Does your DAC sound better through the S/PDIF or I2S inputs than through USB?

@soix 

Good points.

My USB and Coax cables are both very good custom jobs by Igor Kuznetsoff of the NJAS, about the best cables can be. I'm not sure the DAC takes the clock from its SPDIF inputs.

I've tried the DAC with two different DDCs now: the U18 ($500) and the iFi iLink (small, inexpensive). My iLink has some custom modifications by Igor and is powered through a very HQ 5V LPS. The iLink probably doesn't have that great a clock. Yet the R26 sounds much better through it than through the R26's USB input.

So I'm not sure the DAC even takes the clock from SPDIF.

I've also tried the U18 with an HDMI cable, an Audiquest Firebird 0.75 M, their 2nd from top of the line. It's probably compromised by being longer than necessary. In any case it sounds way better through Coax, maybe because Igor's coax cable is better.

The R26 is fantastic in some ways, but it has a quality control problem and it's the second R26 I've owned that seems to be changing sound over time. I've read some stuff on R-2R DACs that indicate they have variable quality because the precision required of the resistors is so demanding and resistors can drift. So my current R26 is brighter than when I first got it... I changed the filter setting to prevent the harshness that it developed. I'm quite sure the harshness wasn't there when I got it, because I was happy with it on one particular filter setting, and over time I started noticing I really didn't like the sound of my system in unmistakable ways. Now it's okay on the NOS setting.

But I'm thinking I want to get away from R-2R DACs.

@rbstehno 

My server is an Aurender N100 and has only USB output. I’ve tried two different DDC’s in order to convert to I2S or SPDIF for my Gustard R26 DAC and the better one is the Gustard U18. (I haven't tried the Iris 12th or other DDCs so I'm not sure how the U18 compares.) I’ve tried the coaxial connection and the HDMI connection. Coaxial sounds best. The I2S through HDMI sounds a bit bright and more sterile.

My HDMI cable is a 0.75 M Audioquest Firebird, a $1600 new cable although it comes up frequently on US Audio Mart for much less than that. My coaxial cable is a custom silver, highly shielded cable made by Igor Kuznetsoff, which is likely better quality that my Firebird.

Is it possible that my Gustard R26 gets the clock in two different ways?

- via internal clock when using S/PDIF inputs like coax? (i.e. it would do reclocking)

- via the input when using I2S?

What happens when I connect an external 10 MHz clock to my R26? Does that not affect the I2S input?

I say this because the S/PDIF coax input sounded similar with two different DDCs and one was small and cheap (probably not a good clock). It sounded worse through I2S from the DDC compared to S/PDIF (perhaps the cable wasn't good enough and the DDC (U18) clock wasn't an improvement on the internal DAC clock).