AUDIO FUTURA
AUDIO FUTURA
AUDIO FUTURA
AUDIO FUTURA
spa
Bellini preamplifier – Virtual Battery off line power supply
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Introduction
To achieve the best performance in a high end chain, firstable 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 his input, that’s
say of course the signal but noise and interferences too! We’ve distinguished noise
and interferences because both work to reduce the performances but they are due to
different mechanisms and must be handled in different ways.
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 phenomenos (thermal noise, 1/f noise, …). So even if we could completely
eliminate interferences what would remain superimposed to the “good signal” is noise.
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 firstable understand which are the sources of the
interferences, how they introduce their unwanted effects 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 part may be very high due to the
increasing number of electronic products using switching power supply and to the
increasing number of electronic products working at very high frequencies.
Even if high frequency interferences work well beyond the audio band,
intermodulation effects can easily drive disturbances in the 20Hz-20KHz range.
Anyway the low frequency components of the AC line fall exactly in the audio band
and 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 problem 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!
