QSC PowerLight Series, Technical & Service Manual

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Technical Service Manual 7
PowerLight 6.0 II, PowerLight 6.0 PFC , and PowerLight 9.0 PFC
1.5 Technical descriptions and
theory of operation
Power supplies
QSC PowerLight amplifiers feature high-frequency switch-mode
power supplies that help reduce noise, increase electrical
efficiency, and lower weight. Two models, the PowerLight 9.0
PFC
and the PowerLight 6.0 PFC , had power factor correction to reduce
the peak current demand from
the AC mains. They accom-
plished this by drawing current
throughout the AC voltage
waveform, instead of just at
the peaks, as most amplifiers
and other electronic equipment
do. The PowerLight 6.0 II was
developed later, and its power
supplies do not have power
factor correction.
All three models have a four-
tier class H system of multiple
rail voltages to boost efficiency.
A power amplifier is most efficient at or near full power, yet the
dynamic nature of music and other typical audio requires much less
than full power most of the time. Thus, this class H scheme creates
essentially four different “full-power” levels within the amplifier
channel. The amplifier circuitry
automatically and instanta-
neously switches to the lowest
rail voltage that will allow the
reproduction of the audio
signal without discontinuity.
Each amplifier channel has its
own power supply. In addition,
each has a small “housekeep-
ing” supply that manages the
turn-on functions before the
main power supply starts up.
Audio circuitry
The audio inputs are balanced to offer a high amount of common-
mode noise rejection. The input balancing is done using an
instrumentation amplifier arrangement, which uses a single op
amp, arranged as a voltage follower or buffer, on each leg of the
balanced input, driving a single op amp differential amplifier. The
degree of common-mode rejection is dependent on the close
matching of the impedance between each leg and ground and
around the differential amplifier. The circuitry uses 1% precision
resistors to ensure at least 40 dB of common-mode rejection.
The differential amplifier circuitry includes a first-order high-
frequency roll-off, down 3 dB at 280 kHz (nearly four octaves above
the high end of the audio spectrum). This makes the amplifier less
susceptible to RF interference,
high-frequency oscillations, etc.
The audio signal passes through
a pre-clipper, which prevents
the audio signal from driving
the output section itself into
actual clipping. This maintains
damping on the channel output
even during clipping so that it
continues to tightly control the
loudspeaker motion, which is
something most amplifiers
cannot do. A defeatable clip
limiter on each channel reduces
signal level when clipping occurs; it does not prevent clipping, but
reduces the amount of distortion to inaudible or barely audible
levels.
An all-pass filter uses group delay to slow the audio signal by 4 µs,
but the class H steps are controlled by the undelayed signal. This
reduces IM distortion by ensuring that the steps are executed
before the audio signal in the output section reaches the transition
thresholds.
The audio signal voltage is converted into current by transistors
Q87 and Q89, to be precisely bifurcated into positive and negative
halves by the current steering circuitry. These current signals are
the controls for the output devices.
The output devices are vertical
MOSFETs, which are commonly
used for very high power
switching because of their
power handling capability and
general nonlinearity. Using
them for linear audio amplifica-
tion requires an unorthodox
approach. In these three
PowerLight amplifier models,
each channel has eight MOSFET
devices arranged in a full bridge
configuration. Each one has a
local management circuit called
a current cell that controls and
linearizes the device by
providing the necessary
compensation to make the
MOSFET’s conductivity track the
signal current.
Figure 1.7. Amplifiers without
PFC draw current only at the
peaks of the AC voltage
waveform.
Figure 1.8. An amplifier with PFC
draws current throughout the
AC voltage waveform.
Figure 1.10. Most amps lose
feedback during clipping,
resulting in loss of damping and
in “clip sticking.”
Figure 1.9. The rail voltages of
the output section switch
among four tiers to reproduce
the signal faithfully while
maximizing efficiency.
Figure 1.11. The pre-clipping
scheme in the PowerLight 6.0
PFC ,
9.0 PFC , and 6.0 II keeps the
output signal clean despite the
flat-topping of the waveform.