A ground loop is the result of multiple paths to ground that are of different electrical potential. That can happen whether the circuits are balanced, or not. If the phono cartridge ground is of different potential than the phono preamp ground, you’ll have a ground loop.
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So, my statement was "Balanced and isolated"The phono cartridge is only "balanced and isolated" if it's fed into a balanced phono preamp. But very few phono preamps are balanced. (Of those that are, not all meet the AES balanced spec, an issue Ralph frequently points out here.)
Once a phono cartridge is connected to an unbalanced input, the cartridge is no longer "balanced and isolated."
If the phono cartridge - which as you note in inherently balanced - is connected to a truly balanced phono input, there should be no ground loop or hum problem. If there is, it suggests an issue with the cartridge itself or the pickup arm wiring that results in unbalancing the phono circuit.
OK ... help me out a little here. But even if it gets grounded at the phono input, that is a single ground reference. Where's' the competing ground that the phono catridge attaches to?
I'm not trying to be difficult here... I'm just genuinely not seeing the whole picture.
Of course, YES, phono cartridges are notoriously susceptible to ground loops. My own mental model just doesn't explain it yet.