Interfacing and Processing 2578.3.4.4 Relativity Rules
The size of common mode (noise) signals is not fi xed or even very predictable; they may
range from microvolts to tens of volts. CMR is just a layer of protection. Forty dB of
protection is not much against 10 V of CMR, but it is defi nitely enough for 1 μ V.
8.3.4.5 Sonic Effects of RF
Radio frequency interference is a common mode noise, and sources of RF go on increasing.
In a competently wired system in premises away from radio transmitters and urban/
industrial electrical hash, a modest rejection no better than 40 dB has previously seemed
good enough to make inaudible induced 50/60-Hz hum and harmonics, and the “ glazey ”
sound of RFI and RF intermodulation artifacts. Unfortunately, RFI artifacts aren’t always
blatant, and when any sound system is in use, they’re the last thing that users are likely to
be listening for the symptoms of. However, even if there are no blatant noises, inadequate
CMR can allow ambient electrical hash to cover up ambient and reverberative detail.
8.3.4.6 System Reality
The CMRs discussed are those cited for power amplifi er input stages. The actual system
CMR is inevitably cumulatively degraded by the cabling and the source CMRs. However,
it can be maintained by ensuring all three have individually high CMRs and have highly
balanced leg impedances. Lines driven from unbalanced sources give numerically
inferior results, but often quite adequate ones (subject to appropriate grounding and
cable connections) in low-EMI domestic hi-fi and studio conditions, where equipment
connections are also compact, and even in outdoor PA systems, in an open countryside.
8.3.4.7 Summary
Generally, 20 dB is a low, poor CMR, 40 to 70 dB is average to good, and 80 to 120 dB or
more is very good and far harder to achieve in a real system. In a world where some audio
measurements have had their credibility undermined, it’s reassuring to know that with
CMR, more dBs remain simply better.
8.4 Subsonic Protection and High-Pass Filtering ........................................................
8.4.1 Rationale
All loudspeakers have a low-end limit; their bass response does not go endlessly deeper.