Phono cartridges are floating sources rather than balanced, and unless there is a low-impedance connection between phono amplifier ground and both sides of the cartridge signal coil, and/or the common mode rejection of the phono stage input stage is extremely high across a wide bandwidth, there most likely will be substantial pickup of electrical noise from the environment (i.e., connecting the phono cartridge in balanced mode will probably yield worse noise performance than an unbalanced connection).
An input transformer magnetically isolates common mode noise, and is a good solution to building a quiet balanced phono stage.
An input transformer magnetically isolates common mode noise, and is a good solution to building a quiet balanced phono stage.