Crown Macro-Tech MA-2402 User manual

POWER AMPLIFIER
SERVICE MANUAL
130445-1
02-00
Rev. A
Macro-Tech®
©2000 by Crown International, Inc., P.O. Box 1000, Elkhart, Indiana 46515-1000 U.S.A.
Telephone: [phone]. Trademark Notice: PIP
™,
Grounded Bridge
™and
SmartAmp
™are
trademarks and Amcron
®
, Crown
®,
Macro-Tech
®,
IOC
®,
ODEP
®, and
IQ System
®are registered
trademarks of Crown International, Inc. Other trademarks are the property of their respective
owners.
Models:
MA-2402
Some models may be exported under the name Amcron
®

MA-2402 Service Manual
II
130445-1 Rev. A
©2000 Crown International, Inc.
À PRÉVENIR LE CHOC
ÉLECTRIQUE N’ENLEVEZ
PAS LES COUVERTURES.
RIEN DES PARTIES
UTILES À L’INTÉRIEUR.
DÉBRANCHER LA BORNE
AVANT D’OUVRIR LA
MODULE EN ARRIÈRE.
TO PREVENT ELECTRIC SHOCK DO
NOT REMOVE TOP OR BOTTOM
COVERS. NO USER SERVICEABLE
PARTS INSIDE. REFER SERVICING
TO QUALIFIED SERVICE
PERSONNEL. DISCONNECT
POWER CORD BEFORE REMOVING
REAR INPUT MODULE TO ACCESS
GAIN SWITCH.
CAUTION AVIS
WARNING
TO REDUCE THE RISK OF ELECTRIC
SHOCK, DO NOT EXPOSE THIS
EQUIPMENT TO RAIN OR MOISTURE!
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 Crown
Technical Support Group.
Mail: P.O. Box 1000 Elkhart IN 46515-1000
Shipping: Plant 2 SW 1718 W. Mishawaka Road Elkhart IN 46517
Phone: [phone]
FAX: [phone]
The lightning bolt
triangle is used to
alert the user to the
risk of electric shock.
The exclamation point
triangle is used to alert the
user to important operating
or maintenance instructions.

III
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
Revision History
Revision Number Date Comments
Rev. A 02-2000 Initial Printing

MA-2402 Service Manual
IV
130445-1 Rev. A
©2000 Crown International, Inc.
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V
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
Table of Contents
1 Introduction ............................................................................ 1-1
1.1 Introduction ................................................................................1-1
1.2 The MA “02” Series Amplifiers ...................................................1-1
1.3 Scope.........................................................................................1-1
1.4 Warranty.....................................................................................1-1
2 Specifications ......................................................................... 2-1
3 Voltage Conversion................................................................ 3-1
4 Circuit Theory ........................................................................ 4-1
4.1 Overview ....................................................................................4-1
4.2 Features .....................................................................................4-1
4.3 Front End Operation...................................................................4-1
4.3.1 Balanced Gain Stage (BGS) .............................................4-1
4.3.2 Variable Gain Stage (VGS)................................................4-1
4.3.3 Error Amp ..........................................................................4-1
4.4 Voltage Amplification .................................................................4-2
4.4.1 Voltage Translators ...........................................................4-2
4.4.2 Last Voltage Amplifiers (LVAs)..........................................4-2
4.5 Grounded Bridge Topology .......................................................4-2
4.5.1 High Side (HS) ..................................................................4-2
4.5.2 Low Side (LS) ....................................................................4-3
4.6 Output Device Emulation Protection (ODEP).............................4-4
5 Maintenance........................................................................... 5-1
5.1 Cautions and Warnings..............................................................5-1
5.2 General Information ...................................................................5-1
5.3 Test Procedures........................................................................5-1
5.3.1 Turn On Delay ..................................................................5-1
5.3.2 Output Bias Adjustment ...................................................5-1
5.3.3 ODEP Voltage Adjustment ...............................................5-1
5.3.4 DC Offset...........................................................................5-2
5.3.5 Quiescent Power Draw.....................................................5-2
5.3.6 Voltage Gain.....................................................................5-2
5.3.7 Current Limit.....................................................................5-2
5.3.8 10 kHz Square Wave Response ......................................5-3
5.3.9 Crosstalk ..........................................................................5-3
5.3.10 Output Power .................................................................5-3
5.3.11 ODEP Limiting ................................................................5-3
5.3.12 Fan Operation ................................................................5-4
5.3.13 LF Protection ..................................................................5-4
5.3.14 Signal to Noise ...............................................................5-4

