Which connection: RCA, XLR, COAX, OPTICAL?

You’re welcome! Regarding putting an optical cable through the pipe, if there are any sharp bends in it I would expect that you should avoid glass cables, and use something like the more flexible Analysis Plus optical cable I linked to earlier. Although I’m not sure that particular cable would be available in a 20 foot length (their website only lists up to 3 meters). I think Monoprice offers long lengths such as 25 feet at very low prices, but I have no knowledge of how well their optical cables perform.

Good luck. Regards,
-- Al

Man, you are a wealth of informations!
But there is one catch which I didn't mention. That length, several meters, runs underground. I put plastic pipes under my wooden floor many years ago, for a situation like this. That cable, to get a signal from PC to amplifier, has to run through the pipe, so it can't be something with massive connectors. I'll try with optical cable first, and if that proves fine, that's it.
But thanks a lot, I learned about some options and things I knew nothing about, that might be useful sometimes somewhere.
All the best to you!

ethernet-to-S/PDIF converter - something like this? http://www.ultrahighendreview.com/simple-design-rendu-ethernet-to-spdif-converter/

Yes, that is one such device. Another much less expensive converter from the same company is this one. As I indicated, though, I have no particular familiarity with such products, and I have not taken the time to study them in any detail. For one thing, I don’t know offhand if those particular products are suitable for playback directly from a computer, or from network attached storage, or both.

Another possibility to consider, if the computer is a desktop model (as opposed to a laptop or tablet), would be to upgrade the sound card to a high quality professionally oriented model such as the Lynx E22 ($699), with which you would also need this $40 analog breakout cable, the combination providing analog XLR outputs having exceptionally good drive capability. You would also need an additional XLR cable, such as Mogami Gold Studio which I mentioned previously, to provide the necessary additional cable length. Or, alternatively, it MAY be possible to connect the E22 to the DAC via S/PDIF using this $30 digital breakout cable plus a BNC to RCA digital cable providing the necessary additional length. (I’m not certain of that, however, as the entries in the description relating to S/PDIF are unclear).

There may be other models from Lynx that are also worth looking into.

Lots of possibilities to consider.

Regards,
-- Al

So, the safest bet would be optical?

No, I didn't say that :-)

While I would definitely rule out USB for a connection of that length, I have no strong convictions about which of the other possibilities to go with.  So I've stated the relevant tradeoffs and concerns that occur to me, and beyond that your guess is probably as good as mine.

Good luck.  Regards,
-- Al
 

ethernet-to-S/PDIF converter - something like this?

http://www.ultrahighendreview.com/simple-design-rendu-ethernet-to-spdif-converter/

Thanks! So, the safest bet would be optical?

For computer, audiophile quality is not so much important, because CD player is my primary source for listening.

Good!  Because implementing a 20 foot or 6 meter computer-to-DAC connection with high quality results is likely to be problematical and/or expensive.

Yes, it's possible that analog RCA connections could work well enough for your purposes.  But I'd have some concern about the possibility of ground loop issues (which can be sensitive to cable length), and I'd also have some concern that the (often unspecified) output impedance of the sound card may be too high to drive the capacitance of the cables without significant rolloff of the upper treble and dulling of transients.  Using a very low capacitance cable, such as inexpensive BlueJeans LC-1, would minimize that possibility.

Regarding USB, both the USB 2.0 standard and the manual for the DIA-250 specify 5 meters max for a USB connection, and I suspect that lengths that are even somewhat shorter than that may tend to result in significant sonic compromises.  And I'd be skeptical of USB hubs or repeaters that I believe are sometimes used to extend those lengths for non-audio purposes.

Given a choice between a 20 foot digital coaxial cable and a 20 foot optical cable, I suspect that more often than not the coaxial cable would be the better choice, although in some cases noise caused by ground loop issues (to which an optical cable would be immune) could tip the balance the other way.

Also, there are some ethernet-to-S/PDIF converters on the market, and I suspect that some of them would provide better results than any of those other choices (with an ethernet cable covering most of the distance, and a S/PDIF cable covering just a short distance from the converter to the DAC).  But while I'm not particularly knowledgeable about such devices, I suspect that good ones will tend to be fairly expensive, and they might necessitate using playback software that you wouldn't otherwise prefer.

Regards,
-- Al
 

* I meant SOUND CARD, not "soundboard"

Also, I didn't mention analog RCA because I tought it won't be a good choice for 6 meters length, but it may still be ok? For computer, audiophile quality is not so much important, because CD player is my primary source for listening.

almarg, you seem really knowledgeable about this, so just one more question, please.

To get audio signal from computer to amplifier across the legth of 6 meters / 20 feet - what would be the best connection?

You can see above the inputs on that Gato Audio amplifier. My computer has USB and optical toslink outputs. It doesn't have coaxial, but if by any chance coaxial would be better than USB or optical, I could change the soundboard to have coaxial output compatible with that coaxial input of Gato amp.

