OK, so you connected the output of a power amplifier to your soundcard and it got fried. Not surprising really.
Actually it didn't get fried. Connecting a fillet steak to an electrical power source would be a different method of cooking entirely.
DON'T try it!
But you got it fixed, so that's OK. Actually you didn't quite get it fixed because it now hums. This is not OK.
Firstly you need to take your soundcard back to the repairer, either for them to fix it properly for no additional charge, or to refund your money because they didn't repair it.
A soundcard that hums is NOT OK.
If the hum is on the inputs, all of your work will be tainted by this hum and it would not be acceptable professionally. You would be in competition with people who can do work that doesn't hum, and guess who would get all the work? Not you.
If the hum is on the outputs, it is unlikely to get onto your recordings, but it will be coloring your judgment on everything that you listen to through it.
So although this is an unfortunate thing to happen, you must do whatever you need to be able to make recordings that are clean and clear.
And this means no hum.
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