MA-2402 Service Manual
VI
130445-1 Rev. A
©2000 Crown International, Inc.
5.3.15 Intermodulation Distortion (IMD) ....................................5-4
5.3.16 Displays..........................................................................5-4
5.3.17 Post Testing....................................................................5-4
6 Parts ....................................................................................... 6-1
6.1 General Information ..................................................................6-1
6.2 Ordering and Receiving Parts ..................................................6-1
6.2.1 Terms .....................................................................................6-1
6.2.2 Shipment ................................................................................6-1
7 Exploded View Parts ............................................................. 7 -1
8 Module and Schematic Information..................................... 8 -1
9 Module Parts.......................................................................... 9-1
10 Schematics ........................................................................ 10-1
Table of Contents

Introduction 1-1
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
1.1 Introduction
This manual contains complete service information
onthe Crown
®MA-2402poweramplifier. Itisdesigned
to be used in conjunction with the Reference Manual;
however, some important information is duplicated in
this Service Manual in case the Reference Manual is
not readily available.
NOTE: THE INFORMATION IN THIS MANUAL IS
INTENDEDFORUSEBYANEXPERIENCEDTECH-
NICIAN ONLY!
1.2 The MA “02” Series Amplifiers
The Macro-Tech
®series is a complete family of am-
plifiersdesignedfor prosoundreinforcement.Macro-
Tech amplifiers are designed to provide enormous
levelsofpure,undistorted power in a rugged low-pro-
file package, utilizing Crown's patented Grounded
Bridge ™ output topology. They also employ Crown's
patented ODEP
®protection circuitry, which keeps the
amplifierworkingunderextremeconditionsthatwould
shutdownalesseramplifier.Crown'snew Macro-Tech
“02” series amplifiers feature Crown's enhanced
PIP2 ™ (Programmable Input Processor) expansion
system. The PIP2 expansion system makes it easy to
tailor the amplifier to a specific application. Providing
high power amplification from 20 Hz to 20 kHz with
minimumdistortion,Macro-Techseriesamplifiers fea-
ture balanced inputs with bridged and parallel mono-
phonic capability. Specific features vary depending
on model.
1.3 Scope
This Service Manual in intended to apply to all ver-
sions of the MA-2402 amplifier. The Parts Listings in-
clude parts specific for the US version and the Euro-
pean version (E13CE). For parts specific only to other
versions contact the Crown Technical Support Group
for help in finding part numbers.
1.4 Warranty
Each Reference Manual contains basic policies as
relatedtothecustomer.Inaddition,itshouldbestated
that this service documentation is meant to be used
only by properly trained personnel. Because most
Crown products carry a 3-Year Full Warranty (includ-
ing round trip shipping within the United States), all
warrantyserviceshouldbereferred to the CrownFac-
tory or Authorized Warranty Service Center. See the
applicable Reference Manual for warranty details. To
find the location of the nearest Authorized Warranty
Service Center or to obtain instructions for receiving
Crown Factory Service, please contact the Crown
Technical Support Group (within North America), or
yourCrown/AmcronImporter(outsideNorthAmerica).
If you are an Authorized Warranty Service Center and
have questions regarding the warranty of a product,
pleasecontacttheFieldServiceManagerortheTech-
nical Support Group.
Crown Customer Service
Technical Support Group
Factory Service
Parts Department
Mailing Address: P.O. Box 1000, Elkhart IN 46515
Shipping Address: Plant 2 S. W.
1718 W. Mishawaka Rd., Elkhart IN 46517
Phone: [phone]
Toll Free: [phone]
Fax: [phone]
http://www.crownaudio.com
1 Introduction

MA-2402 Service Manual
1-2 Introduction
130445-1 Rev. A
©2000 Crown International, Inc.
Figure 1.1 MA-2402 Front and Rear Views

