JBL A6000GTi User manual

A6000GTi
Car Audio
Power Amplifier
Service Manual
2003 by Crown Audio, Inc., P.O. Box 1000, Elkhart, Indiana 46515-1000
U.S.A. Telephone: [phone]. Trademark Notice: Crownis a
registered trademark of Crown International, Inc; JBLis a registered
trademark of JBL. Other trademarks are the property of their respective
owners. 137159-1
11/03
Rev. 1

137159-1 Rev. 1 A6000GTi Service Manual
2003 Crown Audio, Inc. II
The information furnished in this manual does not include all of the details of
design, production, or variations of the equipment. Nor does it cover every
possible situation, which may arise during installation, operation or
maintenance. If you need special assistance beyond the scope of this
manual, please contact the JBL Technical Support Group.
This document contains data proprietary to Crown Audio, which was
developed at private expense. Crown Audio reserves all rights in
connection with this document and in the subject matter presented therein.
The recipient hereby acknowledges these rights and shall not, without
Crown Audio’s prior written permission, copy, disclose or divulge this
document in whole or in part to third parties or use it for any purpose other
than that for which it was delivered to the recipient.
TO PREVENT ELECTRIC
SHOCK DO NOT REMOVE TOP
OR BOTTOM COVERS. REFER
SERVICING TO QUALIFIED
SERVICE PERSONNEL.
TO REDUCE THE RISK OF ELECTRIC
SHOCK, DO NOT EXPOSE THIS
EQUIPMENT TO RAIN OR MOISTURE!
The exclamation point
triangle is used to alert
the user to important
operating or maintenance
instructions.

137159-1 Rev. 1 A6000GTi Service Manual
2003 Crown Audio, Inc. III
Revision History
Revision Number Date Comments
Rev. 1 11-2003 Initial Draft

137159-1 Rev. 1 A6000GTi Service Manual
2003 Crown Audio, Inc. IV
This page intentionally left blank

137159-1 Rev. 1 A6000GTi Service Manual
2003 Crown Audio, Inc. V
Table of Contents
1. INTRODUCTION ..................................................................................................... 1-1
1.1. THE A6000GTI CAR AMPLIFIER ........................................................................... 1-1
1.2. SCOPE............................................................................................................... 1-1
1.3. WARRANTY......................................................................................................... 1-1
2. SPECIFICATIONS................................................................................................... 2-1
2.1. OUTPUT POWER ................................................................................................. 2-1
2.2. PERFORMANCE................................................................................................... 2-1
2.3. CONTROLS &CONNECTORS................................................................................. 2-2
2.4. INDICATORS........................................................................................................ 2-2
2.5. INPUT/OUTPUT.................................................................................................... 2-2
2.6. PROTECTION ...................................................................................................... 2-3
3. THEORY OF OPERATIONS ................................................................................... 3-1
3.1. AUDIO SIGNAL PATH............................................................................................ 3-1
3.1.1. Input Stage................................................................................................ 3-1
3.1.2. Error Amplifier............................................................................................ 3-2
3.1.3. Modulator .................................................................................................. 3-2
3.1.4. Triangle Generator..................................................................................... 3-2
3.1.5. Output........................................................................................................ 3-3
3.1.6. Output Filter............................................................................................... 3-4
3.2. PROTECTION AND CONTROL CIRCUITRY ................................................................ 3-5
3.2.1. Current Monitor.......................................................................................... 3-5
3.2.2. Current Limiter........................................................................................... 3-5
3.2.3. Display Circuitry......................................................................................... 3-5
3.2.4. Thermal Limit Control (TLC) ...................................................................... 3-6
3.3. POWER SUPPLY.................................................................................................. 3-7
3.3.1. Switching Power Supply Operation............................................................ 3-7
3.3.2. Switching Power Supply Protection Circuit ................................................ 3-7
3.3.3. (LVPS) Low Voltage Power Supply Operation ........................................... 3-7
4. MAINTENANCE ...................................................................................................... 4-1
4.1. WHERE TO BEGIN................................................................................................ 4-1
4.2. SURFACE MOUNT TECHNOLOGY........................................................................... 4-1
4.3. DISASSEMBLY FOR INSPECTION &SERVICE............................................................ 4-2
4.3.1. Top Cover Removal................................................................................... 4-2
4.3.2. Power Supply Discharge............................................................................ 4-4
4.3.3. Power Connection Housing Removal ........................................................ 4-5
4.3.4. Pip Assembly Removal.............................................................................. 4-6
4.3.5. Power Supply Assembly Removal............................................................. 4-7
4.3.6. BCA Assembly Removal............................................................................ 4-7
4.3.7 BCA Disassembly...................................................................................... 4-8
4.4. TROUBLESHOOTING............................................................................................. 4-9
4.4.1. Power Supply ............................................................................................ 4-9
4.4.2. Audio Section Trouble Shooting............................................................... 4-10

