Spark amp with a terrible hum problem and not working anymore

  • After 7months my spark stop working. there was a terrible hum and when I plug the guitar nothing happened... and on Gain the hum was even higher so high that I couldn't keep the amp on.. I got it replaced, it work for 6 days and than again same problem and I had the new grounded power supply... I sent back (and I am waiting the new one replaced) and in the meantime I bought another one and I got it... it worked for 5 hours than the same problem again...
    I know it is not electricity problem because if I plug all my others digital amplifier from Yamaha nothing ever happened
    does this problem happened to anyone? the customer service doesn't really help explaining to me what can be the issue.
    thank you for any help