knows what device is on.

Well, actually one may perform double blind test even in one person. For example, computer (or whatever) may randomly change the devices´ setup.

Your blindly connecting may be close to it. I just do not know, how do you get know what the random setup was in each case.

At the end of the day, it is important to know that psychoacoustic phenomena exist and that many of the audio myths are simply nonsense.

Performing blind tests or in the way KaiS is doing, plus the knowledge of psychoacoustic is way better compared to the usual testing and claims you hear in audio related forums.

I thought so. You get the information, what device has been used during the testing session. It still may deviate the results. But sure, it is way, way better than no blind test, or possibly even blind test when other person changes the setup.

I guess one could use Totalmix and Digiface USB to switch devices routing. And controll Totalmix using computer and MIDI.

I fully agree that to make propper test is time consuming/costly.....

By the way, what was the result of mp3 testing? Which bitrate is transparent for general folks?

Definitely, and they only concentrated on these. You must still have exceptional hearing to be able to do so, at that rate.

Still not heard about the Bit test or what's the value of the "Bit test"? Then please read about it in the manual.
You can download the test files from RME free of charge.

Perform the Bit test for both of your use cases.
If it succeeds in both cases, then audio is unaltered from the audio source up to the DSP of the ADI-2/4 Pro SE.

If you still hear differences, although the Bit test clearly shows that the digital data stream was not altered in the digital domain, then the differences can only be due to level differences (louder sounds better) or other psychoacoustic phenomena (your expectations, pre-bias).

I am not sure whether Qobuz supports playing of RME test files (WAV files).
If not, simply use another music player, like e.g. MusicBee and use the MADIface ASIO driver to exclude any issues of Windows Sound system. Take special care, that the volume of the MusicBee player is set to 100% otherwise the Bit Test will fail, as it would alter the bit pattern by such a volume reduction at the player.

Both configurations USB–ADI-2 and the Apogee–ADI-2 passed the RME Bit Test, you told us here:
https://forum.rme-audio.de/viewtopic.ph … 24#p226924

This 100% excludes any processings inside Apogee, and even excludes driver issues with all config's that pass the test.

Maybe you repeat Bit Test now, then only compare the sound of config's, including the player app, that pass the test.
This makes sure the difference isn’t happening in the digital domain.

On ADI-2/4 check that SRC is off for the optical input:
SETUP / SPDIF/Remap Keys / SRC: OFF

To exclude other config issues, run ADI-2/4 Pro SE in DAC- instead of Auto-Mode.
This way ADI-2 keeps the same internal configuration when switching inputs.
Still don’t forget to unplug USB when running Optical.
Optical BTW can stay plugged, it’s ignored when USB is selected as input.

SETUP / Device Mode/DSD / Basic Mode: DAC

BTW: what about the connection ADI-2 ->power amp, balanced or single ended?