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manual abstract
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! Bellini preamplifier – Virtual Battery off line power supply 2/5 A AAAAU UUUUD DDDDI IIIIO OOOOF FFFFU UUUUT TTTTU UUUUR RRRRA AAAAspa Virtual battery Obviously 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 a “AC filter”. But the solution 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 very not useful for a preamplifier or audio source too. To solve the problem we have designed a virtual battery power supply. That’s say a circuit that isolates completely the preamplifier power supply from the AC main. How can you see in the schematic (Fig. 1), the AC main is filtered just as it reaches the board to eliminate any high frequency disturbance. Then the AC voltage is reduced by the transformer to be usable 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 possible, designing two discrete components voltage regulators (voltage regulators 1 and 2) with very high 50Hz/60HZ noise rejection. Until now nothing of new under the sun, because these solutions are present with better or worse results in every good audio equipment, the virtual battery comes beyond. What we want to do is to never connect in any way the preamplifier supply stage (that supplies the preamplifiers stage, 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 156000uF(!!!!) via solid state bipolar switches. The circuit works 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. In such way after a delay all the capacitors in the four banks are charged. • Then the control open the switches 1 and 3 and closes the switches 5 and 7.So the Area2 is powered by the banks B1 and B3 but is disconnected from Area1. The voltage regulators 3 and 4 works consuming the charge stored in the capacitors of the banks B1 and B3. So the voltage at point V1 and V2 decrease. • The control observes the voltage V1 and V2, and when they are too low the control switches off the switches 5 and 7 disconnecting the banks B1 and B3 from the Area2, switches off the switches 2 and 4 disconnecting the banks B2 and B4 from the Area1 and then switches on the switches: 6, 8 (connecting banks B2 and B4 to voltage regulators 3 and 4). Then the control close the switches 1 and 3 connecting the banks B1 and B3 to the Voltage regulators 1 and 2 and charging again the capacitors of the two banks. Bellini preamplifier – Virtual Battery off line power supply 3/5 A AAAAU UUUUD DDDDI IIIIO OOOOF FFFFU UUUUT TTTTU UUUUR RRRRA AAAAspa During the switching time the voltage at the input of Voltage regulator 1 and 2 is supplied by the capacitor C1 and C2. Of course the story goes on cycle by cycle setting the appropriate voltage at the input of Voltage regulators 3 and 4. Fig. 1 -Schematic The virtual battery works supplying all the preamplifiers circuits separately from the 50Hz/60Hz area, eliminating in such way the 50Hz/60Hz interference. The high frequency interferences are stopped by the AC filter and also reduced by the virtual battery. In the measurements beyond we can see the noise spectrum measured at the RCA output of the Bellini Virtual Battery Preamplifier with no signal applied and input 2 selected. The scale of the frequency axis is logarithmic to evidence the low frequencies range. FFT ...