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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 signals connect with either XLR connectors or TRS (tip, ring, sleeve)
connectors. Your Echo card uses TRS connectors for connecting balanced line
level signals.
The three sections of a TRS connector are used to transmit the three components of
a balanced signal (T = plus, R = minus, S = ground).