@Kairosman - from my reading the conventional wisdom is most DACs when fed sources like I2S, s/pdif and AES that contain a synchronous word clock will use that embedded clock signal untouched so theoretically at least a master clock connected to the DAC wouldn’t be used to reclock the stream so would be of no benefit. And that conversely with asynchronous sources like USB and ethernet they are of necessity reclocked by the recipient DAC/ streamer and so if the DAC/streamer is being slaved to a master clock you’ll get the benefit of its greater temporal precision. Indeed from memory Gustard’’s R26 product blurb that appears on vendors’ sites is consistent with this referring to the USB and Ethernet inputs being reclocked.
So far so logical. But then it gets rather less clear cut with:
a) a clarification Gustard provided over email to a Headfi member (copied verbatim below, ambiguous punctuation and all) that suggests - depending on how you read the punctuation - that all the R26’s inputs benefit from an external clock with NOS & DSD DIRECT OFF (ie over sampling ON) but with NOS/DSDD ON only the asynchronous inputs, USB and ethernet, benefit; and
b) at odds with a) my observation from careful testing that there is a satisfying audible benefit of toggling an EXT Clock ON for I2S and s/pdif regardless of the NOS/DSDD setting. Fairly sure a number of other Gustard *26 owners who use I2S and NOS/DSDD also find a good benefit with an external clock. As to the mechanism of action/benefit from the external clock if these synchronous inputs are not buffered and reclocked (which would seem unlikely), your guess is as good as mine!
It has become apparent there’s substantial language barrier/translation error risk factor with any comms with Gustard, so a pinch of salt with all their comms and publications especially on nuanced technical points is wise. Eg. They accepted the manual had a translation error re DSD processing.
Credit to HF member MMWMM:
This is an answer from Gustard regarding the use of a 10Mhz external clock with the R26 and when its clock signal is used to reference the internal K2 synthesizer and when is not used.
“When playing PCM
1. PCM NOS: OFF
The improvement provided by the external clock applies to all R26 inputs.
2. PCM NOS: On
The improvement provided by external clock applies to USB and LAN inputs, IIS AES coaxial optical Bluetooth does not apply.
When playing DSD
1.DSD DIRECT: OFF
The improvement provided by the external clock applies to all R26 inputs.
2.DSD DIRECT: ON
The improvements provided by the external clock apply to USB and LAN inputs, IIS AES coaxial optical Bluetooth does not apply.”