Specifications 2-1
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
6 Specifications
Note: Specifications relate to 120-volt,60-Hz units in Stereo mode with 8-ohm loads and
an input sensitivity of 26-dB gain at 1-kHz rated power unless otherwise specified.
Specifications for units supplied outside the U.S.A. may vary slightly at different AC
voltages and frequencies.
Power
Output Power:
Load Impedance: Safe with all types of loads. Rated for 2 to 16
ohms in Stereo, 4 to 16 ohms in Bridge-Mono and 1 to 4 ohms in
Parallel-Mono mode.
Voltage Gain to 1-kHz, 8-ohm rated output;
83:1 ±12% or 38 dB ±0.5 dB at 0.775-volt sensitivity;
46:1 ±12% or 33 dB ±0.5 dB at 1.4-volt sensitivity.
20:1 ±3% or 26 dB ±0.25 dB at 3.1-volt sensitivity at the maximum
level setting.
Required AC Mains: 50/60 Hz; 100-, 120-, 220- and 240-VAC
(±10%) units are available. All draw 100 watts or less at idle.
Current, voltage and frequency requirements are provided on the
unit’s back-panel.
AC Line Connector: NEMA 5-20P (20A).
Performance
Frequency Response: ±0.1 dB from 20 Hz to 20 kHz at 1 watt (see
Figure 2.5).
Phase Response: ±10 degrees from 10 Hz to 20 kHz at 1 watt (see
Figure 2.3)..
Signal-to-Noise Ratio, A-weighted:
Better than 105 dB below rated full bandwidth power.
20 Hz to 20 kHz:
Better than 100 dB below full bandwidth power.
Total Harmonic Distortion (THD): full bandwidth power, Less than
0.05% from 20 Hz to 1 kHz increasing linearly to 0.1% at 20 kHz.
Intermodulation Distortion (IMD): (60 Hz and 7 kHz at 4:1) Less
than 0.05% from 163 milliwatts to full bandwidth power.
Damping Factor: Greater than 1,000 from 10 Hz to 400 Hz (see
Figure 2.6).
Crosstalk: See Figure 2.4.
Slew Rate: (Slew rates are limited to useful levels for ultrasonic/RF
protection). Greater than 13 volts per microsecond.
Controls
Enable: A front-panel push button used to turn the amplifier on and
off.
Level: A front-panel 31-detent rotary control for each channel used
to control the output level.
Stereo/Mono: A three-position back-panel switch used to select
Stereo, Bridge-Mono or Parallel-Mono mode.
Sensitivity: A three-position switch inside the PIP compartment
used to select the input sensitivity for both channels: 0.775 volts or
1.4 volts for standard 1-kHz power, or a 26 dB voltage gain.
Reset: A back-panel push button for each channel used to reset the
circuit breaker that protects each power supply.
Indicators
Enable: An amber front-panel indicator that shows the on/off status
of the low-voltage power supply.
Signal/IOC: A green front-panel indicator for each channel that
flashes to show amplifier output. If a channel’s output waveform
differs from its input by 0.05% or more, the indicator flashes
brightly to show distortion. This function provides proof of
distortion-free performance. In Parallel-Mono mode, the Channel 2
light stays on.
ODEP: An amber front-panel indicator for each channel that shows
thermal-dynamic energy reserve. Normally, each ODEP indicator is
lit to show available reserve energy. In the rare event that a channel
has no reserve, its indicator will dim in proportion to ODEP limiting.
An ODEP indicator may also turn off under other conditions.
Input/Output
Input Connector: Two balanced ¼-inch(6.35-mm) phone jacks and
two balanced three-pin female XLR connectors on the factory-
installed PIP2-FXQ.
Input Impedance: Nominally 20 k ohms, balanced. Nominally 10 k
ohms, unbalanced.
Maximum Input Level: 9 Vrms.
Input Sensitivity: 0.775 volts for standard 1 kHz power, 1.4 volts
for standard 1 kHz power, or a 26 dB voltage gain.
Output Connectors: Two sets of color-coded 5-way binding posts
(for banana plugs, spade lugs or bare wire).
Output Impedance: Less than 10 milliohms in series with less than
2 microhenries (see Figure 2.7).
DC Output Offset: (Shorted input) ±10 millivolts.
Output Signal
Stereo: Unbalanced, two-channel.
Bridge-Mono: Balanced, single-channel. Channel 1 controls are
active; Channel 2 should not be used.
Parallel-Mono: Unbalanced, single-channel. Channel 1 controls are
active; Channel 2 controls are bypassed.
Protection
Macro-Tech amplifiers are protected against shorted, open or
mismatched loads; overloaded power supplies; excessive tempera-
ture; chain destruction phenomena; input overload damage; and
high-frequency blowups. They also protect loudspeakers from
input/output DC and turn-on/turn-off transients.
If unreasonable operating conditions occur, the patented ODEP
circuitry will proportionally limit the drive level to protect the output
2-ohm Dual (per ch.)
1 kHz
Power
1,050W
800W
520W
2,070W
1,585W
850W
750W
505W
1,670W
1,485W
8-ohm Bridge-Mono
4-ohm Bridge-Mono
8-ohm Dual (per ch.)
4-ohm Dual (per ch.)
*1 kHz Power: refers to maximum average power in watts
at 1 kHz with 0.1% THD.
**20 Hz–20 kHz Power: refers to maximum average power
in watts from 20 Hz to 20 kHz with 0.1% THD.
MA-2402 *20 Hz– 20 kHz
Power
**
2 Specifications

