avoiding grid buzz on the inputs, how? ( contact mic )

I sample with a cheap contact mic, plugged directly to the input of the MPC One,
and capture a lot of buzz from the grid.
I have to turn up the input level to max since I use the mic without preamp, and normalize afterwards to get usable levels. ( not exactly your audio engineers way )

How could I avoid or reduce the buzz? I doubt a preamp will make things much better,
since I had the problem before with an audio interface and PC.
But since I only recorded line level I could turn the output levels up and it wasnt a huge problem when I recorded.

I unplugged everything from the MPC to make sure its not a ground loop with other gear.

Right now I have to use matched FFT filtering in Audacity to reduce the buzz.
Otherwise the recording is unusable.

Any ideas or suggestions?

cheapest ( battery powered) microphone preamp I found is 71,- €.

Anybody knows a cheaper one? Given that is just a small transistor and some I dont want to spend
much more than 15,- Euro. I only need it for contact mic so no phantom power required.

I had a Novation BassStation lying around which has an audio in and can be USB powered,
so I tried this for a pre amp, but the buzz is still there.

The microphone cable isnt shielded ( I think) and when I touch it the buzz gets louder.

I wonder if its just the microphone and cable thats picking up the buzz and if a proper microphone would solve it.