AUDIO FUTURA
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Bellini preamplifier – Virtual Battery off line power supply
3 / 5
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.
