Guitar Amplifier noise when a DC adaptor used.

[bzomb]

Hello!

I've built a guitar amp with two LM386...its acctually a kit that I bought to put together. Its powered by a 9V battery but also you can plug it to the wall. The thing is that when I plug the power adapter there is a stong humming signal. I tried one cheap 9V 1A and a good quality 9V 2A and its all the same, noisy. I've been told that it must be the power adapter thats causing the problem and I need to add a "Noise filter" but I have no idea how to (very new in this)

Anyone can provide any information about how can I solve this issue?

Thank you!