QSC PowerLight3 Series Operating manual

Technical Service Manual
PowerLight3
Series
▲▲
▲▲
▲PL325
▲▲
▲▲
▲PL340
▲▲
▲▲
▲PL380
*TD-000274-00*
*TD-000274-00*
Rev. A
QSC Audio Products, LLC
Costa Mesa, CA 92626 USA
www.qscaudio.com

PL3 Series Service Manual 1
TD-000274-00 Rev. A
QSC Audio Products, LLC
Technical Services Group
Phone: 1-800 QSC AUDIO (1-800-772-2834) USA only
[phone]
Fax: [phone]
Postal: 1665 MacArthur Blvd.
Costa Mesa, California 92626 USA
E-mail: [email protected]
Web: http://www.qscaudio.com (product information and support)
http://www.qscstore.com (parts and accessory sales)
PL325
PL340
PL380
PowerLight 3 Series
Technical Service Manual
Copyright © 2008 QSC Audio Products, Inc. All rights reserved.

2QSC Audio Products, LLC
PL325 and PL340 amplifier, rear view
PL380 amplifier, rear view

PL3 Series Service Manual 3
TD-000274-00 Rev. A
Table of Contents
Tables and Figures ........................................................................................................................................................................ 4
Specifications................................................................................................................................................................................ 6
1. Introduction ................................................................................................................................................................................ 7
1.1 Restriction of Hazardous Substances Directive (RoHS) ............................................................................................................................................. 7
1.2 Service bulletins and updates .................................................................................................................................................................................... 7
1.3 The well-equipped service bench .............................................................................................................................................................................. 7
1.4 Working with surface-mount components................................................................................................................................................................. 8
1.5 PL380 Service Fixture ............................................................................................................................................................................................... 10
2. Technical Descriptions and Testing .................................................................................................................................... 14
2.1 PL380 Circuit Description ......................................................................................................................................................................................... 14
2.2 PL380 Major Circuit Blocks ...................................................................................................................................................................................... 16
2.3 PL380 Test Procedure ............................................................................................................................................................................................... 21
2.4 PL340 Test and Calibration Procedure ..................................................................................................................................................................... 24
2.5 PL325 Test and Calibration Procedure ..................................................................................................................................................................... 27
3. Troubleshooting ....................................................................................................................................................................... 31
3.1 PL380: Symptoms, causes, and remedies ................................................................................................................................................................ 31
3.2 PL325 and PL340: Symptoms, causes, and remedies .............................................................................................................................................. 39
PLC Power Supply Restoration Kit ................................................................................................................................................................................. 43
4. Printed Circuit Boards............................................................................................................................................................ 53
4.1 PL325 and PL340 Amplifier ...................................................................................................................................................................................... 53
Main module circuit board assembly ............................................................................................................................................................................................................ 53
Input module circuit board assembly ............................................................................................................................................................................................................ 56
4.2 PL380 Amplifier ........................................................................................................................................................................................................ 58
Main module circuit board assembly ............................................................................................................................................................................................................ 58
5. Schematic Diagrams .............................................................................................................................................................. 64
5.1 PL325 Schematics .................................................................................................................................................................................................... 64
5.2 PL340 Schematics .................................................................................................................................................................................................... 72
5.3 PL380 Schematics .................................................................................................................................................................................................... 80
6. Replacement parts .................................................................................................................................................................. 89
6.1 Semiconductor package descriptions and pinouts .................................................................................................................................................. 89
6.2 PL325 parts and assemblies ..................................................................................................................................................................................... 93
PL325 Power Amplifier (120V) (QSC part # FG-032500-00) ......................................................................................................................................................................... 93
PL325 Power Amplifier (100V) (QSC part # FG-032500-01) .......................................................................................................................................................................... 93
PL325 Power Amplifier (230V) (QSC part # FG-032500-02) ......................................................................................................................................................................... 93
Chassis Assembly PL325 (120V) (QSC part # WP-032500-00) ...................................................................................................................................................................... 93
Chassis Assembly PL325 (100V) (QSC part # WP-032500-01) ...................................................................................................................................................................... 94
Chassis Assembly PL325 (230V) (QSC part # WP-032500-02) ...................................................................................................................................................................... 95
PCB Assembly PL325 (120V) (QSC part # WP-032501-00) ............................................................................................................................................................................ 95
PCB Assembly PL325 (100V) (QSC part # WP-032501-01) ............................................................................................................................................................................ 98
PCB Assembly PL325 (230V) (QSC part # WP-032501-02) .......................................................................................................................................................................... 101
Input Assembly PL325 (QSC part # WP-032502-00) ................................................................................................................................................................................... 105
6.3 PL340 parts and assemblies ................................................................................................................................................................................... 106
PL340 Power Amplifier (120V) (QSC part # FG-034000-00) ........................................................................................................................................................................ 106
PL340 Power Amplifier (100V) (QSC part # FG-034000-01) ........................................................................................................................................................................ 106
PL340 Power Amplifier (230V) (QSC part # FG-034000-02) ........................................................................................................................................................................ 106
Chassis Assembly PL340 (120V) (QSC part # WP-034000-00) .................................................................................................................................................................... 106
Chassis Assembly PL340 (100V) (QSC part # WP-034000-01) .................................................................................................................................................................... 107
Chassis Assembly PL340 (230V) (QSC part # WP-034000-02) .................................................................................................................................................................... 108
PCB Assembly PL340 (120V) (QSC part # WP-034001-00) .......................................................................................................................................................................... 108
PCB Assembly PL340 (100V) (QSC part # WP-034001-01) .......................................................................................................................................................................... 112
PCB Assembly PL340 (230V) (QSC part # WP-034001-02) .......................................................................................................................................................................... 115
Input Assembly PL340 (QSC part # WP-034002-00) ................................................................................................................................................................................... 118
6.4 PL380 parts and assemblies ................................................................................................................................................................................... 120
PL380 Power Amplifier (120V) (QSC part # FG-038000-00) ........................................................................................................................................................................ 120
PL380 Power Amplifier (100V) (QSC part # FG-038000-01) ........................................................................................................................................................................ 120
PL380 Power Amplifier (230V) (QSC part # FG-038000-02) ........................................................................................................................................................................ 120