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4.4.3. Fault Modes............................................................................................. 4-10
4.5. REQUIRED TEST EQUIPMENT.............................................................................. 4-12
4.6. ELECTRICAL CHECKOUT PROCEDURE ................................................................. 4-12
4.6.1. Quiescent Current Draw .......................................................................... 4-13
4.6.2. Sensitivity ................................................................................................ 4-13
4.6.3. Bridge Mono Operation............................................................................ 4-13
4.6.4. Frequency Response............................................................................... 4-13
4.6.5. Short Circuit Test..................................................................................... 4-13
4.6.6. Output Power........................................................................................... 4-13
4.7. CALIBRATIONS .................................................................................................. 4-14
4.7.1. Amplifier Power up:.................................................................................. 4-14
4.7.2. DC offset adjustment: .............................................................................. 4-14
4.7.3. Overlap correction: .................................................................................. 4-14
4.7.4. Current Monitor adjustment: .................................................................... 4-14
5. PARTS .................................................................................................................... 5-1
5.1. GENERAL INFORMATION....................................................................................... 5-1
5.2. ORDERING AND RECEIVING PARTS........................................................................ 5-1
5.2.1. Terms........................................................................................................ 5-1
5.2.2. Shipment ................................................................................................... 5-1
5.3. MECHANICAL PARTS............................................................................................ 5-1
5.4. CIRCUIT BOARD PARTS........................................................................................ 5-2
5.4.1. Circuit Board and Schematic Part Numbers............................................... 5-2
6. SCHEMATICS......................................................................................................... 6-1

137159-1 Rev. 1 A6000GTi Service Manual
2003 Crown Audio, Inc. Introduction
1-1
1. Introduction
This manual contains complete service information on the JBL A6000GTi power amplifier.
It is designed to be used in conjunction with the A6000GTi Owner’s Manual; however,
some important information is duplicated in the Service Manual.
NOTE: THE INFORMATION IN THIS MANUAL IS INTENDED FOR USE
BY AN EXPERIENCED TECHNICIAN ONLY!
1.1. The A6000GTi Car Amplifier
The A6000GTi is an extremely high output audio amplifier designed for automotive use,
providing high power amplification from 20Hz –400Hz with minimum distortion. The
A6000GTi features Crown’s patented, awarded-winning BCA(Balanced Current
Amplifier) engineering, for superior power output, increased efficiency, legendary Crown
sound and extraordinary reliability. Other features include parallel mode operation for
driving loads below 2 Ohms, selectable 12/24 dB/octave crossover slope, phase
adjustment and DBO (Dynamic Bass Optimization).
1.2. Scope
This Service Manual is intended to apply to all versions of the A6000GTi amplifier. The
Parts Listings include parts specific for all versions. Parts are available from the Crown
Parts Department.
1.3. Warranty
Each Reference Manual contains basic policies as related to the customer. In addition, it
should be stated that this service documentation is meant to be used only by properly
trained personnel. All warranty service should be referred to the Crown Factory or JBL
Authorized Warranty Service Center. See Figure 1.1 for a copy of the Warranty. To find
the location of the nearest Authorized Warranty Service Center or to obtain instructions for
receiving Factory Service, please contact the JBL Technical Support Group. If you are an
Authorized Warranty Service Center and have questions regarding the warranty of a
product, please contact the JBL Technical Support Group.
JBL Technical Support
Phone: [phone]JBL
Phone: [phone]
Web Page: http://www.jbl.com
Customer Support
Factory Service
Parts Department
Mailing Address: P.O. Box 1000, Elkhart, IN 46517
Shipping Address: Plant 2 S. W.
1718 W. Mishawaka Rd., Elkhart IN 46517
Phone: [phone]
Fax: [phone]
http://www.crownaudio.com