MA-2402 Service Manual
2-2 Specifications
130445-1 Rev. A
©2000 Crown International, Inc.
Figure 2.1 Dimensions
transistor stages, particularly in the case of elevated temperature.
Transformer overheating will result in a temporary shutdown of the
affected channel; when it has cooled to a safe temperature, the
transformer will automatically reset itself. Controlled slew-rate
voltage amplifiers prevent RF burnouts. And input overload
protection is provided by current-limiting resistance at the input.
Turn On: Four-second delay with no dangerous transients. Delay
time can be changed (contact Crown’s Technical Support Group).
Accessories: Crown PIP and PIP2 modules including IQ-PIP
modules.
Construction
Steel chassis with durable black finish, aluminum front panel with
Lexan overlay, and specially designed flow-through ventilation from
front to side panels.
Figure 2.4 Typical Crosstalk
TEF
®
+45˚
0˚
–45˚
100 1 K 10 K 20 K
FREQUENCY (Hz)
100 1 K 10 K 20 K
FREQUENCY (Hz)
dB
TEF®
Figure 2.3 Typical Phase Response
Cooling: Internal heat sinks with forced-air cooling for rapid,
uniform heat dissipation.
Dimensions: EIA Standard 19-inch (48.3-cm) rack mount width
(EIA RS-310-B), 3.5-inch (8.9-cm) height, 16-inch (40.6-cm) depth
behind the mounting surface and 2.5-inch (6.3- cm) protrusion in
front of the mounting surface.
Approximate Weight: Center of gravity is 6 inches (15.2 cm) behind
front mounting surface.
120 VAC, 60 Hz Units: 51 pounds, 12 ounces (23.5 kg) net; 65
pounds (29.5 kg) shipping weight.
International Units: 48 pounds, 15 ounces (22.2 kg) net; 57 pounds,
6 ounces (26.0 kg) shipping weight.
–60
–66
–72
–78
–84
–90
–96

Specifications 2-3
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
Figure 2.5 Typical Frequency Response
Figure 2.6 Typical Damping Factor
Figure 2.7 Typical Output Impedance
10 100 1 K 10 K 100 K
FREQUENCY (Hz)
+2
+1
0
–1
–2
–3
–4
dB
–5
–6
–7
4 ohm
8 ohm
1 watt
504.0
126.8
31.8
MILLIOHMS
8.0
2.0
6 dB
20 100 1 K 10 K 20 K
400
200
100
0
600
800
1000
1200
1400
FREQUENCY (Hz)
8 ohm
10 100 1 K 10 K 100 K
FREQUENCY
(
Hz
)
2 ohm

