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The reason Grado rejected that is because

All magnetic and MC cartridges are already balanced sources. They don't need 6 connections!

If a center tap were added, it would reduce the CMRR (Common Mode Rejection Ratio) of the differential amplifier that the cartridge drives.

We sorted out all this about 30 years ago so when we introduced our MP-1 preamp; it was the first balanced line preamp that could properly accept the phono signal in the balanced (and differential) domain.

That ground wire that all single-ended phono systems use? That's the shield connection that ties to pin 1 of the XLRs of both channels. That wire (which no other single-ended system seems to need) is one way you can tell that you are working with a balanced source while using it single-ended. You have to do something with the ground; like any true balanced source, the ground is ignored and is only used for shielding.

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