4QSC Audio Products, LLC
Tables and Figures
Chassis Assembly PL380 (120V) (QSC part # WP-038000-00) .................................................................................................................................................................... 120
Chassis Assembly PL380 (100V) (QSC part # WP-038000-01) .................................................................................................................................................................... 121
Chassis Assembly PL380 (230V) (QSC part # WP-038000-02) .................................................................................................................................................................... 122
PCB Assembly PL380 (120V) (QSC part # WP-038001-00) through January 2008 ..................................................................................................................................... 122
PCB Assembly PL380 (100V) (QSC part # WP-038001-01) through January 2008 ..................................................................................................................................... 126
PCB Assembly PL380 (230V) (QSC part # WP-038001-02) through January 2008 ..................................................................................................................................... 130
PCB Assembly PL380 (120V) (QSC part # WP-038001-00) from February 2008– ....................................................................................................................................... 134
PCB Assembly PL380 (100V) (QSC part # WP-038001-01) from February 2008– ....................................................................................................................................... 138
PCB Assembly PL380 (230V) (QSC part # WP-038001-02) from February 2008– ....................................................................................................................................... 142
Table of Contents (continued)
PL325 and PL340 amplifier, rear view ..................................................................................................................................................................... 2
PL380 amplifier, rear view ....................................................................................................................................................................................... 2
Table 1.1. Load resistor bank switch truth table ..................................................................................................................................................... 7
Figure 1.1. Load resistor bank ................................................................................................................................................................................. 7
Figure 1.2. Use two irons......................................................................................................................................................................................... 8
Figure 1.3. Soak up solder ....................................................................................................................................................................................... 8
Figure 1.4. Apply new solder ................................................................................................................................................................................... 8
Figure 1.5. Place component ................................................................................................................................................................................... 8
Figure 1.6. Solder one end of the component ......................................................................................................................................................... 8
Figure 1.7. Solder other end .................................................................................................................................................................................... 8
Figure 1.8. The PL380 service fixture .................................................................................................................................................................... 10
Figure 1.9. Locations of Test Point A and Test Point B .......................................................................................................................................... 10
Figure 1.10. Jumpers to be removed from Test Point B ........................................................................................................................................ 11
Figure 1.11. Connecting the hookup leads ............................................................................................................................................................ 11
Figure 1.12. Schematic diagram of the PL380 service fixture .............................................................................................................................. 12
Figure 1.13. Proper IGBT drive signals (with chassis ground reference ) ............................................................................................................. 13
Figure 1.14. Proper FET gate drive signals with FETs installed (with chassis ground reference) ....................................................................... 13
Figure 2.1. Signal with 250 kHz switching noise .................................................................................................................................................. 21
Figure 2.2. Signal with switching noise filtered out ............................................................................................................................................. 21
Figure 2.3. Burst sine wave signal for 2 Ω power testing ..................................................................................................................................... 21
Figure 2.4. Noise and distortion residual with bias properly set .......................................................................................................................... 25
Figure 3.1 IGBT gate drive waveforms .................................................................................................................................................................. 32
Figure 3.2 FET gate drive waveforms .................................................................................................................................................................... 33
Figure 3.3 Dead time between one pulse turning off and the other turning on should be about 20–30 ns at the 5 V level. ............................. 33
Figure 3.4 The clock drive logic signals................................................................................................................................................................. 33
Figure 3.5 The power supply sync pulse. .............................................................................................................................................................. 34
Figure 3.6 Triangle wave at comparator inputs (pin 2 of U8 and U28). ................................................................................................................ 35
Figure 3.7 Triangle wave with supply rails energized. .......................................................................................................................................... 35
Figure 3.8. Switching pulse at node N401 (pin 3 of U19) ..................................................................................................................................... 39
Figure 3.9: Switching signal with dead time at nodes N397 and 398 (pins 11 and 14 of U19). .......................................................................... 40
Figure 3.10. Typical crossover residual from distortion analyzer output .............................................................................................................. 41
Figure 3.11. Identify damaged transistors by measuring resistance across the collector and emitter. .............................................................. 43
Table 3.1. Clamping voltage troubleshooting ....................................................................................................................................................... 44
Table 3.2. Troubleshooting clamp malfunctions ................................................................................................................................................... 45
Figure 3.12. The overcurrent detection circuit for power supply cutback. ........................................................................................................... 46
Table 3.3. Troubleshooting clamp transistors ....................................................................................................................................................... 46

PL3 Series Service Manual 5
TD-000274-00 Rev. A
Table 3.4. Troubleshooting short circuit cutback clamping ................................................................................................................................... 47
Figure 3.13. Output signal and positive rail steps. ............................................................................................................................................... 48
Figure 3.14. Output signal and negative rail steps. .............................................................................................................................................. 48
PL325 and PL340 main circuit board, with major circuit sections shown ............................................................................................................ 53
PL325 and PL340 main circuit board, top layer ..................................................................................................................................................... 54
PL325 and PL340 main circuit board, bottom layer, mirror image ........................................................................................................................ 55
PL325 and PL340 input board, top layer ............................................................................................................................................................... 56
PL325 and PL340 input board, bottom layer, mirror image ................................................................................................................................... 57
PL380 main circuit board, with major circuit sections shown .............................................................................................................................. 58
PL380 main circuit board, detailed view ............................................................................................................................................................... 59
PL380 input board, main circuit blocks shown ...................................................................................................................................................... 59
PL380 main circuit board, top layer ....................................................................................................................................................................... 60
PL380 main circuit board, mid1 layer .................................................................................................................................................................... 61
PL380 main circuit board, mid2 layer .................................................................................................................................................................... 62
PL380 main circuit board, bottom layer ................................................................................................................................................................ 63
PL325 schematic notes .......................................................................................................................................................................................... 64
Schematic sheet INPUT, PL325 ............................................................................................................................................................................. 65
PL325 wiring diagram ............................................................................................................................................................................................ 66
PL325 schematic guide .......................................................................................................................................................................................... 67
Schematic sheet AMP CH-A, PL325...................................................................................................................................................................... 68
Schematic sheet AMP CH-B, PL325 ...................................................................................................................................................................... 69
Schematic sheet PROTECT/CONTROL, PL325 ...................................................................................................................................................... 70
Schematic sheet POWER SUPPLY, PL325 ............................................................................................................................................................. 71
PL340 schematic notes .......................................................................................................................................................................................... 72
Schematic sheet INPUT, PL340 ............................................................................................................................................................................. 73
PL340 wiring diagram ............................................................................................................................................................................................ 74
PL340 schematic guide .......................................................................................................................................................................................... 75
Schematic sheet AMP CH-A, PL340...................................................................................................................................................................... 76
Schematic sheet AMP CH-B, PL340 ...................................................................................................................................................................... 77
Schematic sheet PROTECT/CONTROL, PL340 ...................................................................................................................................................... 78
Schematic sheet POWER SUPPLY, PL340 ............................................................................................................................................................. 79
PL380 schematic notes .......................................................................................................................................................................................... 80
PL380 wiring diagram ............................................................................................................................................................................................ 81
PL380 schematic guide .......................................................................................................................................................................................... 82
Schematic sheet AMP CH-A, PL380 through January 2008 ................................................................................................................................. 83
Schematic sheet PROTECT/CONTROL, PL380 ...................................................................................................................................................... 84
Schematic sheet AMP CH-B, PL380 through January 2008 ................................................................................................................................. 85
Schematic sheet POWER SUPPLY, PL380 through January 2008 ......................................................................................................................... 86
Schematic sheet INPUT, PL380 through January 2008 ........................................................................................................................................ 87
Schematic sheet AMP CH-A, PL380 from February 2008– ................................................................................................................................... 88
Schematic sheet AMP CH-B, PL380 from February 2008– ................................................................................................................................... 89
Schematic sheet POWER SUPPLY, PL380 from February 2008–........................................................................................................................... 90
Schematic sheet INPUT, PL380 from February 2008– .......................................................................................................................................... 91
Tables and Figures (continued)

