Dersom årsaken til dårlig lyd ikke skyldes spenningskvaliteten,
http://www.hifisentralen.no/forumet...72643-sentralnettet-og-spenningskvalitet.html
http://www.jensen-transformers.com/an/generic seminar.pdf
Treating the power
line to get rid of noise is like using a shotgun instead of a silver bullet to target the
problem!
5.4 - POWER ISOLATION, FILTERS, AND “BALANCED POWER”
Because system noises are most frequently coupled from the power line, solutions which
somehow “cleanse” or “purify” it have great intuitive appeal. However, in this author’s experience,
such treatments usually produce marginal improvements, if any. Generally, the most cost-effective
solution is to identify and eliminate the ground loops or other problems which allow the noise to
couple into signal paths. This approach solves the real problem. Treating the power
line to get rid of noise is like using a shotgun instead of a silver bullet to target the
problem!
First, when any line filter, conditioner, or
isolation transformer is used, Code requires
that the device as well as its load still be
connected to safety ground as shown.
Because transformer winding capacitances
and/or filter capacitors divert additional 60 Hz
and high-frequency noise currents into the
safety ground system, they frequently
aggravate the problem they claim to solve.
Second, the touted noise attenuation figures
for virtually all these power line devices are
very unrealistic. Measurements are made with all the equipment (generator, detector, and device
under test) mounted on a large metal ground plane. Although the resulting specs are impressive,
they simply don’t apply to performance in a real-world situation where grounding is via safety
ground wires or conduit. But such devices can be very effective when installed at the power
service entrance, where all system safety grounds are physically very close to a common
reference point (usually the ground rod).
“Balanced power” or, more properly, symmetrical ac power is a seductively appealing concept.
However, its proponents often mistakenly assume that equipment has precisely matched
capacitances from each leg of the power line to chassis (C1 and C2 or C3 and C4) when they
explain how it itcancelsly ground noise. Of course, if this were true, capacitive noise currents from
each 60-volt leg would then be of equal magnitude and opposite polarity, and completely cancel.
But the assumption is not valid for the overwhelming majority of real-world equipment, whose
capacitance ratios are often 3:1 or 4:1. Even balanced power proponents admit that actual noise
reduction is usually under 10 dB and rarely exceeds 15 dB (recall that 10 dB noise reductions
are generally described as “half as loud” by listeners). And it’s not likely that equipment
manufacturers will ever adopt expensive power transformers with capacitively “balanced” primary
windings or RFI filters that use precision capacitors.
But balanced power can make cost-effective sense in some systems. One example might be a
video duplication facility having 50 unbalanced (coax) video interconnects and hum bars at an
unacceptable level of !30 dB. Here, the 10 dB improvement likely from balanced power (or
“technical grounding” for that matter) might reduce the hum bars to a more acceptable level of
!40 dB, effectively solving the problem. But, for audio, a 10 dB improvement will rarely make the
difference between unacceptable and acceptable performance!
In reality, many of the benefits often ascribed to “power treatment” schemes are simply
due to plugging all system equipment into the same outlet strip or dedicated branch
circuit. For obvious reasons, this is always a good idea!
A simple outlet wiring error that swaps the neutral and safety ground conductors allows load
current to flow in the safety ground wiring. The abnormally high voltage drops created can cause
severe ground noise in systems using safety-grounded equipment. This error cannot be found
with simple outlet
testers, but measuring
current at points A, B,
and C with a clamp-on
ammeter will clearly
reveal it. [30]
