How to accomodate a balanced only phono stage ?

Just keep in mind that the typical "RCA" cable will have only one conductor for the signal. The "ground" connection in an RCA-terminated cable will typically be carried on the shield that surrounds the "hot" wire. In balanced configuration, there is no "ground". The "hot" wire of an RCA cable fed a balanced signal will carry only the positive half of the signal, and the negative half of the signal will travel on the shield of the cable to the outer ground connection at the RCA plug. Even if you use RCA to XLR adapters, this is the case. Some contend this degrades the signal. Better, IMO, to do a little soldering and change over to XLR, 3-conductor cables all the way.

Elizabeth, Keep your shirt on. Nothing I wrote was incorrect. And it may help the OP, if the OP is concerned with the details of this hobby. And most of us here are already nuts enough to be that way. On the other hand, your comment that cartridges do not have three-wire outputs may reflect a lack of understanding on your part. As others wrote, nearly all cartridges are inherently balanced devices, and the two sides of the coil deliver a balanced signal. What you are grounding in an SE phono stage is what would otherwise be the negative phase of the signal PLUS the shield on the cable, ALL going to chassis ground at the phono stage, typically.

I should have added that IF one does use an RCA cable to carry a balanced signal, then one must lift the chassis ground connection on the RCA jacks, for sure. Else, like Dave said, you will lose the negative half of the signal to ground. This equivalent to a 6db loss in signal strength.

Al, Interesting point you make re not losing signal strength. That runs counter to my actual experience when I accidentally grounded one half of the signal in a full balanced set-up. But now that you say it (and now that I realize I knew it, and you are correct), I can think of another reason why I definitely lost at least 6db of signal strength until I un-grounded the 2nd of the two signal-carrying conductors. (Note I have avoided using the terms "positive" or "negative", which are merely conventions and could also be confusing.)

As to my statement regarding the "negative half of the signal", I know very well what is happening. I had just been criticized for being too pedantic, so I decided not to elaborate on that statement for fear of confusing someone. Perhaps my over-simplification could also be confusing, so thanks for mentioning it.