Can I ask how a cartridge output is balanced? I thought balanced was where a 'negative' signal was produced and sent with the original signal then a circuit used both of these to identify the interferance picked up by the cable and subtracted this from one of the signals. It uses summation of the signals, I think, to void the added RFI. I had this explained clearly to me once but unfortunately I cannot remmember it fully.
A balanced input amplifies, or detects, only the difference between the two signals, usually named 'in-phase' and 'return'.
Any interference - unless of extremely high frequency - will affect both conductors and inputs equally, and so be rejected by the input circuit.
The ability of the balanced input to do this is called its common-mode-rejection-ratio (CMRR), the bettter the system, the higher the number. Any manufacturer that doesn't quote a CMRR is probably a little embarassed by the performance in this area.
Balanced lines are very commonly used on mobile/DJ/PA equipment for another reason - to eliminate ground-loop problems. That is the main reason why such equipment has XLR connectors. A relatively poor noise rejection figure is achieved, but that is not an issue for them, as long as they have eliminated the potential for ground loops. (1)
Balanced cables usually have an outer braid to add extra screening. An almost-as-good solution can be achieved by twisting the two wires together very tightly, as is done on telephone connections and ethernet computer networks (ethernet uses USTP cable = unshielded twisted pair)
The balanced line receiving can be done by means of a transformer (centre tapped or not, as described later). Or it can be done by means of a differntial amplifier circuit. These usually need to be more complex than a single op-amp, because all practical op-amps present a different load impedance on their '+' and '-' inputs, which cannot be 'fixed' purely by feedback resistors. This impedance disparity completely defeats the purpose of a balanced system. Some form of buffer circuit must preceed the differencing op-amp, and the whole configuration of 3 op-amps is then called an 'instrumentation amplifier'.
It is convenient to *imagine* the cartridge winding as centre-tapped, this being a ground reference. Each half winding then generates an equal, but opposite phase signal with respect to this ground reference.
However, if the various currents are measured - or computed theoretically, you find that there is zero current flowing in the ground reference - no current flows into or out of the 'centre tap' of the cartridge. If there is no current flow, you can sever the connection with no effect.
You can infer from this that the halfway point of the cartridge winding - when driving a balanced load - always remains at ground potential.
Anyone familiar with 3-phase power distribution will know you can send three phase power over 3 wires - no neutral conductor is required - if the load is balanced. The reasons and principle are exactly the same for a cartridge.
(1) a ground loop (often called a hum loop since it mostly injects 50- or 60Hz hum into the system) is formed when two connected pieces of equipment share more than one ground connection; either their cabinets are touching, or they share a mains earth through the building wiring, or they just have two screened audio cables. The two ground connections then form a complete loop, which acts as a single-turn transformer, susceptible to any magnetic fields, especially those of the transformers in the equipment.