Looking for help understanding clock usage in my system

@sls883  These are very good questions, and you’re far from the first to run into them. I found an instance where Gustard customer service provided clarification to this question, which was published in a Facebook post in a Gustard group. Relatively few DACs have a PLL, and some like the Holo Audio May enable this feature to be toggled on or off by the user, but not in your case.

This is not like in a recording studio, where everything is synchronized to the same clock. Assuming Gustard’s clarification is correct, it seems the only place in this system where the OCK-2 would be the master clock is with the N18 Pro. As for the U26, the OCK-2 should make no difference, but it’s free to try. 

@ted_b thanks again for the reply.

The Gustard X-30 and the U-18 allow you to switch between external clock and internal clock.  But, that doesn't mean that the external clock is doing anything when using the i2S input.

I've reached out to Innuos when I purchased the X-30 and they suggested that I use the ddc rather than going USB straight into the dac.  Gustard concurred.  I didn't ask them specifically about the use of an external clock.  I bought the U-18 and LYH when I was using a Gustard R-26 dac.  I wasn't sure if I needed them with the X-30.  It has a better internal clock.

I also reached out to Gustard and asked if the U-26 would produce better results than the U-18 with the LYH ock-2 and their response was "the U-26 will be an improvement".  I kinda expected them to say that.  

@sls883 If my understanding is correct, this default internal PLL feature appears to apply to R/X/A-26 and later flagship Gustard DACs, so your R26 was the same (whether you realized it or not). 

Innuos has a very good understanding of USB audio concerns, and the Zen Mk 3 has "ultra low noise USB audio output." What you could do is try a direct USB connection from the Zen Mk 3 into the X-30 while using the OCK-2 as an external DAC clock and see how that compares to using the DDC. 

I‘d frankly do away with the DDC, use USB via LHY audio UIP-1 ( an excellent USB decrapifier) and have the switch, DAC and UIP use the OCK-2 clock. That way your INNUoS streamer would be slaved to your DAC. USB in my mind is wrongly maligned. It is an excellent interface provided you have clean 5V power and galvanic isolation of the signal, hence the UIP recommendation

The U-26 just delivered a little bit ago.  I have it hooked up without the LHY ock-2 connected to anything except the N18 Pro ethernet switch.  I don't know if it's worth keeping the LHY solely for that purpose.  I guess I can compare listening with and without the clock to see if it's having any impact.

So far, it's sounds excellent.  I'll have more time to listen after work tonight.