Owner`s manual

61
Unbalanced and Balanced Inputs and Outputs
An unbalanced signal, commonly used for guitars and consumer electronics,
contains two components, a ground signal and a “hot” or active signal. The
ground is the barrel of a ¼” connector and the shell of an “RCA” style
connector.
A balanced signal contains two active signals instead of one in addition to
the ground. These are referred to as the “plus” and “minus” signals. A
balanced input amplifier amplifies the difference between these two signals.
Any extraneous noise picked up from power lines or other sources will
appear equally on both the plus and minus inputs. This is called “common
mode” noise since it is common to both signals and the input amplifier will
subtract the noise on the minus input from the noise on the plus input.
If the input amplifier is perfectly balanced and the noise on both plus and
minus is precisely equal, the noise will completely cancel out. In the real
world this is not the case and some of the common mode noise will still
make it through, although at a much reduced level. How well an input
amplifier rejects this common mode noise is called the “common mode
rejection ratio” (abbreviated as CMRR) and is expressed in dB.
Balanced outputs typically drive the plus and minus components of a
balanced signal in one of two ways:
A “differentially” balanced output signal is one where the minus output is
the mirror image of the plus output. That is, if the “plus” output is at +1
volt, then the “minus” output is at –1 volt.
An “impedance” balanced output, which is used in Mia, is different in that
only the plus component actually carries the signal. The minus component
is tied to ground through a resistor so that the output impedance matches
that of the plus component. Since the impedances are matched, any noise
added to the minus component will still be approximately the same as that
picked up by the plus component and will be subtracted out by the input
amplifier.