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2003 Crown Audio, Inc. Introduction
1-2
Figure1.1 JBL Warranty

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2003 Crown Audio, Inc. Specification 2-1
2. Specifications
2.1. Output Power
A6000GTi Power Chart
Maximum power @ 100Hz With 0.1% THD
2 ohm Dual 3000W
4 ohm Bridged 6000W
1 ohm Parallel 6000W
Load Impedance: Safe with all types of loads. Rated for 2 ohms in dual mode, 4 ohms in
Bridged and 1 ohm in Parallel.
Required Battery Voltage: 10.0-16.0 VDC
DC Line Current:
For 6000W: 700A;
At idle: Amp draws 12.0A and no more than 200 watts.
2.2. Performance
Frequency Response: ±3dB from 10Hz to 200Hz at 1 watt.
Phase Response: 0 to 180 degrees deviation from 20Hz to 320Hz.
Signal to Noise Ratio, A-Weighted, 20Hz to 320Hz: Better than 100dB below rated
100Hz power.
Total Harmonic Distortion (THD): 100Hz rated power 0.1% or less THD
Damping Factor: Greater than 1000 from 10Hz to 400Hz
Common Mode Rejection (CMR): Better than 70dB from 20Hz to 100Hz.

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2.3. Controls & Connectors
Power: REM voltage of 5.0VDC will power up the amplifier. REM voltage of less than
3.0VDC will power down the amplifier.
Mode: Turn power off before switching. A two-position switch located in the middle of the
input panel, when switched to 2CH/BR operates the amplifier either in two “mono”
channels or one bridge channel. When switched to PARALLEL, the amplifier joins the two
output channels to deliver twice the output current.
Sensitivity: A continuously variable rotary level control that when turned to the maximum
setting with an input of .250mV RMS the amplifier will deliver maximum output power.
When turned to the minimum setting with an input of 8.0Vrms the amplifier will deliver
maximum output power.
Slope: Two-position switch used to control filter rolloff. Switch in 12 pos yields 12dB per
octave rate of rolloff. Switch in 24 pos yields 24dB per octave rate of rolloff.
LP Freq: Crossover frequency adjustment from 32Hz to 320Hz.
DBO: Dynamic Bass Optimization. Two-position switch turns DBO on and off.
Continuous rotary adjustment to increase/decrease bass level. Another adjustment for HP
rolloff.
2.4. Indicators
Signal: A blue LED, which flashes when a very low-level signal is present at input.
Clip: A red LED, which turns on when distortion becomes audible in the amplifier output.
Fault: Normally off, this red indicator will light if amplifier output stage becomes non-
operational.
Power: A blue LED that turns on when the amplifier has been turned on and has power.
2.5. Input/Output
Input Connector: One RCA connector for each channel.
Input Impedance: 25k ohms
Output Connectors: Custom gold plated terminals with set screws.
Power Connectors: Custom gold plated terminals with set screws accepting 0 gauge
hookup wire.
Wiring Diagram: See A6000GTi Owner’s Manual

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2.6. Protection
A6000GTi amplifiers are protected against shorted, open or mismatched loads, overloaded
power supplies, excessive temperature, input overload damage and high frequency
blowups. They also protect loudspeakers from input/output DC, large or dangerous DC
offsets and turn-on/turn-off transients.
Cooling: 3 fans
Dimensions: Width 23.250”, Length 29.350” and Height 6.50”
Weight: Net weight is 90lb(40.8kg). Shipping weight is 143 lbs (64.9kg).

