At what distance are balanced XLR cables needed?

Al - I'm not sure if metal purity or dielectric absorption or capacitance play role in any particular setup either, but if they do - I can't see any reason why it would be different for balanced vs unbalanced cable.

Balanced versions are always slightly more expensive. If noise is the only concern balanced cable can be more effective for less money. Shield on single ended cable protects from capacitive coupling and high frequency EMI, but might be less then perfect in lower range of the radio frequencies where it works still as an antenna (poor one) while skin effect (that causes induced EMI to run on the outside - shield)is not strong yet. Balanced configuration suppresses common mode signals in this range of frequencies, while twisted conductors provide further noise rejection (cancellation thru even exposure).

IME, more than 15ft.

Kijanki -- I agree 100% with your post just above.

I would additionally mention the fact that balanced interfaces will be much less susceptible to ground loop issues than unbalanced interfaces. That susceptibility can be length-dependent, because it is sensitive to the resistance of the inter-chassis connection path. But the length-dependency will be system-dependent and unpredictable.

Best regards,
-- Al

Anything over 3 meters, but I would still keep the interconnects as short as possible

Mariv26: Anything over 3 meters, but I would still keep the interconnects as short as possible

I don't think you can make such a general recommendation. It really depends on the EMI environment. OTOH, if you believe that there are no potential disadvantages, only potential advantages, to balanced interconnects, you might want to use them regardless of length.

Kal