balanced pre amp

Was reading a review of the Marsh 2000 balaned pre amp. What does "balanced" mean? What does it add. Are all pre amps balanced. Does this change the interconnectors you have to use.
Balanced refers to a different circuit design. Others will fill in the details on that. All pre-amps are NOT balanced. Balanced circuits require cables with XLR connectors. One known advantage is that balanced i/cs are less susceptible to noise on long runs. Some amps require and/or sound much better when run balanced. Some items wiht XLR inputs or outputs are not "Truly balanced" but actually function as a built-in adaptor. Stand alone adapters are available.
A true balanced design is a differential design, with both positive and negative legs going through mirror image gain stages, output stages, etc. To do balanced right is expensive, because you require duplicate circuits. A regular preamplifier has an input, buffer, gain and output stages, but only for the positive leg of the input. The balanced preamp will have identical circuits for both the positive and negative legs.
As stated previously, it is more expensive because of the additional circuitry. The benefits are basically a blacker background, but are really only noticeable when running fully balanced, from source through amplifier. If either the source, preamp or amp are not fully differential designs, then you will not be listening to 'balanced' output.

John not in all instances.I have a McIntosh C200 preamp that will convert an unbalanced signal to a balanced signal according to the owners manual.
Yes the C200 will convert an unbalanced input signal to a blanced one. I beleave it splits the signal so that half the wave form goes to one set of circuits and the other half goes to the other.

If at some point in the chain it is unbalanced then the signal is murged into a single wave form.

If you want a fully balanced system you would want the pre-amp and amp and even the source to have the same circuit design to fully utilize it.

I hear a big difference using a balanced signal directly from my fully balanced McIntoh MDA1000 to my fully balanced Cary CAD-211's
When I use the unbalanced cables between the two it is does not dance and sing the same way as it does with the balanced cables.
Phono cartridges are a naturally balanced source too. It really seems to help running them balanced!