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2003 Crown Audio, Inc. Theory Of Operations 3-1
3. Theory of Operations
3.1. Audio Signal Path
3.1.1. Input Stage
Signal is presented to the A6000GTi through one of two RCA connectors. Each RCA
connector is a balanced input. Each input is immediately treated with a ferrite, to block any
high frequency noise. A TVS (Transient Voltage Suppressor) and DC blocking cap are
also included to protect the amplifier against high voltage and DC voltage levels. The
optoisolator (U103, U203) acts to compress the input level if needed to protect the
amplifier from dangerous conditions such as over temperature, clip conditions or DCLF
(DC or low frequency). The signal is then converted from balanced to unbalanced in the
Balanced Input Stage (U100).
After the two channels have been converted to unbalanced, they are summed together to
form a single mono signal that will continue through the amplifier. At this same point in the
circuit, the channel one and two signals are also sent to the op amp buffer stage of U104,
where some gain is added. The signals are then passed thru the photomos relay K100.
When the amplifier is powered on, the relay closes and routes the signal to the pass thru
RCA connectors on the input panel.
The signal is then passed through the low pass Butterworth filter. The pot (R131) is used
to adjust the signal phase from 0to 180. The switch (S100) is used to switch in a
second filter stage, making a fourth order filter. After the filter stage, the signal is passed
thru the DBO (Dynamic Bass Optimization) stage. In this stage S101 is used to turn DBO
on and off, R141 is used to adjust the DBO frequency between 20Hz and 80Hz, and R143
is used to adjust the DBO boost (gain) between +1dB and +12dB.
Following the DBO circuitry, the signal passes thru the VCA control (U105). This is where
the remote level control is used to adjust the gain. The VCA is followed by U100, which
acts as a buffer/driver and sends the signal to both the channel 1 and channel 2 circuitry
on the input card.
For the sake of simplicity, from this point on only channel one of the audio signal path is
described.
Inherent in all PWM amplifiers is a rise in gain at higher frequencies. Because of this, a
800Hz 6th order Gaussian low pass filter has been included in the input stage of the
A6000GTi (on the input PWA). The Gaussian filter-type is unique in that it has minimal
ringing and excellent phase response so even a high-order filter such as this one does not
adversely affect the sonic excellence of the product. U201-A, -B, -C and –D comprise this
filter.

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3.1.2. Error Amplifier
The signal next enters the main amplifier error amp (U500-D) where it is mixed with a
small portion of the output voltage and current in such a way as to control the amplifier’s
overall output performance. From the error amplifier, the signal is divided and fed to the
modulator. Since the modulator circuit is balanced, the drive signal for the positive
modulator is inverted by U500-A.
3.1.3. Modulator
U502 and U505 are high-speed differential comparators. U502 is the positive comparator
and U505 is the negative comparator. The comparator section has two outputs: inverting
and non-inverting. The output is therefore balanced. The audio signal is applied to the
inverting input of both differential comparators (with the positive modulator receiving the
audio out of phase from the negative modulator). The 125kHz triangle wave (described
below) is applied to the non-inverting input of both differential comparators. With no audio
signal, the bipolar triangle wave is therefore compared to a zero-volt signal and this results
in a 125kHz square wave pulse train that is passed to the NAND gate section of the
differential comparator.
The balanced output of U502 forms the positive portion of the output waveform (Vp). The
output of U505 is also balanced and forms the negative portion of the output waveform
(Vn). These two balanced signal lines are routed to the output stage drivers, U301 and
U302. If an audio signal is present at the inputs of the modulators, the triangle wave will
be compared to a varying signal at the comparators and the outputs of the NAND gates
will be a 125kHz pulse train in which the widths of the pulses vary with the audio
amplitude.
This operation is described as Pulse Width Modulation (PWM), as used in the BCA
amplifier.
3.1.4. Triangle Generator
The 125 kHz triangle wave has its origins from the 4MHz crystal oscillator (Y40). A seven-
stage counter (U49) is used as a divide by. The output Q3 is a divide by 8 (500kHz), Q4 is
a divide by 16 (250kHz) and Q5 is a divide by 32 (125kHz). The clock is then buffered by
the NAND gate (U47).
After U509-A divides the 250kHz square wave down to 125kHz, the square wave is sent
thru a parallel combination of hex inverters (U505-A thru –F). Multiple inverters are
required to provide sufficient drive to the next stage. The transistors Q502 –Q509 form a
low noise discrete op amp. This circuit converts the signal from a 0V-5V square wave to a
–5V to +5V triangle wave that is extremely accurate. The triangle wave is finally routed to
the modulators.

