Advice on field mixer for documentary. Newbie

Hi all-

Any advice would be greatly appreciated. I bought some audio gear today and am worried that I got the wrong mixer. I'm using a Sennheiser MKH-416 shotgun mic with a "Rolls MX54 field Mixer"... I'm going into a Panasonic DVX-100 digital video camera. I hear line noise or "white noise"... I can't tell where it's coming from. Is this what I can expect from a $130 dollar mixer?

If I take this mixer back I can spend no more than $500 dollars and will buy used. I just want to get the BEST quality for my money. Any advice.

Thank you,

michael