Ugly buzz in sterio recordings. New to me!

I have been using n-track for several years with no problems at all.
A few days back I did a sterio recording and it was no good.
The sound was like a speaker with a tear in it.
I used Audio Technica sterio mic and ran it through a preamp.
I also tried a sterio preamp out from a line-6 with the same result.
When I use the mic at home I can not hear any problem but at the gig a horrible buzz ruins the recording.
The problem exists with very quiet parts as well as loud parts.
I will use a different board, cords, etc next time but I am a little concerend and do not want to make a fool of myself again.
Any advice would be great.