Originally Posted by dsperber
You're correct in everything you've theorized.
The only digital input supported is HDMI (discrete multi-channel LPCM via HDMI). And the only digital output supported is optical (2-channel stereo PCM). Using either or both of the digital input/output methods will of course eliminate the need to convert to/from digital when using analog input/output... thus allowing a presumably superior external ADC or DAC to perform the function otherwise performed by the built-in functionality in the Realiser for the same purpose.
In actuality, however, both the ADC and DAC in the Realiser are superb! Would a "superior" (and expensive) high-end external DAC provide better sounding results than the analog headphone outputs from the Realiser (using its own built-in DAC)? Probably, but the incremental improvement is essentially small.
It's hard to do a real A/B comparison because you'll very likely feed the XLR output of an external DAC to a high-end headphone amp that accepts XLR (e.g. the high-end Stax amps), and balanced XLR is "twice as loud" as unbalanced RCA (say from the same external DAC, feeding the RCA inputs of the same headphone amp). Since "louder always sounds better" you really won't be able to do an honest comparison, at least not unless you have (as I do) an EQ in between DAC and amp on the RCA path, which can apply "gain" to the RCA output to make it "as loud" as the XLR.
If you feed the analog RCA audio output of the Realiser to the RCA input of the headphone amp, you'll probably again have an issue of balancing the levels between the XLR output of the DAC and the RCA output of the Realiser, most likely handled by the volume control on the DAC to equalize things.
In other words, it's more likely that XLR output from external DAC to the headphone amp is really where you'll get observable improved "sound quality" over any RCA approach, rather than any significant difference in the Realiser's internal DAC vs. use of a "high quality" external DAC. As far as the two DAC's go, the difference that you will be able to hear in this multi-channel virtual sound is honestly small, probably.
And, much the same type of argument can be made on the input side. The Realiser's analog-to-digital conversion is really excellent. Eliminating it by using HDMI input directly again produces a small improvement, perhaps. It is possible to do an A/B comparison using two presets on the Realiser (one set for HDMI input and the other set for analog). But honestly, the difference (if any) that you will notice is small at best. It will definitely not be large.
At least that's my opinion.
Thanks for your answers/opinions, and others too! One thing however, neither my DAC nor headphone amp has balanced XLR applications. They're all RCA inputs & outputs, and single-ended 1/4" headphone jack.
Here's my question then: what would be the best method in my current setup?
#1) iMac -> mini dp to HDMI adaptor -(digital input via HDMI)-> Realiser A8 -> headphone amp -> headphones
(DA conversion takes place within Realiser, thus excluding my external DAC off the grid entirely)
#2) iMac -> external DAC -(analog interconnects)-> Realiser A8 -> headphone amp -> headphones
(DA conversion by external DAC, AD conversion & another DA conversion within Realiser...)
#3) iMac -> mini dp to HDMI adaptor -(digital input via HDMI)-> Realiser A8 -(Toslink out)-> external DAC -> headphone amp -> headphones
I think each has its advantages and disadvantages, but not sure what would be the smartest choice.
#1 chain is the simplest, yet it doesn't take advantage of a superior external DAC (then again, they say the Realiser A8's DAC is good too)
#2 chain takes advantage of the external DAC, but it does DA->AD->DA conversion... which may result in some loss.
Speaking of which, if Realiser supports up to 24/192 via HDMI digital input (utilized in #1 chain), then to which sampling rate does the AD converter convert analog signals?
#3 chain sounds good too, but is it true the Toslink out only does up to 24/48? Does it mean it won't play hi-rez files like 96kHz or 192kHz, or does it simply downsample them and still play them OK?
Lastly, since you seem to favor #1 chain, could you recommend some decent audiophile-grade HDMI cable? Also, since I'm an iMac user, I'd need a mini dp-to-HDMI adaptor as well. Does anyone know where to buy one, which gets its job done without compromising audio signal quality during mini dp-to-HDMI conversion?