Audio Analogue BELLINI User manual

AUDIO FUTURA
AUDIO FUTURAAUDIO FUTURA
AUDIO FUTURA spa
Bellini preamplifier – Virtual Battery off line power supply 1 / 5
Issue Date: June 2008
AUDIO FUTURA spa
Research & Development
E-Mail (support):
sblan a@au ioanalogue.com
po esta@au ioanalogue.com
saggini@au ioanlogue.com
elia@au ioanalogue.com
Bellini preamplifier
Virtual Battery off line power supply

AUDIO FUTURA
AUDIO FUTURAAUDIO FUTURA
AUDIO FUTURA spa
Bellini preamplifier – Virtual Battery off line power supply 2 / 5
Intro uction
To achieve the best performance in a high en chain, firstable noise an interferences
must be re uce in all the parts an especially in the stages before power amplifier.
In fact power amplifier will a his high gain to everything it sees at his input, that’s
say of course the signal but noise an interferences too! We’ve istinguishe noise
an interferences because both work to re uce the performances but they are ue to
ifferent mechanisms an must be han le in ifferent ways.
Interferences are cause by external sources that inject some kin of unwonte signal
into the unit, while noise is intrinsically generate in every electronic circuit by a lot of
physical phenomenos (thermal noise, 1/f noise, …). So even if we coul completely
eliminate interferences what woul remain superimpose to the “goo signal” is noise.
To re uce noise we must choose appropriate components an appropriate
components values an work on the schematics to re uce the effects of the noise on
the signal.
To re uce the interferences we must firstable un erstan which are the sources of the
interferences, how they intro uce their unwante effects an at what frequencies are
generate .
Often the main interferences in an au io equipment are riven in by the main power
supply that intro uces interferences at both low an high frequencies. In fact the AC
power supply is a fun amental 50Hz (60Hz) signal with a lot of high frequencies
components. Nowa ays, the high frequency part may be very high ue to the
increasing number of electronic pro ucts using switching power supply an to the
increasing number of electronic pro ucts working at very high frequencies.
Even if high frequency interferences work well beyon the au io ban ,
intermo ulation effects can easily rive isturbances in the 20Hz-20KHz range.
Anyway the low frequency components of the AC line fall exactly in the au io ban
an they are the main responsible of the annoying “hum” in the lou speakers.
To re uce the “hum” problems the printe circuit boar must be well esigne an
the power supply of the circuit filtere an regulate appropriately. Anyway is very
har to completely eliminate the 50Hz/60Hz component. Perhaps that’s why the scale
of the frequencies axis in the au io measurements is often linear, in such way the low
frequency part is simply invisible on the graphic!!
To re uce the high frequency components, the filtering process usually suffice to
eliminate the problem on the circuit power supply but coupling capacitive effects an
irra iations, which effects epen on the frequency, can make the interference appear
where we on’t want!

AUDIO FUTURA
AUDIO FUTURAAUDIO FUTURA
AUDIO FUTURA spa
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, an this is what is one with a “AC filter”. But
the solution oesn’t help in any way to eliminate the low frequencies problems.
The perfect solution woul be to not connect the unit to the AC line an take the
supply from a battery, but this is impossible for power amplifier for evi ent reasons
an very not useful for a preamplifier or au io source too.
To solve the problem we have esigne 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 filtere just as it reaches
the boar to eliminate any high frequency isturbance. Then the AC voltage is
re uce by the transformer to be usable on the boar , rectifie by the bri ge an
then filtere . Then we foun two voltage regulators one for the positive voltage an
the other for the negative. The Area1 is “contaminate ” by the 50Hz (60Hz) presence.
What we have one in this part is trying to eliminate the interference as possible,
esigning two iscrete components voltage regulators (voltage regulators 1 an 2)
with very high 50Hz/60HZ noise rejection.
Until now nothing of new un er the sun, because these solutions are present with
better or worse results in every goo au io equipment, the virtual battery comes
beyon . What we want to o is to never connect in any way the preamplifier supply
stage (that supplies the preamplifiers stage, the input selection circuit an the volume
circuit) to the 50Hz/60Hz area.
This is implemente supplying the Voltage regulators 3 an 4 by a capacitor bank of
156000uF(!!!!) via soli state bipolar switches.
The circuit works as follows:
•at the turn on the control, that governs every switches an can sense the
values of the voltage at the points V1 an V2, opens the switches 5, 6, 7 an 8
leaving the Voltage regulators 3 an 4 (an so all the preamp circuits)
unpowere , while closing the switches 1, 2, 3, an 4. In such way after a elay
all the capacitors in the four banks are charge .
•Then the control open the switches 1 an 3 an closes the switches 5 an 7.So
the Area2 is powere by the banks B1 an B3 but is isconnecte from Area1.
The voltage regulators 3 an 4 works consuming the charge store in the
capacitors of the banks B1 an B3. So the voltage at point V1 an V2 ecrease.
•The control observes the voltage V1 an V2, an when they are too low the
control switches off the switches 5 an 7 isconnecting the banks B1 an B3
from the Area2, switches off the switches 2 an 4 isconnecting the banks B2
an B4 from the Area1 an then switches on the switches: 6, 8 (connecting
banks B2 an B4 to voltage regulators 3 an 4). Then the control close the
switches 1 an 3 connecting the banks B1 an B3 to the Voltage regulators 1
an 2 an charging again the capacitors of the two banks.

AUDIO FUTURA
AUDIO FUTURAAUDIO FUTURA
AUDIO FUTURA spa
Bellini preamplifier – Virtual Battery off line power supply 4 / 5
During the switching time the voltage at the input of Voltage regulator 1 an 2 is
supplie by the capacitor C1 an C2.
Of course the story goes on cycle by cycle setting the appropriate voltage at the input
of Voltage regulators 3 an 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 stoppe by the AC filter an also re uce by the virtual
battery.
In the measurements beyon we can see the noise spectrum measure at the RCA
output of the Bellini Virtual Battery Preamplifier with no signal applie an input 2
selecte . The scale of the frequency axis is logarithmic to evi ence the low
frequencies range.
FFT Spectrum Analysis at Volume 0

AUDIO FUTURA
AUDIO FUTURAAUDIO FUTURA
AUDIO FUTURA spa
Bellini preamplifier – Virtual Battery off line power supply 5 / 5
FFT Spectrum Analysis at Volume maximum
How you can see in the graphics the 50Hz component an the 50Hz harmonics have
been re uce to -120 B, probably that value is ue much more to the environment
influence on the cables use , a stan ar one, then to the preamplifier.