Thanks!

For what they may be worth, the statements in the manual for the Gato DIA-250 about its DAC circuit and its analog input circuits seem confidence inspiring, as do the reviews of it that I've seen.  And the PCM1794(A?) DAC chip it uses, while fairly inexpensive, is also used in the Modwright Elyse DAC, which is also confidence inspiring IMO.

Of course as I indicated earlier which interface approaches would turn out to be preferable will also depend on the designs of the particular players and cables, and on how well the technical and sonic characteristics of everything synergize.

My instinct would be to go with a 5 or 6 foot length of coaxial S/PDIF for one of the connections; and for the other (at least initially) I would suggest purchasing and trying both the fairly inexpensive Analysis Plus Toslink cable I linked to earlier, in whatever length is suitable; and a pair of fairly inexpensive 2 or 3 foot Mogami Gold Studio analog XLR cables; and an inexpensive pair of 2 or 3 foot analog RCA cables as well.

Then try each of those possibilities on each of the players.  Then, after doing that, if you see fit to do so consider upgrading to more expensive cables providing the functionalities that you settle on.  Although I wouldn't be surprised if some of those choices turned out to provide excellent results in the long term.

Good luck.  Regards,
-- Al
 

Also, I think I will go for different head-amp, the one which has only one analog RCA input, so I will feed it from the integrated amp. That way I will have less cables and less complications.
So you can ignore the questions related to connection between players and head-amp.
All that matters is in which one of the 4 inputs should a CD player go (XLR, RCA, coax or toslink), and in which one should Blu-ray player go (RCA, coax or toslink).

Wow, almargh, thank you very much for your effort!
XLR is analog, but a picture is worth a thousand words:
http://www.gato-audio.com/images/DIA-250_3.jpg
- that will be my new integrated amp, most likely!

I was reorganizing my system and found my RCA digital cable was too short for the new arrangement. I used a Toslink cable to tide me over and found the sound was better than the RCA. Have since purchased a Wireworld multi-filament glass fiber Toslink and am quite happy!

One reason that "opinions are somewhat contradictory and confusing" is that which interconnection scheme will be most optimal is highly dependent on the specific components and cable that are involved, and in particular on design dependent variables that are generally unspecified and unpredictable.

For example, if the quality of the DAC in a player can be presumed to be inferior to the quality of the DAC in an amp, it could conceivably still be preferable in some cases to use the DAC in the player instead of the one in the amp if the digital interface in one or both components has particularly high levels of jitter (short term random or pseudo random timing fluctuations), or if the impedance match between the digital interfaces of both components and the cable happens to be less than good, or if the design of the two components is more susceptible than average to effects on their digital interfaces that may result from ground loop issues. Etc. etc.

I’m not sure from your post if the XLR inputs and outputs you referred to are analog or digital. A digital XLR interface would most commonly be referred to as an AES/EBU interface. Also, assuming the coaxial inputs and outputs you referred to are digital, they would most commonly be referred to as coaxial digital or coaxial S/PDIF. A coaxial connection via RCA connectors could also refer to an analog interface.

Also, for AES/EBU or coaxial S/PDIF interfaces, more often than not the lengths you specified will not be optimal. Somewhat counter-intuitively a length of 5 or 6 feet will more often than not be preferable to those lengths, or if a very short length is practicable (e.g., around 8 inches or less), that would probably be even more preferable. Intermediate lengths such as 1 meter, or even 0.4 meters, are often (although not always) less preferable. See this paper for further explanation. In the case of analog interfaces though, and I suspect also in the case of Toslink interfaces (although I’m not totally certain about Toslink), the shorter the better (assuming the goal is to minimize the sonic effects of the cable).

Regarding analog XLR interfaces vs. analog RCA interfaces, which is preferable depends on the designs of the particular equipment. In theory, assuming equal quality of implementation, an XLR analog interface is preferable. However XLR interfaces are in many cases are not as well implemented as RCA interfaces, especially at moderate or lower price points.

But to address your question specifically: My perception, based on many anecdotal reports I have seen as well as my own experience, would be that more often than not the greatest likelihood of optimal results would result from the following, in descending order of preference:

If the player is described as providing analog XLR outputs and as having a balanced internal signal path from its DAC circuit to those outputs, and if that internal DAC is expected to be of reasonably good quality:

Analog XLR > Analog RCA > AES/EBU > coaxial S/PDIF > Toslink

If the player is not so described:

AES/EBU > coaxial S/PDIF > analog RCA > analog XLR > Toslink.

The ranking of Toslink may be improvable by using a relatively expensive (but presumably more fragile) glass cable, rather than a plastic one, or by using this relatively inexpense (and non-fragile) cable from Analysis Plus (which I have used with fine results in a relatively non-critical application).

FWIW, also, I have had fine results with this relatively inexpensive Mogami AES/EBU cable.

Other opinions will certainly differ in many cases.

Hope that helps. Regards,
-- Al