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.
Malfunctioning SMSL SU-9n
I recently purchased a used SMSL SU-9n on audiogon and long story short the optical/toslink input is malfunctioning but the USB works well. I tested multiple cables, sources, and I looked through the menus for simple solutions to no avail. All of my other cables and components work as intended with the SU-9n taken out of the system. However, when the optical input is selected on the SU-9n the audio cuts in and out at different intervals. Have any of you had this issue?
Here is a link to a YouTube video so you can see and hear the malfunction.
A list of the gear I used for testing.
Outputs:
Rogue Sphinx v2 amp
Elac UB5 speakers
Inputs:
Oppo 105d
Caldigit TS3 plus - USB hub
Macbook Pro
LG C3 - TV