MA-2402 Service Manual
2-4 Specifications
130445-1 Rev. A
©2000 Crown International, Inc.
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Voltage Conversion 3-1
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
Due to Crown's distribution of Macro-Tech amplifiers all
over the world, not all of these amplifiers are built with
thesame power supply components. MA-2402amplifi-
ers are built in one of the following two versions:
120VAC, 60 Hz Units
TheseNorthAmericanunitshavededicatedtransform-
ers for 120 VAC, 60 Hz power mains. These units are
notconvertableforuseatanyothervoltageorfrequency.
3 Voltage Conversion 100 – 240VAC, 50/60 Hz Units
Theseunitshavemult-taptransformersthataccept100,
120,220/230,or240VAC,and50or60Hzpowermains.
Jumpers are used to configure the power supplies for
different voltages. Note that the circuit breakers, fuse,
fan motor and low voltage transformer may need to be
changed to accommodate different voltages. Refer to
Figure 3.1 for Main Transformer jumper locations and
part numbers for all voltage configurations. Figure 3.2
shows correct wiring of the Low Voltage Transformer
for 100-120VAC and 220-240VAC operation.
Figure 3.1 Voltage Jumper Settings and Parts
Figure 3.2 Low Voltage Transformer Wiring

MA-2402 Service Manual
3-2 Voltage Conversion
130445-1 Rev. A
©2000 Crown International, Inc.
This page intentionally left blank

Circuit Theory 4-1
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
4 Circuit Theory
4.1 Overview
ItshouldbenotedthatovertimeCrownmakesimprove-
ments and changes to their products for various rea-
sons. This manual is up to date as of the time of writing.
For additional information regarding these amplifiers,
refer to the applicable Technical Notes provided by
Crown for this product.
This section of the manual explains the general opera-
tion of a typical Crown grounded bridge power ampli-
fier. Topics covered include Front End, Grounded
Bridge,andODEP. Due to variations in design from vin-
tage to vintage (and similarities with other Crown prod-
ucts) the theory of operation remains simplified.
4.2 Features
Macro Tech amplifiers utilize numerous Crown innova-
tions including grounded bridge and ODEP technolo-
gies. Cooling techniques make use of the what is es-
sentially air conditioner technology. Air flows bottom to
top, and front to side. Air flows a short distance across
awideheatsink. Outputtransistorsareofthemetalcan
type rather than plastic case. This allows for a signifi-
cantly higher thermal margin for the given voltage and
currentratings. Alldevicesusedaretestedand graded
to ensure maximum reliability. Another electronic tech-
nique used is negative feedback. Almost all power
amplifiers utilize negative feedback to control gain and
provide stability, but Crown uses multiple nested feed-
back loops for maximum stability and greatly improved
damping. Most Crown amplifiers have damping in ex-
cess of 1000 in the bass frequency range. This feed-
back, along with our compensation and ultra-low dis-
tortion output topology, make Crown amplifiers supe-
rior.
FeaturesspecifictotheMacroTechSeriesincludethree
seperatepowertransformers(oneforeachchanneland
one for low voltage), a full time full speed fan, slew rate
limiting,andCrown's “Quad-Mute” protectioncircuit for
mutingdelayor protectiveaction.Thisamplifiercanop-
erate in either a Bridged or Parallel Mono mode as well
as dual (stereo). A sensitivity switch allows selection of
input voltage required for rated output. Level controls
are mounted on the front panel and are of the rotary
type. Front panel indicators let the user know the status
of the low voltage power supply (enable), an ODEP in-
dicator for each channel which shows the reserve en-
ergy status, and a SPI/IOC indicator for each channel
whichindicatessignaloutputanddistortion.Ingeneral,
the packaging of this model is designed for maximum
watt/price/weight/size value with user friendly features.
For additional details refer to the specification section,
or to the applicable Reference Manual.
4.3 Front End Operation
The front end is comprised of three stages: Balanced
Gain Stage (BGS), Variable Gain Stage (VGS), and the
Error Amp. Figure 4.1 shows a simplified diagram of a
typical front end with voltage amplification stages.
4.3.1 Balanced Gain Stage (BGS)
Input to the amplifier is balanced. The shield may be
isolatedfromchassisground by an RC network to inter-
rupt ground loops via the Ground Lift Switch. The non-
inverting (hot) side of the balanced input is fed to the
non-inverting input of the first op-amp stage. The in-
verting (negative) side of the balanced input is fed to
the inverting input of the first op-amp stage. A potenti-
ometer is provided for common mode rejection adjust-
ment. Electrically, the BGS is at unity gain. (From an
audioperspective,however,thisstageactuallyprovides
+6dB gain if a fully balanced signal is placed on its
input.) The BGS is a non-inverting stage. It’s output is
delivered to the Variable Gain Stage.
4.3.2 Variable Gain Stage (VGS)
From the output of the BGS, the signal goes to the VGS
where gain is determined by the position of the Sensi-
tivity Switch, and level is determined by the level con-
trol. VGS is an inverting stage with the input being fed
to its op-amp stage. Because gain after this stage is
fixed at 26 dB (factor of 20), greater amplifier sensitivity
is achieved by controlling the ratio of feedback to input
resistance. The Sensitivity Switch sets the input imped-
ance to this stage and varies the gain such that the
overall amplifier gain is 26 dB, or is adjusted appropri-
ately for 0.775V or 1.4V input to attain rated output.
4.3.3 Error Amp
The inverted output from the VGS is fed to the non-in-
verting input of the Error Amp op-amp stage through
an AC coupling capacitor and input resistor. Amplifier
outputisfed backviathenegativefeedback(NFb)loop
resistor. The ratio of feedback resistor to input resistor
fixes gain from the Error Amp input to the output of the
amplifier at 26 dB. Diodes prevent overdriving the Error
Amp. Because the Error Amp amplifies the difference
between input and output signals, any difference in the
twowaveformswillproduceanearopen loopgaincon-

