Samplepad 4 noise output

I'm having an issue with the output of this device. when connected to an amp the signal has a constant buzzing noise, I'm pretty sure it's electrical interference, like ground noise or somithing related. I have a lot of equipment and I haven't experiece anything like this in my studio, I also have an alesis dm6 which is fine, clean signal. Strangly it does not happen when I connect it via the headphones out. Can you help? Is it worth to return it or all of them will do that? Thank you
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