6QSC Audio Products, LLC
PL325 PL340 PL380
Stereo Mode (both channels driven)
8Ω/ EIA 1 kHz / 1% THD
4Ω/ EIA 1 kHz / 1% THD
2Ω/ EIA 1 kHz / 1% THD
500 W
850 W
1250 W
800 W
1250 W
2000 W
1500 W
2500 W
4000 W*
Bridge Mono Mode
8Ω/ EIA 1 kHz / 1% THD
4Ω/ EIA 1 kHz / 1% THD
1700 W
2500 W
2600 W
4000 W
5000 W
8000 W*
Typical Distortion
(20 Hz–20 kHz, 3 dB below clip, or
20 Hz–5 kHz, 10 dB below clip, or
20 Hz–20 kHz, 20 dB below clip)
8Ω
4Ω
2Ω
0.002–0.01%
0.005–0.01%
0.02%
0.002–0.01%
0.005–0.01%
0.02%
0.01–0.03%
0.03–0.06%
0.10%
Maximum Distortion
(20 Hz–20 kHz, 1 dB below rated power)
4–8Ω0.05% 0.05% 0.20%
Frequency Response (8Ω) 20 Hz–20 kHz, ±0.2 dB 20 Hz–20 kHz, ±0.2 dB 20 Hz–20 kHz, ±0.2 dB
Noise (20 Hz–20 kHz, 32 dB gain) -106 dB -105 dB -104 dB
Dynamic Headroom (4Ω) 2 dB 2 dB 2 dB
Damping Factor (8Ω) 500 500 200
Output Circuitry Class H (2-tier) Class H (2-tier) Class D
Input Sensitivity @ max gain
(26 dB setting)
(32 dB setting)
3.28 V (+12.5 dBu)
1.60 V (+6.3 dBu)
3.92 V (+14 dBu)
1.96 V (+8.1 dBu)
5.27 V (+16.7 dBu)
2.67 V (+10.7 dBu)
Maximum Gain (1.2 V setting) 34.5 dB 36.4 dB 39.1 dB
Input Impedance >10 kΩ, balanced or unbalanced >10 kΩ, balanced or unbalanced >10 kΩ, balanced or unbalanced
Maximum Input Level
(1.2 V setting)
(32 dB setting)
(26 dB setting)
11 V (+23 dBu)
14.6 V (+25.5 dBu)
25 V (+30 dBu)
11 V (+23 dBu)
14.6 V (+25.5 dBu)
25 V (+30 dBu)
10 V (+22 dBu)
22 V (+29 dBu)
25 V (+30 dBu)
Controls and LEDs—Front Panel Common: AC Power Switch, Power LED (blue), Br Mono LED (yellow), Par LED (orange)
Each Channel: Signal -35 dB LED (green), -20 dB LED (green), -10 dB LED (orange), Clip/Prot LED (red),
Gain Control (21 detents, 1 dB steps)
Controls and LEDs—Rear Panel Common: Input Mode Parallel LED (orange), Stereo LED (green), Br Mono LED (yellow); Sensitivity 26 dB
LED (orange), 32 dB LED (green), 1.2 V LED (yellow)
Each Channel: LF Filter Switch off–30 Hz (yellow LED)–50 Hz (red LED); Clip Limiter Switch off–on (yellow LED)
Input Connectors Common: HD15 DataPort (inputs parallel with XLR inputs)
Each Channel: Male XLR, Female XLR, 3-Pin Terminal Block
Output Connectors Each Channel: 5-Way Binding Posts, Neutrik Speakon® (upper has both channel outputs)
Amplifier and Load Protection Short circuit, open circuit, thermal, RF protection. On/off muting, DC fault shutdown, active inrush current limiting
AC Power** / Cordset
120 V, 50–60 Hz
230 V 50 Hz
8.5 A / NEMA-15
7.5 A / Euro 16 A
12 A / NEMA-15
7 A / Euro 16 A
18 A / L5-30 (30 A twist-lock)
11 A / Euro 16 A
Dimensions Height: 2 RU 3.5" (8.9 cm); Width: 19" (48.3 cm); Depth: 15.63" (39.7 cm) from front mounting rails
Weight Net / Shipping 22 lb (10 kg) / 31.5 lb (14.3 kg) 22 lb (10 kg) / 31.5 lb (14.3 kg) 24 lb (11 kg) / 33.5 lb (15.2 kg)
Agency Approvals UL, CE, RoHS / WEEE compliant, FCC Class B (conducted and radiated emissions)
* Burst mode testing required due to AC service current limitations
** Representative of current draw with tyical music program material with occasional clipping
All specifications are subject to change without notice.
Specifications

PL3 Series Service Manual 7
TD-000274-00 Rev. A
Figure 1.1. Load resistor bank
Table 1.1. Load resistor bank switch truth table
1. Introduction
1.1 Restriction of Hazardous Substances Directive (RoHS)
All PowerLight 3 Series amplifiers are manufactured to conform to the European Union’s RoHS Directive, which reduces the amount of
hazardous substances allowed in products for sale within its member nations. In electronic equipment such as audio power amplifiers, this
applies primarily to certain toxic heavy metals, such as lead, which may be present in electronic components, solder, and other parts.
RoHS-compliant materials
When servicing RoHS-compliant amplifiers, it is important for the service technician to use only RoHS-compliant components and solder
(lead-free). All replacement parts provided by QSC for RoHS-compliant products are certified as RoHS compliant.
RoHS-compliant tools
Soldering irons and desoldering apparatus used on RoHS-compliant products must also not be contaminated by hazardous substances, such
as lead. Therefore, you cannot use the same soldering and desoldering tools for RoHS-compliant products and solder as you do for non-
compliant products and solder. You must either use separate soldering irons, desoldering tools and braid, etc., or at the very least designate
separate tips and braids and use only the appropriate ones. If you contaminate a tip or braid, even inadvertently, by using it on a non-
compliant product or solder, you can no longer use it with RoHS-compliant products or solder.
1.2 Service bulletins and updates
Contact QSC Technical Services to make sure you have the most up-to-date service bulletins, schematics, and parts lists for PowerLight 3
Series amplifiers. Service materials may be distributed in hard copy, via fax, and electronically (Adobe Acrobat PDF) via CD-ROMs, FTP from
the QSC web site (www.qscaudio.com), and e-mail.
At the time of this manual’s publication, one service bulletin, PL30001 (PL380 and Standby), had been released for the PowerLight 3
amplifiers.
1.3 The well-equipped service bench
To properly service PowerLight 3 amplifiers, a technician needs the right tools. The
technician’s service bench should have the following equipment:
• Digital multimeter with RMS AC voltage and current
• Digital clamp-on ammeter
• Dual-trace oscilloscope
• Low-distortion audio sine wave generator
• Audio distortion analyzer
• Switching amplifier measurement filter, such as Audio Precision AUX-0025
• Bank of non-inductive load resistors, configurable as 8 ohms
(min. 1500 watts capacity), as 4 ohms (min. 3000 watts), and 2 ohms
(min. 4000 watts). See Figure 1.1.
• Variable AC voltage source, such as a Variac or Powerstat
variable transformer;
For PL325 and PL340: rated current capacity of up to 25 A (for
120 V models) or 12 A (for 230 V models)
For PL380: rated current capacity of up to 50 A (for 120 V models)
or 25 A (for 230 V models)
• Philips and flat screwdrivers
SW1 SW2 SW3
SW4 8Ω8Ω8Ω8Ω
R1, R2: 8 , 1500W non-inductiveΩ
R3, R4: 8 , 1000W min. non-inductiveΩ
R1 R2 R3 R4
SW1 SW2 SW3 SW4
∞Ω (no load) OFF•••
8ΩON OFF OFF OFF
4ΩON ON OFF OFF
2ΩON ON ON OFF
0Ω(short circuit) ON • • ON