MA-2402 Service Manual
4-2 Circuit Theory
130445-1 Rev. A
©2000 Crown International, Inc.
dition which in turn results in high peak output voltage.
Theoutputof theErrorAmp,calledtheError Signal(ES)
drives the Voltage Translators.
4.4 Voltage Amplification
The Voltage Translator stage separates the output of
theError Ampintobalancedpositiveandnegativedrive
voltagesforthe Last VoltageAmplifiers (LVAs), translat-
ing the signal from ground referenced ±15V to ±Vcc
reference. LVAs provide the main voltage amplification
and drive the High Side output stages. Gain from Volt-
age Translator input to amplifier output is a factor of
25.2.
4.4.1 Voltage Translators
A voltage divider network splits the Error Signal (ES)
intopositiveandnegativedrivesignalsforthebalanced
voltagetranslatorstage.Theseoffsetreferencevoltages
drive the input to the Voltage Translator transistors. A
nested NFb loop from the output of the amplifier mixes
with the inverted signal riding on the offset references.
This negative feedback fixes gain at the offset refer-
ence points (and the output of the Error Amp) at a fac-
tor of –25.2 with respect to the amplifier output. The
Voltage Translators are arranged in a common base
configuration for non-inverting voltage gain with equal
gain. They shift the audio from the ±15V reference to
VCCreference. TheiroutputsdrivetheirrespectiveLVA.
Also tied into the Voltage Translator inputs are ODEP
limitingtransistorsandcontrol/protectiontransistors.The
ODEP transistors steal drive as dictated by the ODEP
circuitry (discussed later). The control/protection tran-
sistors act as switches to totally shunt audio to ground
duringtheturn-ondelay, orduringaDC/LForFault pro-
tective action.
4.4.2 Last Voltage Amplifiers (LVAs)
The Voltage Translator stage channels the signal to the
Last Voltage Amplifiers (LVA’s) in a balanced configura-
tion. The +LVA and -LVA, with their push-pull effect
through the Bias Servo, drive the fully complementary
output stage. The LVAs are configured as common
emitteramplifiers.Thisconfigurationprovides sufficient
voltage gain and inverts the audio. The polarity inver-
sion is necessary to avoid an overall polarity inversion
from input jack to output jack, and it allows the NFb
loop to control Error Amp gain by feeding back to its
non-inverting input (with its polarity opposite to the out-
putofthe VGS).Withtheadded voltageswingprovided
by the LVAs, the signal then gains current amplification
through the Darlington emitter-follower output stage.
4.5 Grounded Bridge Topology
Figure4.2isasimplifiedexampleofthegroundedbridge
output topology. It consists of four quadrants of three
deepDarlington(composite)emitter-followerstagesper
channel:oneNPNand one PNP on the High Side of the
bridge (driving the load), and one NPN and one PNP
on the Low Side of the bridge (controlling the ground
reference for the rails). The output stages are biased to
operate class AB+B for ultra low distortion in the signal
zero-crossing region and high efficiency.
4.5.1 High Side (HS)
The High Side (HS) of the bridge operates much like a
conventional bipolar push-pull output configuration. As
the input drive voltage becomes more positive, the HS
NPNconductsand deliverspositivevoltagetotheload.
Eventually the NPN devices reach full conduction and
+Vcc is across the load. At this time the HS PNP is bi-
ased off. When the drive signal is negative going, the
HS PNP conducts to deliver –Vcc to the load and the
HS NPN stage is off.
+
-+ -+ -
BGS VGS Error
Amp
Audio
Inputs
Voltage Divider
NFb Loop
+
-
ODEP Mute
+15V
-15V
+VCC
-VCC
NPN Outputs (+HS)
PNP Outputs (-HS)
Q100
Q103
Q121
Q122
Q101
Q102
Q105
Q110
Voltage
Translators
LVA's
Figure 4.1 Typical Amplifier Front End and Voltage Amplification Stages.