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3.1.5. Output
The PWM modulated 125kHz signals exit the modulators and enters a voltage translator
formed by Q301 –Q310. This string translates the balanced PWM signals into a single
ended waveform with increased amplitude for the FET driver (U301 and U302). The FET
driver’s output is increased by a two transistor discrete amplifier stage for improved fan out
capability.
The FET driver (U301) output is referenced to Vp+14 and Vp. The Vp+14 floating supply
is derived from +Vcc on the main PWA. Therefore, it is proportional to Vcc and varies as
the +/- Vcc levels vary.
We pause now for a qualitative description of output stage operation (refer to Figure 3.1).
All parts to the left of “Iout” are positive or “p” side parts and all parts to the right of “Iout”
are negative or “n” side parts. The switches Swp and Swn are analogous to the FETs.
Quiescent operation will be considered first. At the start of a switching cycle Swp and Swn
are both turned on. Current flows from +Vcc, through Swp, through Lp, through Ln,
through Swn, and down to –Vcc. The current rises at a controlled rate in the Lp and Ln
inductors (see the current diagram to the right in Figure 3.1). Halfway through the cycle,
the switches turn off but the inductors have reached a certain current flow (the peaks on
the diagram) and now must continue pushing current in the same direction. The current
continues to flow in the same direction through the inductors but comes through the diodes
because the switches are open. Specifically, current flows from –Vcc through D1, through
Lp, through Ln, through D2, and to +Vcc while ramping down. At quiescent, the Lp and Ln
currents cancel so the net voltage developed at Iout across the capacitor is zero. For
positive voltage output, Swp is left on longer than Swn and for a negative output voltage,
the opposite occurs. Note that for any output voltage, the “on” time of both switches will
overlap, even if one is on longer than the other.
Figure 3.1 BCA Operation

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Now, back to the description of output circuit flow: after the FETs, the signal is then routed
to the BCA filter inductors (Lp and Ln). The combining point (B1) is then sent to the Output
Filter.
With no audio modulation, the PWM pulse train that is sent to the gates of each set of
FETs is the same and the duty cycle is approximately 50%. This means that for no audio
input, the positive FETs (Q301, Q302, Q303, Q304) turn on and off at the same time as
the negative FETs (Q305, Q306, Q307, Q308). As mentioned before, the Lp and Ln
inductors control the rate of rise of the current during this time so that this does not act like
a short during the couple of microseconds that they are on. The time that the FETs are on
ramps current up in the inductors, which stores energy in them. This energy is released
through the diodes (D301-A, D301-B, D302-A, D302-B) when the FETs are turned off.
The Vp and Vn nodes swing between –Vcc and +Vcc with a duty cycle of approximately
50%.
Operating the output stage in this way causes the current in the Lp and Ln inductors to
completely cancel at the B1 node. This drastically reduces the filtering requirements
necessary after the output stage.
If an audio signal is present at the input of the amplifier, the widths of the pulses at Vp and
Vn change but the amplitude of the pulses stays at 2*Vcc. Positive audio signals will
cause the positive side of the output stage to be on more than 50% of the time while the
negative side will be on less than 50% of the time. The total “on-time” will add to
approximately 100%. Also, the on-time of the two polarities will overlap so that when the
negative side is on, the positive side is also. For negative-going signals, the action is
reversed (swap positive and negative). The result at the summing junction B1 is an
amplified version of the input signal, with some noise superimposed on it. The B1 signal is
then routed to the Output Filter.
3.1.6. Output Filter
The output filter is made up of two individual filters: a 62.5kHz filter (L800,C823-C831) and
a 125kHz filter (L801, C832-C836). The signal passes through these two filters to
eliminate both residual 125kHz and 62.5kHz signals. The audio is then sent out the 6-wire
cable to the output terminator board. The output terminator board connects the output
signal to the amplifier output terminals.