8QSC Audio Products, LLC
Figure 1.6. Solder one end
of the component
Figure 1.7. Solder other
end
Figure 1.5. Place
component
Figure 1.4. Apply new
solder
Figure 1.3. Soak up solder
Figure 1.2. Use two irons
Solder braid
• Soldering iron with a fine tip (25–60 W recommended)
• RoHS-compliant rosin-core solder
• Long-nose pliers
• Diagonal cutters
• Wire strippers
• PL380
Automated test equipment, such as an Audio Precision workstation, is very useful for servicing QSC amplifiers.
Contact QSC Technical Services to obtain applicable AP test files.
1.4 Working with surface-mount components
PowerLight 3 amplifiers, like many modern electronic products, use surface-mount technology (SMT) compo-
nents where appropriate in order to make high-density circuitry that is reliable and economical to manufac-
ture.
SMT components are used in the amplifiers’ small-signal and control circuits, so they do not handle significant
amounts of power; therefore, they are subject to very little stress and should seldom fail. Sometimes they do
fail, or they require replacement for a performance upgrade or modification. Thus, it is important to know how
to work with SMT components.
Specialized tools and equipment exist for soldering, unsoldering, and removing SMT components quickly and
efficiently, but they are often expensive. Most SMT repairs, though, can be handled reasonably well with common
tools and equipment, such as tweezers, solder braid, and fine-tip soldering irons.
Two-terminal components (resistors, capacitors, diodes, etc.)
Removal
1Use two soldering irons, preferably about 25 to 40 watts, with fine tips.
2With a soldering iron in each hand, hold one tip on the solder at one end of the component and the other
tip on the other end (Figure 1.2).
3Once the solder melts on both ends, grip the component between the two tips and lift it from the circuit
board.
4Use solder braid and a soldering iron to remove the solder from the two pads (Figure 1.3).
Insertion
1With a soldering iron and RoHS-compliant solder, melt just enough solder onto one pad to create a small
mound (Figure 1.4).
2Grasp the component in the middle with tweezers. Melt the small mound of solder with the iron and place
the component across the two pads (in the correct orientation, if the component is sensitive to direction)
and press it flat against the circuit board, with one end of the component immersed in the melted solder
(Figure 1.5).
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the compo-
nent in place.
4Fully solder the other end of the component to its pad. Let the solder harden (Figure 1.6).
5Fully solder the tacked end of the component to its pad (Figure 1.7).
Solder
Solder
Tweezers
Solder
1.3 The well-equipped service bench (continued)

PL3 Series Service Manual 9
TD-000274-00 Rev. A
Three-terminal components (transistors, etc.)
Removal
1With a soldering iron and solder braid, remove as much solder as possible from the middle terminal of the component.
2With a soldering iron in each hand, hold one tip on the solder at the terminal at one end of the component and the other tip on the
terminal at the other end.
3When the solder on both ends melts, grip the component between the two tips and lift it from the circuit board. You might need to
quickly touch the pad on the middle terminal with a soldering iron to melt any remaining solder that might be holding the component
down.
4Use solder braid and a soldering iron to remove the solder from the three pads.
Insertion
1With a soldering iron and RoHS-compliant solder, melt just enough solder onto one pad to create a small mound of solder.
2Grasp the component with tweezers. Melt the small mound of solder with the iron and place the component in the correct orientation
across the three pads and press it flat against the circuit board, with one terminal of the component pressed into the melted solder.
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the component in place.
4Fully solder the other terminals of the component to their pads. Let the solder harden.
5Fully solder the tacked terminal of the component to its pad.
Multi-pin components (ICs, etc.)
Removal
Removing a multi-pin SMT component is a delicate procedure. Ideally, you should use a soldering iron with an attachment that allows you to
heat all the pins simultaneously.
If such a soldering device is not available, use this procedure:
1Use a soldering iron and solder braid to remove as much solder as possible from the pins of the component.
2With fine tweezers, carefully try to lift each pin to see if it’s free. If it’s not, touch it with the tip of the soldering iron and if necessary, use
the solder braid to remove the remaining solder.
3Repeat the process until all the pins are free and you can remove the component.
Insertion
1With a soldering iron and RoHS-compliant solder, melt just enough solder onto one pad to create a small mound of solder. It is usually
easiest to use a pad that corresponds to one of the end or corner pins of the component.
2Grasp the component with tweezers. Melt the small mound of solder with the iron and place the component in the correct orientation
upon its pads and gently press it flat against the circuit board, with the appropriate terminal of the component pressed into the melted
solder.
3Hold the component in place and take the soldering iron away. Let the solder harden to tack the component in place.
4Fully solder the other terminals of the component to their pads. Let the solder harden.
5Fully solder the tacked terminal of the component to its pad.
1.4 Working with surface-mount components (continued)

