Audio Analogue SRL Bellini preamplifier manual Introduction

AUDIO FUTURA s.r.l.

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!

Bellini preamplifier – Virtual Battery off line pow er supply 2 / 5