Unbalanced to Balanced ?

yep, you can try first like you said. But if there is a hum in the line because of some ground loop, it will be better to use a DI-box. This box contains a small transformer, so both amps are galvanically isolated.

Chris, are you sure the amp input instructions are really talking about a true balanced input, or maybe just a normal single-ended input with a separate shield? The shield would then be connected to ground at only one end to prevent ground loops.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

I wouldn't do it that way. Connect tip-to-tip unbalanced-to-balanced, connect the sleeve of the preamp to the ring of the main, and leave the sleeve of the main floating at the preamp. Use shielded twisted-pair for the connection. As another poster mentioned, the best way is to use transformer isolation, but I think you don't have that option. If you tie "earth" and "return" together in the connecting wire, you will probably have a ground current loop, and hence, hum.

A clarification - you can use triax, if that's what you already have. The point is that you use a three-conductor transmission line. I don't think cable characteristic impedance will be a problem with your planned low frequencies and short runs.