10 QSC Audio Products, LLC
Figure 1.8. The PL380 service
fixture
Figure 1.9. Locations of Test Point A and Test Point B
1.5 PL380 Service Fixture
With its class D output section, the PL380 amplifier differs from all
previous QSC stand-alone models, which up to now have used
linear output circuitry in either a class AB configuration or a class
AB-based class G or H one.
To the end user, these differences should not be apparent, except
that he or she may notice that
such a high-power amplifier does
not generate much heat and
appears to consume much less
electricity than might be
expected. The PL380 amplifier
should behave sonically like a
high-quality, high-power audio
amplifier.
Being a class D amplifier, the
PL380 uses pulse-width modula-
tion to allow output transistors that are either fully on or fully off to
produce varying output voltages. To reduce noise, the clock for the
output sections’ modulators is synchronous with the power supply’s
clock. However, that interdependence makes testing and trouble-
shooting one section of the amplifier without the other impossible.
This is the reason for the PL380 service fixture (Figure 1.8). It is
necessary for many of the procedures described in Chapter 2’s
section on the PL380 test procedure, and in Chapter 3’s sections on
PL 380 troubleshooting.
Figure 1.12 shows the schematic for the PL380 service fixture. The
fixture is available for purchase from QSC Technical Services.
Functions of the service
fixture
•Substitutes for the amplifier’s house-
keeping supply. The housekeeping supply
powers the clock, power supply switching,
and modulation circuitry. The fixture allows
you to operate and check these key areas of
the amplifier’s circuitry even without its being
connected to the AC mains.
It also allows you to operate the amplifier for
testing and troubleshooting at low AC mains
voltages that would be less likely to cause
damage if a fault exists.
•Monitors the ±15-volt rail currents. The
terminal strip on the service fixture provides
precision voltages that are analogous to the
currents drawn by the positive and negative 15-
volt supply rails. The voltages are scaled to 1
volt = 1 ampere. This is useful for detecting
abnormal situations such as defective op amps or other circuitry
that could cause abnormally high or low current demand.
•Monitors the +5-volt suppy. The 5-volt regulated supply
powers the clock and logic circuitry. Because it is derived from
other higher voltage DC supplies, the presence of the voltage on
the screw terminal indicates that they also are functioning.
•Verifies clock switching. The “Sync Sig” terminal should
carry a 250 kHz pulse train signal. Its presence verifies that the
clock oscillator and divider circuits are operating. Because of the
cables connecting the fixture to the amp, the pulse train will
tend to be messy, with significant ringing. Therefore, the signal
is useful only to verify the operation of the amp’s circuitry.
Hooking up the service fixture
These steps describe how to set up the service fixture and connect
it to the PL380.
Prepare the service fixture
1. Set the service fixture on the right side of your test bench work
area, with the screw terminals and hookup leads facing toward
you. Setting the fixture this way makes it nearly impossible to
connect the hookup leads the wrong way.
TEST POINT A
TEST POINT B
Test Point B
Test Point A

PL3 Series Service Manual 11
TD-000274-00 Rev. A
Figure 1.10. Jumpers to be removed
from Test Point B
Figure 1.11. Connecting the hookup leads
2. Turn off the service fixture’s power switch and connect it to the
AC mains. The service fixture has a universal power supply that
can operate on any AC voltage from 100 to 240 volts.
Prepare the amplifier
3. Disconnect anything connected to the amplifier’s inputs and
outputs.
4. Disconnect the amplifier from the AC mains and allow at least
five minutes for the
internal voltages to
bleed down.
5. Remove the
amplifier’s bottom
cover.
6. Set the amplifier on
the test bench next
to the service
fixture, with its
open side up and
front panel facing
you.
7. Locate the two test points on the amplifier’s main circuit board.
Test Point A is located near the output board, while Test Point
B is in front of the power transformer (Figure 1.10).
8. Remove the two jumpers from Test
Point B (Figure 1.11). Set them
aside to be re-installed later.
Connect the fixture and
amplifier
9. Connect the service fixture’s two
hookup leads to the corresponding
test points. Make sure you orient
the housings correctly (as shown
in Figure 1.12) when you plug
them onto the test point headers.
As the lengths of the leads
suggest, the long one goes to Test
Point A and the short one to Test
Point B .
Initial tests with the service fixture
These initial tests with the service fixture will allow you to deter-
mine whether the amplifier’s clock and control circuitry are working
properly, with no risk of damaging high-power devices or circuits.
Tools and equipment needed
• Oscilloscope (2 channels minimum) and probes
• Digital multimeter (frequency counter is a plus)
Service fixture tests
1. Start-up sequence—Turn on the service fixture’s power
switch and watch the amplifier’s LED display on the front
panel. The clip/protect indicator LEDs should light briefly and
then go out. (If they stay on, then it is usually because an audio
signal is being put into the amplifier inputs. Disconnect any
input signals for this stage of testing.)
The amplifier should start with its normal sequence. You
should hear a small relay click after about two seconds,
followed by a large relay click about two seconds after that.
2. Regulated +5 volts—Check for the regulated 5 volts DC at
the “+5V REG.” terminal (reference to the fixture’s ground
TEST POINT B
Pin 1
Remove jumpers on
pins 3–4 and 5–6 before
connecting service fixture.
Replace jumpers for
normal operation.
1.5 PL380 Service Fixture (continued)
TEST POINT A
TEST POINT B
Remove two jumpers
from Test Point B.
Connect long lead to
Test Point A and short
lead to Test Point B.
To AC mains
100–240 VAC
50–60 Hz
Do not connect amplifier
to AC mains yet.
Test Point A
Test Point B

12 QSC Audio Products, LLC
Figure 1.12. Schematic
diagram of the PL380 service
fixture
QTY ITEM NO. PART NO. DESCRIPTION VENDOR

PL3 Series Service Manual 13
TD-000274-00 Rev. A
Figure 1.13. Proper IGBT drive signals (with chassis ground
reference )
Figure 1.14. Proper FET gate drive signals with FETs installed (with
chassis ground reference)
1.5 PL380 Service Fixture (continued)
terminal). The voltage is derived from other low-voltage power
supplies, so its presence indicates that the other supplies are
probably good.
If the voltage is not present, then check the other supplies and
associated circuitry to find the fault. For more detailed
troubleshooting, see Troubleshooting Serious Failures in
Section 3.1.
3. Power supply sync signal—Connect an oscilloscope to the
sync signal terminal on the service fixture. You can use the
ground terminal on the fixture for the scope probe’s ground clip.
You should see a steady 250 kHz pulse train. The purpose of
this terminal is simply to verify that the clock and divider
circuits are working; the signal here is not clean and has a
significant amount of ringing, but that is not important.
If the signal is not present, then check the clock oscillator
circuit (74HC4060 ripple counter U1, Y1, C3, C8, R4, R10, et al;
on the schematic, see sheet “AMP CH-A,” zones C-8 and D-8),
the dividers and complementary logic generators (74HC74 flip-
flops U4:1, U4:2, U7:1, and U7:2; same sheet, zones C-7 and C-
6), and the Schmitt inverters U5:1 and U5:2. For more detailed
troubleshooting, see Troubleshooting Serious Failures in
Section 3.1.
4. Low-Voltage Supply Current—Measure the voltages on the
+15 V and -15 V current monitoring points. These terminals
present voltage analogs of the current demand on the low-
voltage supply rails, scaled to 1 volt = 1 ampere. For example, a
voltage of 0.55 volt on the -15 V terminals would indicate that
0.55 A was being drawn from that supply rail.
Typically, the currents drawn should be approximately 400 mA
on the negative rail and 500 mA on the positive when the
amplifier is muted (during the start-up sequence, for example).
When the output FET gates are being driven, the current should
increase to about 500 mA on the negative rail and 800 mA on
the positive one.
If the currents drawn appear to differ substantially from these
figures, there may be a fault in the low-voltage circuitry.
5. IGBT drive—The service fixture allows you to use an
oscilloscope to check the drive signals to the gates of the
power supply IGBTs, Q68 and Q69. Figure 1.13 shows what the
signals should look like. For more details on the IGBT drive
waveforms, please see the Troubleshooting Serious Failures
section in Chapter 3.
6. FET drive—The service fixture also makes it possible to check
and verify the drive signals to the gates of the output FETs, Q10,
Q11, Q58, and Q59 (see Figure 1.14). However, to get at the FET
gates you must remove the amplifier module (the main circuit
board assembly) from the chassis.
For more details on the FET drive waveforms, please see the
Troubleshooting Serious Failures section in Chapter 3.

