First you say it doesn't work when connected to a PC...The problem is that it has the broken sound while connected to pc. Before it was perfectly working.
Now only amp section does the job, seems the usb, dac section is broken.
...then you talk about connecting to a smartphone.It would not be worth sending it to them but at list they could tell us how would this be possible: broken usb socket after connecting to smartphone via a Hama otg cable.
I'm confused. Which is it?
Problems with USB connection to a PC usually come down to one of two things:
1) You messed with the iFi driver settings and changed the USB Streaming Mode or the ASIO Buffer Size to values that don't work;
2) It's the cable (99% of all such cases are eventually boiled down to a faulty cable).
The USB inlet on the iDSD can be a bit wobbly, even with iFi's own supplied cable on which I wrap tape around the plug for a more secure fit.
Try it with another cable and the supplied adapter.