A typical stereo cartridge has 4 pins, L and R, + and -. The signal appears "across" the +/- pins.
How is that not suitable for balanced operation?
If you ground the - (or the +) the signal appears on the other pin.
That can have the interpretation of unbalanced.
Take your pick, for whether you have a differential or single-ended input to your phono preamp.
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An MC cartridge is inherently a balanced device, as the impedance to ground is pretty much equal on both wires of each channel, there may be a tiny capacitative and inductive difference due to the routing of the wires from the coils, but this is small.
However, a MM cartridge isn't balanced because the coils are large, and the outside of the coil, nearest the screening can, has a different impedance to the inner part which is sheiled from the can by the bulk of the coils. Furthermore, in many cartridges, the screening can is connected to the -ve pin of one channel, usually the left, so inherently unbalancing the cartridge.
Nevertheless, it's a continuing mystery to me why in home HiFi, MC phono stages which are the only part of a system which would benefit from a balanced input, very seldom is.
S.