14 QSC Audio Products, LLC
2.1 PL380 Circuit Description
The PL-380 uses an unregulated, open-loop switchmode power
supply that provides DC power to a pair of high-power Class D
amplifier channels. A master clock circuit synchronizes and
regulates the switching frequencies of all power supply and audio
amplification sections. Various input circuits, controls and displays,
protection systems, and output monitoring connect to the various
subsystems.
Power Supply
There are three distinct power supply domains in this amplifier.
• The main power supply provides ±185 volts at high current to
the main rails of the output section.
• The auxiliary power supply is derived from the main supply,
providing ±16-volt rails that are used by the cooling fan and
various low-voltage circuit blocks. Note that this supply tracks
the voltage and on/off status of the main supply.
• The "housekeeping" supply (labeled on the schematic as a
“keep-alive” supply) uses a separate low-power flyback system
to provide power to functions that need to be active during
standby.
The main power supply uses a large, center-tapped primary-side
reservoir capacitor bank (C209, C210, C213, C214, C216, and C217)
charged to DC voltages of about ±165 V by an offline (direct from
the AC mains) rectifier, BR1. The rectifier is wired as a voltage
doubler for 120 V AC operation, and as a full-wave bridge for 230 V,
and thus either configuration results in about the same DC voltage
on the reservoir.
The main switching devices, D68 and D69, are mounted along with
the rectifiers on one of the two smaller heat sinks and operate at a
frequency of 125 kHz to alternately couple the positive and negative
reservoir voltages (reference to the center tap of the reservoir, node
PRICAPCT) through T2:1, which are the primary windings of the
main power transformer, an E55-core isolation type. Capacitors
C231 and C232, together with the inductance of the primary, form a
series resonant tank that shapes the voltage waveform across the
primary into a pseudo-sinusoid instead of a square wave.
The transformer’s secondary voltages are rectified into a bank of
eight capacitors (Channel 1: C247–C250; Channel 2: C253, C254,
C259, and C260), which store nominal DC voltages of about ±185 V.
The secondary HF (fast) rectifiers D82–D85 are mounted on a
second, smaller heat sink.
The auxiliary supply uses single-turn taps on the main transformer
T2, with surface-mount rectifiers and small electrolytic capacitors,
to produce a bipolar pair of DC rails of approximately ±16 V.
The housekeeping flyback supply operates independently of the
high-power main supply, and powers certain circuit functions that
allow the main supply to operate.
Turn-On Sequence
The AC power switch spans points W17 and W18 and controls
power only to the housekeeping supply, which has a much smaller
DC reservoir (C190 and C191) than the main does. When switched
on, this supply starts in about a half second and provides secondary-
side DC power to the crystal-controlled clock circuit (U1, Y1, and
associated components) and main supply controller IC (U49),
resulting in 125 kHz drive to the main power supply switches.
After a short delay, the inrush-current relay, K2, closes and charges
the main supply’s reservoirs through large NTC resistors, R262 and
R266, which prevent drastic inrush current surges. During this time,
the primary and secondary reservoirs charge smoothly in unison
because the power supply switches are already operating. After
about one second, the main power relay, K1, closes and couples the
AC rectifiers directly to the primary reservoir, bypassing the inrush
resistors and providing full power. There is an additional 1-second
delay to allow all internal voltages to settle before the amplifier
comes out of muting.
Turn-off behavior
When power is removed, the amplifier mutes promptly, but the
power supply switches continue operating until primary and
secondary DC voltages have discharged to about 20%. There is an
OR-diode from the main reservoir to the housekeeping reservoir that
maintains voltage to the aux supply during this period. When this
DC voltage drops below about 45 V, the “housekeeping” supply
stops switching and the amp shuts off fully. This sequence of delays
and controlled inrush current will occur on all starts and restarts,
without causing adverse transients.
AC Cordsets
The 230 V version of the PL380 uses a 20 A/250 V rated Neutrik
Powercon® AC inlet, and the provided cordset uses 1.5 mm² × 3
wiring with a 16 A/250 V plug. The 120 V version uses a fixed 12/3
jacketed cable with a 30 A twist-lock plug. (We expect to use the
30 A 120 V PowerCon connector when it becomes available.)
Due to the high efficiency of Class D circuitry, there is a greater
difference between peak operating current and average operating
current at 1/8 of maximum power, so there is an extra margin of
current carrying capacity in the AC components.
Amplifier Channels
Each of the two amplifier channels uses two large switching FETs
operating as a half-bridge at a clock frequency of 250 kHz. The FETs
2. Technical Descriptions and Testing

PL3 Series Service Manual 15
TD-000274-00 Rev. A
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
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
2.1 PL380 Circuit Description (continued)

