Solved by ptrblck in post #2 
              
              
                Could you make sure that the log operation returns valid outputs, e.g. that x is not zero? 
Given that spec(x) can be zero, would it also be possible that complex_norm and mel return zeros? 
If so, then torch.log(torch.zeros(1)) would return tensor([-inf]), which might raise this issue in the backwa…