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manual abstract
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 power supply 2/5 A AAAAU UUUUD DDDDI IIIIO OOOOF FFFFU UUUUT TTTTU UUUUR RRRRA AAAAs.r.l. Virtual battery The target is to not have power supply interferences at all. The first step is to clean out the AC line, and this is what is done with an “AC filter”. But the solution is always expensive and cumbersome if we use a filter for all the audio chain and anyway doesn’t help in any way to eliminate the low frequencies problems. The perfect solution would be to not connect the unit to the AC line and take the supply from a battery, but this is impossible for power amplifier for evident reasons and not very useful for a preamplifier or audio source too. To solve the problem we have designed a virtual battery power supply. That’s said a circuit that insulate completely the preamplifier power supply from the AC main. How can you see in the schematic, the AC main is filtered just as it reaches the board to eliminate any high frequency disturbs. Then the AC voltage is reduced by the transformer to be used on the board, rectified by the bridge and then filtered. Then we found two voltage regulators one for the positive voltage and the other for the negative. The Area1 is “contaminated” by the 50Hz (60Hz) presence. What we have done in this part is trying to eliminate the interference as more as possible, designing two discrete components voltage regulators (voltage regulators 1 and 2) with very high 50Hz/60HZ noise rejection. Until now nothing new under the sun, because these solutions are present with better or worse results in every good preamplifier, the virtual battery comes beyond. What we want to do is to never connect in any way the preamplifier power supply stage (that supplies the preamplifiers, the input selection circuit and the volume circuit) to the 50Hz/60Hz area. This is implemented supplying the Voltage regulators 3 and 4 by a capacitor bank of 156000mF(!!!!). The circuit worsk as follows: • at the turn on the control, that governs every switches and can sense the values of the voltage at the points V1 and V2, opens the switches 5, 6, 7 and 8 leaving the Voltage regulators 3 and 4 (and so all the preamp circuits) unpowered, while closing the switches 1, 2, 3, and 4...