16 QSC Audio Products, LLC
frequency-sensitive limiting will prevent full power operation
above about 15 kHz. A backup system mutes the amplifier
quickly in the event of runaway oscillations.
• Clip Limiting. While clipping is not harmful to the amplifier, the
clip limiter will minimize distortion when it occurs. A user-
selectable switch engages the clip limiter on each channel.
2.2 PL380 Major Circuit Blocks
The following notes cover the same areas noted before in further
detail, with references to voltages, part numbers, and PCB locations.
All locations and directions are described though the amplifier is
placed upside down with the cover removed, with the front panel
facing the observer. This section should be read with the PL380
schematic at hand for reference.
Power Supply
Unless otherwise noted, this section refers to the schematic sheet
SUPPLY, PL380.
AC Entry
AC power enters the amplifier through an chassis-mounted cordset
or AC inlet, circuit breaker, and line filter located on an auxiliary PCB
in the right rear corner (L15 and L16; Y-caps C234 and C235; and X-
caps C294, C295, and C293, which are discharged by R348, R349,
and R350 within 2 seconds after power is disconnected). Sleeved
wires couple the two sides of the AC voltage to selected terminals
on the main PCB: J19 to J20 (all voltages), and J15 to J21 (230 V) or
J15 to J22 (120 V). These lines lead to power control relays
discussed below (schematic zone D-8).
Housekeeping Supply
A smaller fuse, F1, protects the inrush limiting resistors in the event
of a downstream load fault. A small amount of AC power is taken
after this fuse to the power switch via W17 and W18, returning to a
housekeeping reservoir C190 and C191 (zone C-8), charged by D64
through a current-limiting resistor bank that comprises R261, R263,
R264, and R267. This reservoir supplies U41, an integrated
TOP244VN flyback switcher that produces auxiliary supply voltages
through transformer T1. The components immediately surrounding
U41 provide feedback, over- and undervoltage sensing, and flyback
clamping.
The circuitry on the secondaries of T1 (zone B-6) provide a +10 V DC
supply (for future low-power accessory circuits), and unregulated
±25 V voltages. The 25-volt rails are reduced by regulators U42,
U44, U45, and U50 to produce clean ±15 V and ±5 V bipolar supply
rails. These regulated rails are labeled +15_TOP, +5_TOP, , and
-15_TOP, -5_TOP. Providing power for the clock and audio switching
circuitry is another set of ±5 V rails, labeled +5:SW and -5:SW,
which are decoupled through L5, L6, C226, and C227 to filter out
switching noise. All circuitry connected to these rails will be
powered as soon as U41 starts operating, which normally occurs
any time the amplifier is turned on.
The +5:SW rail powers the crystal-controlled clock and divider
circuitry (schematic: see sheet AMP CH-A, PL380, zone C-7 and D-7),
which is centered around U1 and U4–U7. The divider sends a sync
pulse train to U49 (schematic: see sheet Power Supply, PL380, zone
D-2), a PWM switch-mode supply controller that delivers switching
pulses with controlled dead time to U46. A specialized gate drive IC,
U46 in turn provides a gate drive signal through transformer T4, to
the pair of isolated-case switching transistors Q68 and Q69. T4’s
two secondary windings have opposing polarities so that the gate
drive pulses will alternately switch the transistors on and off.
The result of the switching is a 125 kHz alternating current through
the primary of the power transformer, T2. The transformer has a
turns ratio of 10:11, so the secondary voltage is about 10% higher
than the primary voltage. Thus, the energy from the primary
reservoir capacitors—C209, C210, C213, C214, C216, and C217
(zone D-5)—couples through to the transformer secondary, where it
is rectified and stored in the secondary reservoirs comprising C247–
C250 and C253, C254, C259, and C260 (zone C-3). These are the
±185-volt rails for the two channels’ output sections.
Ordinarily, at startup there is little or no voltage on the primary
reservoir, and therefore, little current flows through the switches.
The aux supply powers a sequence of delays that causes the inrush
and main relays to close progressively, ramping the entire main
supply up to full voltage in a controlled manner.
Power Supply On-Off Sequencing
The relay control circuitry is shown in zone C-5 of the schematic sheet
PROT/CNTRL, PL380 . When the amplifier is turned on, the following
sequence must occur to make the main power supply turn on:
1. The AC voltage detector (schematic: see sheet “Power Supply,
PL380” zone B-7 and B-8) connects to the incoming AC line
through resistors R258 and R259. It must sense that the amplifier
has AC mains voltage coming in to the power supply.
If it does, the AC voltage signal will turn transistor Q66 on, which
turns Q67 off. This allows current to flow through the LED side of
the optocoupler, U40, and pulls the AC-ON bus high to +5 V.
Resistor R265 adds a small amount of hysteresis, so that the
turn-off threshold is slightly lower than the one for turn-on.
2. In the relay control circuitry, the AC-ON bus turns transistor Q34
off. The IGBTs in the power supply should be receiving gate drive
pulses, which would cause bus IGBT-SW (a safety interlock to
prevent the relays from closing if the main power switches are
inactive) to charge capacitor C104. The bus AC-OFF-LO is a
remote control line that permits C104 to be remotely discharged
to shut down the main supply.
3. When the capacitor charges to greater than 3.3 V, the compara-
2.2 PL380 Major Circuit Blocks (continued)

PL3 Series Service Manual 17
TD-000274-00 Rev. A
tor U24:1 will swing low, turning on Q35 and Q37. These actuate
relay K2 (schematic: sheet Supply, PL380 , zone C-6), which
couples AC through large NTC resistors R262 and R266 to the
main AC rectifier BR1. The resistors limit inrush current as the
primary reservoirs charge.
4. Transistor Q35 also charges C107 through R142. The voltage on
pin 7 of U24:2 reaches +5 V (the threshold set by bus +5V:LIN) in
about one second, at which point the comparator will trip low
and turn on transistor Q43. This turns on relay K1, through bus
50A-RY, to bypass the inrush-limiting resistors. The main supply
will have now now reached full operating voltage and at normal
AC line voltage, the secondary DC rails should measure ±185 V.
5. The final timing delay is established by C108. When Q43 turns
on to energize K1, transistor Q48 turns off. This allows C108 to
charge through R163, D35, R166, and D43, controlling the rate of
charge of capacitors C111 and C114. When C111 and C114
reach +5 V thresholds (set by the +5_TOP bus), which takes
about one second, comparators U24:3 and U24:4 respectively
switch the RUN-A-LO and RUN-B-LO buses low, which enables
the audio switching on channels 1 and 2. This allows the
channels to pass audio signals.
6. When AC power is removed, including when the amplifier is
switched off, the AC detector circuit mentioned earlier shuts
down the AC-ON bus within several AC cycles. This lets Q34
quickly discharge C104, which triggers a rapid discharge of all
the other timing capacitors as well. This ensures that the delay
intervals are all reset when the amplifier is turned on again. The
RUN-A-LO and RUN-B-LO buses disable the audio switching,
which immediately mutes the audio.
Meanwhile, the housekeeping supply operated by U41 keeps
running, powered through D70 by energy stored in the primary
reservoir, and so the clock and power supply switching continue
until the reservoir voltage drops to about 45 V. Through this
after-shutoff switching action, the primary and secondary
reservoirs discharge together. This ensures that inrush current is
managed properly when the amplifier is switched on again.
7. The red Clip LEDs on the faceplate will light as long as auxiliary
power is present and the amp is in muting. On amplifiers
manufactured in January 2008 or earlier, it is normal for the clip
LEDs to remain lit for about 15–20 seconds after the amplifier is
switched off. On amplifiers manufactured in February 2008 or
later, the clip LEDs will flash briefly at turn-off and then remain
dark.
Amplifier Channels
NOTE: Most components are duplicated in the two amplifier
channels, so these descriptions will limit themselves to channel 1’s
circuitry (schematic: see sheet “Amp Ch-A, PL380” ) unless there is
an actual difference to be noted.
High Current Switching
The amplifier channel uses two large switching FETs, Q10 and Q11,
operating as a half-bridge at 250 kHz. They are mounted under the
single large heat sink, together with HF diodes D14 and D15 and
steering diodes D12:1 and D12:2, which clamp the output voltage
when their respective FET turns off.
Power for Gate Drives
Each FET is driven by a gate drive circuit that consumes appreciable
power and also must be referenced to its respective FET’s source
terminal. Therefore, each gate drive circuit has an unregulated 25-
volt power supply that suitable for its location in the overall channel
circuit. FET Q11’s source is connected to the fixed -185 V rail;
therefore, power for its gate drive circuitry comes from the LO-
SIDE GATE-A supply. The source terminal on the high-side (positive)
FET, Q10, is connected through D12:1 to the switched PWM audio
bus SW-A, so its power supply, HI-SIDE GATE-A, floats with respect
to the audio circuitry.
Gate Drives and Signal Isolation
In the high-side gate drive power supply, the regulator U20 drops
the unregulated 25 volts down to 12 volts for the gate driver IC, U17,
so it can produce 12-volt gate drive pulses. Regulator U15 reduces
the 12 volts to 5 volts for the optocoupler U12, which isolates the
gate drive circuitry from the ground-referenced modulator and logic
circuitry. The components D4, C29, and R43 delay the negative-
going transitions out of U9:2 to control the dead time.
Close Synchronization of FET Switching
Each FET of the pair switches on within tens of nanoseconds after
the other switches off. These transitions must be coordinated
closely to avoid both overlapping conduction—which would cause
destructive “shoot-through” currents—and excessive dead time,
which would increase audio distortion. Gate drive circuit disorders
that cause an FET to stay on too long are likely to result in both FETs’
destruction.
Output Current Clamping
The PWM signals also separately enable current sources that
sample each FET’s source terminal voltage during its “on” state to
determine the current through the FET. Transistor Q8 is the current
source associated with Q10; when it is switched on by U6:6, it
forward-biases D16 and D17, resulting in a signal voltage that
corresponds with the current through the FET. Transistor Q12
converts the voltage into a current on the OC-LIM-A bus.
The negative FET, Q11, has a similar current sensing circuit
comprising Q9, U6:5, D18 and D19, Q13, and their associated
components. This circuit also feeds bus OC-LIM-A.
The Overcurrent Feedback network (schematic: see sheet Amp Ch-A,
2.2 PL380 Major Circuit Blocks (continued)