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3.2. Protection and Control Circuitry
3.2.1. Current Monitor
Audio output current levels are monitored by the use of transformer T801. A small primary
winding is in series with the output current and the secondary develops a voltage across
the input of U512-C. This op amp provides a current monitor signal that is scaled to
approximately 5Amps/1Volt. This output current is used for the following purposes:
1. High Frequency Feedback
2. Low Frequency Feedback
3. Current Limit LED
3.2.2. Current Limiter
To prevent excessive output current, the A6000GTi incorporates an active current limit
circuit for each channel. Resistors (R809-R811, R812-R814) in series with the Lp and Ln
output coils are used to sense the output current. The voltage created by the output
current thru the resistors is compared to a reference voltage set by R801 & R802. These
voltages are compared on U801. If either the Lp or Ln current reaches the maximum
permitted output current, the output pin of the comparator goes low, which pulls the I LIMIT
signal low thru an optocoupler (U800). This signal goes to the front-end board and is used
to turn the channel off. When the current out of the channel fails below the limit, the
channel is enabled. This method of current limit allows the amplifier to play thru current
limit situations without severely degrading the sound.
3.2.3. Display Circuitry
After the input signal leaves the input board, it is routed to the signal detection circuit. With
a signal approximately 45 dB below that required for full output, the signal; LED will
illuminate. Note that this LED will indicate whether signal is present at the amplifier but not
whether it is being output from the amplifier.
The compressor circuit is initiated by either the Input-Output Comparator (IOC) circuit or by
the Thermal Limit Control (TLC) circuit. The IOC circuit is on the front-end board, and
monitors the error amp signal. If the error amp signal becomes too large, indicating too
much distortion on the output, the IOC signal will drop to –15 volts. This will cause the
compressor drive circuit on the input board to compress the inputs of the amplifier. U102
on the input board turns on Q100, which causes U103 to become active. U103 shunts the
input signals together, thereby limiting the input to the amplifier. IOC will cause the CLIP
LED to light only if it occurs long enough to be audible. The TLC circuit lights the
THERMAL LED when the audio section is beginning to overheat. The LED can light before
the TLC circuit begins to compress. The TLC circuit will be discussed in another section.
The FAULT LED indicates that something catastrophic has happened to the amplifier.
When the amplifier senses a fault condition, the audio section and power supply are both

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2003 Crown Audio, Inc. Theory Of Operations 3-6
disabled, and a crowbar circuit is employed to drain the rail voltage to prevent any further
damage.
3.2.4. Thermal Limit Control (TLC)
The TLC circuit is used to control the output of the amplifier while monitoring the
temperature of the amplifier output devices and of the power supply switching devices.
When the devices begin to overheat, the TLC circuit begins to compress the input signal,
scaling back the output power until the unit is capable of cooling itself again.
On the input board, U104-C and U104-D form a window comparator with the input signal
and two DC voltages created by U110-A and U110-D. As the unit begins to heat up, the
window narrows, and when the input signal reaches the upper or lower limit on the window
comparator, the compressor drive circuit is activated and the input is compressed in the
same manner as IOC.

