I'm not sure you can even do this. Those phono signals are extremely tiny and don't do well with long cables. Are you using this long a cable already? If you are in the Graham Phantom world then you likely will be using a LOMC which makes things even worse at less than 1 millivolt output. I even noticed sound degradation from going to a 2.5 meter phono cable with 5 millivolt output cart.