18 QSC Audio Products, LLC
PL380, zone B-1) passes the current through resistor R87 to produce
a signal voltage, which is compared to ±5 V reference voltages. If
the signal exceeds about ±6.2 volts the network sends a correction
signal through R82 to bus OC-FB-A. This is a feedback bus for the
modulator, and the correction signal reduces the amount of
modulation to limit the FET current to about 70 A peak when the
FETs are cold, and less at higher temperature.
PWM Modulator
Comparator U8 is the pulse-width modulator that converts the audio
signal into a PWM stream that controls the output FETs. At its non-
inverting input, pin 2, a triangle wave develops from C21 integrating
the 250 kHz square wave current from bus CLK-A through R34. The
triangle wave is averaged about ground potential, so that with no
audio signal the comparator produces two 50/50 pulse trains that
switch between the negative and positive rails. One output, Q, is the
complement of the other, Q. If these pulse trains were averaged by
an integrator, the resulting voltages would be zero. PWM negative
feedback from SW-A through R35 and R36, as well as audio
negative feedback from OUT-A through R29 and R24, help keep any
DC offset to minimal amounts.
Audio signal voltage from U3:2, through R30, combines with the
triangle wave voltage, thereby changing the points, with respect to
the triangle wave, at which pin 2 crosses zero volts. This varies the
duty cycles of the Q and Qso they are proportional to the audio
signal voltage.
Amplifier Muting
The amplifier mutes protectively during turn-on and turn-off, as well
as when various overstresses occur. It mutes by disabling the PWM
pusle trains to the output FETs; when both FETs are off, the channel
passes no audio and power dissipation is minimal.
When bus RUN-A-LO is low, the output of U6:1 is high. It feeds flip
flop U7:2’s D input, and thus when the flip flop’s Q output goes high
it will allow U9’s four NAND gates to pass the PWM stream from
AMP_TRIG_A. RUN-B-LO, U6:2, U7:1, and U30 do the same for
channel 2.
Output Filter Network
A passive low-pass network comprising L1 and C64 integrate the
PWM stream on SW-A back into a fairly smooth audio waveform
with the switching-frequency components reduced about 40 dB. The
resulting audio spectrum deliverable to the loudspeaker load is
reasonably flat to beyond 20 kHz.
Inductor L2 and capacitor C71 form a parallel-resonant 250 kHz trap
that adds about another 15 dB of attenuation of the switching noise.
A zobel network, which comprises C78 and two high-power
resistors, R85 and R86, further stabilizes the output impedance. The
resistors, mounted under the large heat sink, are protected against
excessive high frequency output signals by the protective limiter
described next.
Protective Limiter
A limiter circuit reduces stresses on the amplifier channel when
certain conditions are detected. For example, a fast-rising stress like
excessive output current will be clamped first by the overcurrent
feedback but will trigger the limiter if it is sustained. Slower-
changing stresses like high temperature trigger limiting as the first
defense, and then muting if the limiter control voltage is driven to
its limit. Muting eliminates all significant stress and heat genera-
tion. When the clip limiter function is engaged, the limiter also
reduces the amount of distortion that occurs during clipping.
The gain reduction cell of the limiter circuit is op amp U3:1 and dual
LM13600M transconductance op amps U2:1 and U2:2, which act as
parallel variable negative feedback loops about U3:1. Normally, the
control current via R7 and R8 is zero, and so the transconductance
op amps are cut off and have no effect on the gain of U3:1.
Bus LIMITER-A is normally held at -5 V by R27. Several control
circuits—detecting excessive FET current, excessive temperature,
zobel network overload, and excessive power supply current—can
pull this voltage upward if necessary. For example, excessive high-
frequency voltage at bus ZOB-A, across R85 and R86 in the zobel
network, will cause Q4 to pull the LIMITER-A bus high.
As the LIMITER-A bus voltage gets pulled more positive, Q1, D1, and
Q2 conduct, feeding up to 2 mA of control current via R7 and R8.
The transconductance op amps U2:1 and U2:2 then will pass signal,
increasing the effective negative feedback around U3:1 and
reducing its gain.
This ciruict offers up to 40 dB of gain reduction. If the control current
exceed 2 mA, though, the voltage across R19 will push A-LIM to a
negative voltage and turn on Q3, which pulls bus A-MUTE-LO low,
muting the amplifier. Muting promptly relieves overloads such as
overcurrent, and the amp will cycle in and out of muting quickly. A
shutdown due to excess temperature will usually take 30 to 60
seconds to return to normal, during which muting will continue.
Thermal Sensing
A precision temperature-to-voltage converter, U10 is located in a
heat sink hole packed with thermal grease near channel 1’s power
devices. The sensor’s output voltage is amplified to a more usable
level by U11:1. The resulting thermal bus voltages A-THERM and
B-THERM change linearly from 6.84 V at 0º C to 3.41 V at 80º C, and
are used for several purposes: fan speed control, protective muting.
Fan Speed Control
In the fan speed control circuitry (schematic: see sheet “Protect/
Control, PL380” zone A-7), buses A-THERM and B-THERM are
buffered respectively by Q41 and Q38. When either channel’s
thermal bus voltage drops to below 4.4 V (at approximately 55° C),
its transistor, Q38 or Q41, will turn Q44 on. This will turn on Q47,
gradually increasing the fan voltage from 10 V. Higher temperatures
will further increase the fan voltage. The maximum fan voltage
available, which would be at temperatures at or about the point of
2.2 PL380 Major Circuit Blocks (continued)