The proper termination of an XLR connector for a tonearm cable depends on the particular type of cable, and how it's wired at the tonearm/cartridge end. Ignoring rare exceptions like old three-wire tonearms and single-DIN terminations found on B&O turntables, there are two types.
Traditional phono output cable is coaxial, that is, a single center conductor surrounded by a shield. At the tonearm end, the cable is connected in a balanced fashion:
- Cartridge red to Right channel center conductor
- Cartridge green to Right channel shield
- Cartridge white to Left channel center conductor
- Cartridge blue to Left channel shield
- Ground wire to tonearm, headshell, and turntable ground
Note that there are no connections between the shields and the turntable or tonearm ground. For (AES-compliant) dual-XLR termination, the shields go to pin 3, the center conductors go to pin 2. The ground wire may be connected to pin 1 of one of the connectors (*shouldn't* matter which channel) or to the ground post on the phono preamp.
The other common type of phono output cable is shielded twisted-pair, where a pair of conductors are surrounded by a shield. In this case, the tonearm end is wired as follows:
- Cartridge red to Right twisted-pair positive
- Cartridge green to Right twisted-pair negative
- Cartridge white to Left twisted-pair positive
- Cartridge blue to Left twisted-pair negative
- Cable shields both connected to tonearm, headshell, and turntable ground
In this case, the twisted-pair positives go to pin 2 of the XLRs, the twisted-pair negatives go to pin 3, and the shield goes to pin 1 (this is identical to pro microphone termination). No extra ground wire is necessary here, as this function is taken by the cable shield.
As far as which type of cable is better . . . of course, it all depends. Speaking generally, shielded-twisted-pair cables have superior hum rejection, and coaxial cables have lower capacitance. For XLR terminations, shielded-twisted-pair is best for low-impedance, low-output MC cartridges, but high-inductance MMs may work better with the lowest possible capacitance, which usually means a coaxial design. This is especially true with RCA terminations.
Complicating the matter a bit is the fact that phono stages with an XLR input tend to have widely varying designs for the input stage, and some may work poorly when the cable impedance isn't well balanced between the positive and negative conductors, with respect to ground . . . as occurs when a coaxial-type cable is terminated with an XLRs. Ironically, designs that have this issue will exhibit it at the highest input impedances, which are used by MM cartridges . . . which have the most to gain with a low-capacitance (coaxial-type) cable.