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Bellini preamplifier – Virtual Battery off line power supply 2 / 5
Introduction
To achieve the best performance in a high end chain, first noise and interferences
must be reduced in all the parts and especially in the stages before power amplifier.
In fact power amplifier will add his high gain to everything it sees at its input, that’s
say of course the signal but noise and interferences too! We’ve spoken of noise and
interferences because both work to reduce the performances but they are due to
different mechanisms.
Interferences are caused by external sources that inject some kind of unwonted signal
into the unit, while noise is intrinsically generated in every electronic circuit by a lot of
physical phenomenons (thermal noise, 1/f noise…). So even if we could completely
eliminate interferences what would remain superimposed to the “good signal” is noise.
It’s very important to differentiate noise and interferences because the treatments
are different. To reduce noise we must choose appropriate components and
appropriate components values and work on the schematics to reduce the effects of
the noise on the signal. To reduce the interferences we must first understand which
are the sources of the interferences, how they introduce their unwanted effects in our
units and at what frequencies are generated.
Often the main interferences in an audio equipment are driven in by the main power
supply that introduces interferences at both low and high frequencies. In fact the AC
power supply is a fundamental 50Hz (60Hz) signal with a lot of high frequencies
components. Nowadays, the high frequency components may be very high due to the
increasing number of electronic products using switching power supply connected to
the AC line and to the increasing number of electronic products working at very high
frequencies. Even if high frequency interferences work well beyond the audio band,
inter-modulation effects can easily drive disturbs in the 20Hz-20kHz range. Anyway
the low frequency components of the AC line fall exactly in the audio band! In fact
they are the main responsible of the annoying “hum” in the loudspeakers.
To reduce the “hum” problems the Printed circuit board must be well designed and the
power supply of the circuit filtered and regulated appropriately. Anyway is very hard
to completely eliminate the 50Hz/60Hz component. Perhaps that’s why the scale of
the frequencies axis in the audio measurements is often linear, in such way the low
frequency part is simply invisible on the graphic!!
To reduce the high frequency components, the filtering process usually suffice to
eliminate the problems on the circuit power supply but coupling capacitive effects and
irradiations, which effects depend on the frequency can make the interference appear
where we don’t want!