Pro-Music-News Know-how

CMR(R) - Common Mode Rejection (Ratio)

The Common Mode Rejection Ratio (CMRR) is a technical term related to differantial inputs of amplifiers. Balanced (=differantial) inputs of amplifiers have a positive (in-phase) and an inverting (or negative /out-of-phase) input leg. Ground is not connected to either of the input legs. Ground is separately connected to the shield of the connecting cable/socket/plug.
If an audio signal transmiited from a balanced output to a balanced input picks up noise it will be picked equally by both the 'positive' and the 'negative' line. Eventually reaching the amplifier's input the 'positive' noise will be added to the 'negative' noise and eliminated.
As the noise is common to both input of the differential amplifier this is called a common mode. Since the common mode cancels itself, it is rejected. A differential input's ability to eliminate a signal sent to both inputs therefore is called Common Mode Rejection Ratio. The CMR (Common Mode Rejection) is measured in dB and is 20 times the common logarithm of the CMMR.
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