Decca cartridge balance output question

As the grounds are shared at the cartridge end, I don't believe it would work as intended.

I think noromance is correct. You probably cannot hook up a 3-pin Decca in balanced mode and still have stereo.  So you have his "I believe" and my "I think" to go on.  For balanced mode, you need two ground pins that are electrically isolated from one another.  Decca's use a common ground, for both channels.

Just email John Wright at Decca. He's really the best. 

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The Decca with the 3-pin output cannot be operated balanced.  The 4-pin versions can.