Electro voice sb120a actspk

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EV Electroil/oice®
® Sb120a/CE Amplifier Service Notes

SERVICE MANUAL

SB120AICE AMPLIFIER

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SB120AICE AMPLIFIER

GE CERTIFIED 53'! 20A SERVICE NOTES

The EV Sb120alCE amplifier consists of two PCB
assemblies; a switching power supply, and a switch-
ing (Class D amplifier). All of the power supply
circuitry on the primary side of the power trans-
former is not isolated from the AC mains, thus an
isolation transformer is HIGHLY RECOMMENDED
when servicing this assembly. The 3:80VDC power
supply output is isolated, so the amplifiers ground
is Earth Ground when used with the power supply.
The amplifiers back plate and output return lug are
also earth ground connections.

Switching Power Supply Description:

The power supply schematic indicates two ground
symbols: (live mains) and (earth). If an isolation
transformer is used, the oscilloscope probe grounds
may be connected to either of these nodes, but not
at the same time as these two grounds are sepa-
rated by high voltage. Oscilloscope measurements
can be meaningless oreven dangerous ifthe scopes
ground is connected to anything other than earth
and an isolation transformer is not used.

The power supply topology is a special type which
corrects the Power Factor of the mains while at the
same time providing an isolated and loosely regu-
lated 18OVDC to the amplifier. The topology is a
unique combination of a boost topology com-
monly used for as a power factor corrected pre-
regulator and a push-pull forward converter
topology. This hybrid topology has the advantage
of supplying an isolated and regulated high power
:BOVDC with a power factor corrected input, but
requiring far fewer components than a traditional
switch-mode solution providing both these
features. As shown in figure 1, the mains voltage is

rectified, then fed through an energy storage power
inductor (L2, used as a boost inductor). The output
of the inductor is fed to the center tap of a push-pull
power transformer. In a normal push-pull converter
the transistors, Qi and 02 are switched on and off
with a specific dead time, feeding the transformer
with a high frequency alternating voltage.

in the push-pull boost topology, the transistors are
never both off at the same time. When operating is
the boost state, the conducting time of the transis-
tors is overlapping and thus shorting the transformer
primary. This is the same condition as when a
standard boost converter has its single transistor on
and is charging the boost inductor, L2.

The switching durations and overlap times (deter-
mined by the control lC, U2) serve to correct the
power factor and regulate the output voltage. in this
case 'power factor does not referto a phase shift of
mains voltage and current, but to the high harmonic
content present in the input currentwaveform caused
by the large electrolytic bulkinput capacitors. (These
capacitors and poor power factor are also present in
amplifiers with traditional linear power supplies).
The Sb120alCE power supply topology serves to
control how the mains input current charges the bulk
capacitors, C3 and C4. it's goal is to make the input
currents shape look exactly the same as the input
voltages shape - sinusoidal. Figure 2 shows two
mains current waveforms for equivalent power de-
livery, one with poor PF, the other with corrected PF.

The secondary voltage ofthetransformer is rectified
and directly fed to the bulk storage capacitors, which
charge to the peak secondary voltage, unlike the
traditional push-pull converter which would need
two large inductors in the output filter.

Figure 1 - Block diagram of Boost Push-Pull Forward Converter with Active Power Factor Correction

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