Malfunctioning SMSL SU-9n

I had the same issue with the SMSL D300. Mine was a month out of warranty.
tried everything to get the thing serviced no one from Amazon / Seller on Amazon or SMSL seemed to care. So now I have a $400 USB Dac only. I’ll never buy SMSL again. 

I’ll stick with Schiit audio for DACs I don’t think Schiit would leave you hanging like SMSL did to me. 

As I've continued to research that is also what I've come to understand. I am frustrated that I was sold a faulty product especially because I specifically purchased it for the functionality of the optical/toslink input. 

Schiit left me hanging,. If you give them good money for a Modi and it fails 1+ years, tough nails. They won’t fix or replace it. I have a couple of S.M.S.L. DACs that always work, one on my TV.

Go to the DPLL settings and change the number.  The default setting is 7. I had the same issue when using optical and the source was my TV.  Changing the DPLL number to 9 fixed the audio cutout