CMRR — Common Mode Rejection Ratio

Common Mode Rejection Ratio

For full balanced vs. unbalanced audio coverage, see fundamentals/balanced-vs-unbalanced-audio. For ground loop hum and noise, see maintenance/av-troubleshooting-methodology.

CMRR (Common Mode Rejection Ratio) is the specification that measures how well a balanced audio input rejects noise that is induced equally on both of its signal conductors (the "common mode" signal). Electrical interference from power lines, fluorescent lights, RF sources, and ground loops induces noise into audio cables. In a balanced system, this noise appears on both the positive (+) and negative (−) conductors simultaneously (common mode), while the audio signal appears as a difference between them (differential mode). A differential amplifier at the input subtracts the two conductors — canceling the common-mode noise while preserving the differential audio signal. CMRR quantifies how completely this cancellation occurs.

CMRR Formula and Specification

CMRR (dB) = 20 × log₁₀ (Differential Gain / Common-Mode Gain)

A CMRR of 60 dB means common-mode signals are rejected by 60 dB — a 1V induced noise signal produces only 1 mV at the output (1000× rejection). A CMRR of 80 dB produces only 0.1 mV rejection.

Typical values:

• 60 dB CMRR — minimum acceptable for professional audio
• 80 dB CMRR — good; many professional microphone preamps and DSP inputs
• 90–100 dB CMRR — excellent; high-quality input transformers and precision differential amplifiers
• Consumer audio — typically 40–50 dB; much lower noise rejection

Why CMRR Matters in AV

Long cable runs: a 50 m balanced microphone cable run through a cable tray alongside power cables induces 50–60 Hz electrical noise from the power cables. With CMRR of 80 dB, that induced noise is 10,000× lower at the DSP input — inaudible. The same signal on an unbalanced cable would appear at full amplitude as audible hum.

Ground loops: when two devices in a balanced system have a ground potential difference (different circuit grounds), 60 Hz current flows through the cable shield, inducing noise. The balanced differential input rejects this as common-mode noise. See the ground loop section in maintenance/av-troubleshooting-methodology.

RF interference: high-frequency RF (from wireless systems, mobile phones, switching power supplies) induced on balanced cables is similarly rejected by high CMRR inputs.

CMRR Limitations

CMRR degrades with frequency — most differential amplifiers have excellent CMRR at 60 Hz but lower CMRR at 10 kHz and above. Transformer-balanced inputs often maintain good CMRR at high frequencies better than electronic balanced inputs. For high-frequency RF rejection, transformer input stages outperform electronic balanced inputs.

CMRR also requires matched impedances on both conductors. If one conductor has a different impedance to ground (e.g., one side of the shield is connected and the other is not, or there is an impedance imbalance in the cable), CMRR degrades significantly. This is why:

• Pin 1 (shield) should be connected at only one end to avoid ground loops while maintaining shield continuity
• Balanced cables must be wired symmetrically (both conductors same length, same wire gauge, properly shielded)

Transformer vs. Electronic Balanced

Transformer balanced (Jensen, Lundahl, Output transformers): provides galvanic isolation — no electrical connection between source and destination grounds. Extremely high CMRR (80–120 dB), excellent RF rejection, and complete ground loop elimination. Used in Jensen ISO-MAX transformers for ground loop breaking, in professional studio equipment, and in mic preamplifiers. Adds slight low-frequency rolloff and phase shift if not properly designed.

Electronic balanced (differential amplifier input, INA217, etc.): lower cost, very flat frequency response, good CMRR (60–90 dB). Provides no galvanic isolation — ground connections are still shared. Adequate for most AV applications; insufficient for situations requiring complete electrical isolation (medical environments, very long runs with severe ground loop issues).

Common Pitfalls

• Connecting a balanced output to an unbalanced input without a proper interface. Connecting pin 2 (hot) and pin 3 (cold) of a balanced XLR to a single-ended (unbalanced) input while grounding pin 3 causes the two halves of the balanced output to work against each other, potentially damaging the output stage. Fix: use a proper balancing/unbalancing adapter or DI box; never simply ground pin 3 without verifying the output's design.

• Degraded CMRR from shield connected at both ends. Connecting the cable shield (pin 1) at both the transmitter and receiver creates a ground loop via the shield — the very problem CMRR is supposed to solve. Fix: connect pin 1 at the destination end only (break ground at the source end); some professional equipment uses a "pin 1 lifted" design.

• Assuming balanced wiring solves all noise problems. Balanced wiring with high CMRR rejects common-mode noise effectively. If the noise is differential (appears as a difference between the two conductors), CMRR cannot reject it. Differential noise sources (pickup from a nearby transformer's stray field on adjacent wires of different lengths) require shielding and physical separation from the interference source, not just balanced wiring.