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3.3. Power Supply
3.3.1. Switching Power Supply Operation
The switching power supply is designed to take 12-volts in and convert it to +/- 150-volts
out using a full bridge topology. The ring of mosfets around the transformer in the middle of
the supply can be divided into 6 parallel H bridges, with each bridge having 2 mosfets per
leg (for a total of 48 mosfets). The low voltage supply is a 48W push-pull. It supplies the
large power supply and audio section with +/- 24-volts.
The large power supply is specially designed to handle the high current demand of the
audio section. The buss bars provide a low impedance path for the current into the full
bridge. The orientation of the power mosfets maintains that low impedance path to
maintain high efficiency.
When the REM signal goes high, the low voltage power supply enables and generates the
+/-24 volt rails. Once the voltage is about +/- 20 volts, the large power supply is enabled.
The 6 parallel H bridges begin switching in unison. The PWM chip for the large power
supply has soft start circuitry to decrease the inrush current demands of the amplifier. The
H bridges create a 24 volts peak-to-peak square wave at a frequency of approximately
1000 Hz. The transformer has a turns ratio of 12:1, and the output voltage is stepped up
to 288 volts peak to peak. The AC voltage is rectified to +/-144 volts DC. These are the rail
voltages for the audio section.
3.3.2. Switching Power Supply Protection Circuit
The power supply is equipped with crowbar circuitry to disable itself and drain the rails if
the amplifier senses DC on the speaker outputs.
The power supply will also protect itself from over voltage and under voltage situations.
The power supply is unregulated, so as battery voltage increases, rail voltage increases.
For 16 volts in, the rail voltage will be 192 volts. The rail capacitors are only rated for 200
volts, so the power supply will shut down to prevent an over voltage situation. The power
supply is also equipped with thermal circuitry to compress the audio section when the
power supply is overheating. This decreases the power demand and allows the power
supply to cool down.
3.3.3. (LVPS) Low Voltage Power Supply Operation
The low voltage power supply is a switch mode power converter that uses a push pull
topology. U8 is a SG3525AP pulse width modulator (PWM). The PWM regulates the
output voltage to +24 volts by controlling the switching times of Q50 and Q54, which are
IRF540N mosfets rated for 100V 33A. The transformer, T2, has a center taped primary
and secondary. The PWM alternates the on time of Q50 and Q54 at a frequency of
125kHz, synchronized with the main system clock, to apply voltage to their respective
sides of the primary. This alternating voltage is multiplied by the turns ratio of the

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transformer, then rectified by the diodes on the secondary (D8, D9, D11, D12), and filtered
by L7 with C43 and by L8 with C49 to give the +/-24 volt outputs. The +24 volt is monitored
by the PWM thru the resistor divider formed by R60 and R59.
The PWM is equipped with soft start circuitry and pulse-by-pulse current limit to prevent
the core from saturating. The current limit is accomplished by comparing the voltage
across R58, which is a 0.04 ohm resistor, to a set voltage limit of 0.65 volts. U18 is the
comparator that performs this function. The output of U18 is tied to the shutdown pin of the
PWM and to a 5-volt pull up resistor.
Note: The supply is designed to operate in continuous current mode (CCM),
meaning it always supplies current on both the plus and minus 24-volt rails. If the
power supply is operated without the audio section connected thru the ribbon cable,
the –24 volt rail goes into discontinuous current mode, and the voltage jumps to –
50 volts. The filter capacitors on the negative rail are only rated to 35 volts, so
operating in this mode for more then a few seconds will require the replacement of
C53 and C54.

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4-1
4. Maintenance
4.1. Where to Begin
Effective repair involves three basic steps:
1) Determine the symptom(s) of the problem
2) Identify the cause(s) of the symptom(s)
3) Repair the unit to eliminate the cause(s)
To determine the symptoms, you will want to get as much information from the user as
possible. Get as much information as you can about the system and how the amplifier is
used. There is always the possibility that the problem will show up only if used in a
specific way.
Once you have all the information about the symptom(s), it is time to inspect the amplifier.
A careful visual inspection is valuable for most problems, which you may encounter. To
inspect the inside of the amplifier remove the cover as described in Section 4.3.1.
Begin the inspection by looking for anything abnormal, like loose connectors, broken wires
and burnt or visibly damaged components. Inspect the printed circuit assemblies for
broken traces and loose connections. Be thorough. The time you spend visually
inspecting the amp is time well spent.
4.2. Surface Mount Technology
The A6000GTi amplifier uses surface mount technology in its design. There are several
advantages to using surface mount technology (SMT), including; (1) surface mount
devices (SMDs) are much smaller, and are mounted to the surface of the board, so more
components can be placed on the board. (2) Components can be attached to both sides
of the board, allowing the board size to be reduced. (3) SMT boards are lighter and
provide better electrical performance and signal speed.
Of course, there are also things to watch out for with SMT. (1) The placement of the
components on the board, not through a hole, makes the components and solder joint
more susceptible to damage. (2) Rework of SMDs can often require specialized tools,
equipment, or training. (3) SMDs are very small and can be difficult to handle, see, and
identify.
Remember that on some of the boards, the SMDs are GLUED to the bottom of the board.
This is done for manufacturing purposes. Take care not to damage components while
trying to remove them from the surface of the module.