Balanced vs RCA between amp/preamp

In a balanced line system there is no need to lift the ground at one end of the cable. 

Try not doing it in a large multi-room facility like a sound stage, dubbing room. It may not be required with the AES-48, but until all gear is to that standard, prudence takes precedence over valor.

Most XLR gear is not transformer coupled. Not all is actually balanced.

In transformer gear, an unbalanced signal is fed through the transformer. The output can be XLR, banana, TRS, multipin, etc., etc.

While the Jensen transformers are excellent, they, like all transformers, still have a sonic signature.

Depending on the issue, adding transformers with the screen carried may not solve the problem.

So finally it goes down to the transformer isolation,it seems the usual way even the XLR output from the pre,they usually get the rca signal first and use the transformer to get the inverse signal and consist the XLR sigjnal. 

If the preamp has both RCA and XLR outputs its quite rare to see a transformer used for the XLR output.

In high end audio in particular, in a situation like this the RCA is one of the phases of the XLR output (which has an inverting and non-inverting output). The problem with this approach is that ground is being used to complete the circuit, and this leaves the circuit vulnerable to ground loops. One of the goals of the balanced line system is to eliminate ground loops.

@atmasphere 

" The problem with this approach" Do you mean Transformer coupled approach?

As far as I know transformer coupled way is to solve the ground loop.

That's the reason we use the transformer isolation instead of RCA to XLR cable,right?

The way to solve a ground loop is to purchase properly or fix improperly designed equipment. Adding a transformer is a bandaid. Bandaids fall off.

My company wired recording and film studios in Los Angeles and New York in the 1980s & 1990s. We had transformer and electronic balanced and unbalanced equipment to interface. Hundreds of connections that could be reconfigured into literally millions of combinations. Correctly done, there is no ground loop.

If you detail out your equipment and connections, perhaps we can solve your issue. By detail, I mean brand, connection, electrical power etc. Since we can now post images, draw a sketch.

" The problem with this approach" Do you mean Transformer coupled approach?

@runwell No. If you are dealing with a ground loop, the transformer can fix that, but take @ieales' post seriously.