Will moving transformer to separate case reduce noise in opamp-based circuit?
You'll have little interference inside the opamp, but maybe some in traces leading to it. More in longer traces, higher impedances, and lower voltages. Keeping the transformer away from the very low voltage input signal (especially for moving-coil) is a good idea. You may want to buy a transformer in a &mu;-metal casing, or build such a casing for it yourself.