Audio Engineering

256 Chapter 8

8.3.4.3 What CMR Cannot Do

Like the stable door, the one thing CMR can’t do is remove unwanted noises that are
already embedded in with the music. It follows that just one piece of equipment with poor
CMR, and in the wrong place, can determine the hum and RFI level in a complex studio
or PA path.

The ingress of common mode noise, called mode conversion , is cumulative, as each
unit in the chain lets some of it leak through. As a result, the CMR performance and/or
interconnection standards of all the equipment in complex systems (e.g., multiroom
studios and major live sets) must be doubly good.

The higher CMR of well-engineered equipment (80 dB or more) provides a safety factor
of 100- to over 1000-fold over the minimum 40 dB that is common in more “ cheerful ”
products. However, the higher CMRs are more likely to vary with temperature and aging,
as with all fi nely tuned artifacts.









(Any) source equipment Balanced output

Vcm1

(Any) destination equipment

Balanced receiver

(field)

Unbalanced

output Ideally

NOT

connected

to source(s)

Only the

DIFFERENCE

between

these 2

terminals

Ov (signal ground) should be reproduced

Groundlift

resistance

Groundlift

switch

Groundlift

switch

Ov (signal ground)

Stray

capacitance Straycapacitance

Equipment

casing

Equipment

casing

Groundlift

resistance

Mains earth wiring

Mains earth wiring

Vcm2

(Mesh conducted)

Superimposed noise currents

in/along mains earth conductors

Distributed resistance and inductance

21

3

Figure 8.5 : Most of the common mode noise that CMR defends against is either RF and

50/60 Hz fundamental intercepted in cabling (Vcml) or 50/60 Hz hum  harmonics caused

by magnetic loop, eddy, and leakage currents fl owing in the safety ground wiring between

any two equipment locations (Vcm2).