I have not had the need to buy a second adapter. I also think NI should sell them (or let the manufacturer sell it…) because people will lose them (or some schlub will rip yours off…) and they will go bad from time to time.
The NI adapter is 9V at 1200mA (1.2A) so a 9V at 2A adapter should be fine as far as powering the unit. My only question is whether the polarity on the aftermarket adapter will be right.
…and after looking at the power supply and turning my S4 around, I see that there is NO polarity indicated. Typically a manufacturer will show you whether it is tip positive or tip negative. Since there is no polarity indicated on either the S4 back panel or the S4 wall wart, it must take care of the polarity internally.
Hope your replacement works out!! Post back and let us all know if it works OK…
If you are picking up weird buzzes in your audio, I would say that you should reverse the polarity of the power supply.