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2.1 PL380 Circuit Description (continued)
are mounted under the single large heat sink, together with HF
diodes that clamp the output voltage when their respective FETs
turn off.
At idle (i.e., no audio signal), the duty cycle of the drive signal is 50/
50 and the output voltage is zero with respect to the secondary
reservoir center tap. As with all Class-D (PWM) amplifiers, turning
on the positive side switch for longer intervals and the negative side
for shorter ones will push the output voltage proportionally towards
the positive rail, and turning on the low-side switch for longer
intervals will lower the output voltage towards the negative rail.
When either switch's "on" time reaches 100%, the amplifier has
reached the equivalent of clipping and the output voltage will be
equal to that supply rail's voltage.
Both switches must be operated in alternation, with very exact
synchronization to prevent cross-conduction (both switches on
simultaneously) or excessive "dead time" (both switches off). A
complex, optically coupled, separately powered gate drive circuit for
each FET receives timing signals from the modulator and provides
several amps of drive current to rapidly charge and discharge the
FET gates. If there is any significant disorder in this circuitry, leading
to both FETs turning on at once, immediate failure is likely.
Output Filter
The pulse width modulated voltage must be filtered before it can be
connected to external loudspeakers. The main low-pass filter uses a
toroidal inductor and high-quality film capacitor (L1 with C64, and L3
with C171) on each channel output, along with additional trap
components to further reduce switching interference at 250 kHz to
approximately 55 dB. Some distortion analyzers may still have
difficulty reading the distortion with this much interference present.
Any available HF filters should be engaged to remove as much of
this interference as possible.
Output Connections
A twisted pair of wires from each channel couple the high-current
output signal to the speaker connectors, which use parallel 30 A
binding posts and 30 A Neutrik Speakon connectors. The output
peak voltage swing can reach 200 V, and output current can peak at
80 A, but internal limiter circuits and the normal dynamics of music
program will prevent long-term currents in excess of the connector
ratings. Technicians should be aware of the amplifier's potentially
hazardous output voltages when making bench connections.
Input Connections
Input connections are balanced XLR in parallel with "euro-block"
screw terminals and the DataPort™ inputs. There are six input panel
switches: a three-position input sensitivity switch (1.2 V input,
32 dB, or 26 dB), which combines with the 21-detent front panel
PL3 Series Service Manual
TD-000274-00 Rev. A
gain controls to regulate overall gain as desired; a three-position
Parallel-Stereo-Bridge Mono switch; two three-position low-
frequency filter (high-pass) settings, and two clip limiter enable
switches. LEDs of different colors indicate certain switch selections,
so the end user can easily check the setup with a quick glance.
QSC DataPort
The PL380 uses the same type of HD-15 connector used on other
QSC DataPort amplifiers to connect to QSControl devices such as
Basis processors. Two changes for the PowerLight 3 series are:
• Input signals from the DataPort are directly parallel with the XLR
and euro-block inputs, allowing the signals to be patched to
other amps for greater flexibility.
• The sensitivity switch settings are visible to the Basis processor.
Protection Systems
As with all audio amplifiers, full rated power is only required for
brief peaks in the audio program, and typical use rarely exceeds 1/8
of full power, when averaged over some time. Therefore the
amplifier must allow high peak power to flow for short periods of
time but also provide longer-term protective systems that limit this
power to reasonable levels. The amplifier's protection relies on peak
clamps for certain instantaneous overstresses, with analog gain
reduction in each channel to reduce long-term overloads, and as a
last resort, muting of the amplifier if stresses continue to build up.
The limiter thresholds and time constants are matched to the
thermal behavior of the systems being protected.
In brief, protection systems are provided for:
• Peak clamping of output current, with internal gain reduction
and/or muting to reduce long-term output current to a reason-
able value. This protection occurs when outputs are shorted, and
gain reduction will also be observed within two seconds, when
attempting full power into 2 ohms, and after about 5 seconds
into 4 ohms.
• AC current limiting. An additional system measures power
supply current and reduces gain of both channels as necessary
to keep currents within the carrying capacity of the overcurrent
protective devices. This prevents nuisance tripping of internal
and external circuit breakers.
• Over-temperature protection. A precision temperature
sensing IC is mounted in the heat sink close to each channel's
output devices, and it controls a DC "thermal bus" for each
channel, whose voltage is used to increase fan speed, cause
gain reduction (thermal limiting) and, if necessary, amplifier
muting, to keep the temperature of the heat sink below 85º C.
• High Frequency Limiting. Certain internal parts are subject to
overload if operated at full powers near 20 kHz, and therefore
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