Circuit Theory 4-3
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
The output of the +LVA drives the base of predriver
device. Together, the predriver and driver form the first
two parts of the three-deep Darlington and are biased
class AB. They provide output drive through the bias
resistor, bypassing the output devices, at levels below
about 100mW. An RLC network between the predriver
and driver provide phase shift compensation and limit
driver base current to safe levels. Output devices are
biased class B, just below cutoff. At about 100mW out-
put they switch on to conduct high current to the load.
Together with predriver and driver, the output device
provide an overall class AB+B output.
The negative half of the HS is almost identical to the
positive half, except that the devices are PNP. One dif-
ferenceisthatthePNPbiasresistorisslightlygreaterin
value so that PNP output devices run closer to the cut-
off level under static (no signal) conditions. This is be-
cause PNP devices require greater drive current.
HS bias is regulated by Q18, the Bias Servo. Q18 is a
Vbe multiplier which maintains approximately 3.2V Vce
understaticconditions.Thepositiveandnegativehalves
of the HS output are in parallel with this 3.2V. With a full
base-emitter on voltage drop across predrivers and
drivers, the balance of voltage results in approximately
.35V drop across the bias resistors in the positive half,
and about .5V across the bias resistor in the negative
half. Q18 conduction (and thus bias) is adjustable.
Adiodestringpreventsexcessivechargebuildupwithin
the high conduction output devices when off. Flyback
diodes shunt back-EMF pulses from reactive loads to
the power supply to protect output devices from dan-
gerous reverse voltage levels. An output terminating
circuit blocks RF on output lines from entering the am-
plifier through its output connectors.
4.5.2 Low Side (LS)
TheLowSide(LS)operatesquitedifferently. The power
supply bridge rectifier is not ground referenced, nor is
the secondary of the main transformer. In other words,
the high voltage power supply floats with respect to
ground,but±Vccremainconstantwithrespect to each
other. This allows the power supply to deliver +Vcc and
–Vcc from the same bridge rectifier and filter as a total
difference in potential, regardless of their voltages with
respectto ground.TheLSusesinvertedfeedbackfrom
the HS output to control the ground reference for the
rails(±Vcc).Both LSquadrantsare arrangedinathree-
deep Darlington and are biased AB+B in the same
manner as the HS.
When the amplifier output swings positive, the audio is
fed to an op-amp stage where it is inverted. This in-
verted signal is delivered directly to the bases of the
positive (NPN) and negative (PNP) LS predrivers. The
negative drive forces the LS PNP devices on (NPN off).
AsthePNPdevicesconduct,VceofthePNPDarlington
drops. With LS device emitters tied to ground, –Vcc is
pulled toward ground reference. Since the power sup-
ply is not ground referenced (and the total voltage from
+Vcc to –Vcc is constant) +Vcc is forced higher above
ground potential. This continues until, at the positive
amplifier output peak, -Vcc = 0V and +Vcc equals the
total power supply potential with a positive polarity. If,
+
-
+Vcc (Positive Rail)
-Vcc (Negative Rail)
Load
(speaker)
Input
signal
HIGH SIDE LOW SIDE
Inverting Op-amp
Figure 4.2 Grounded Bridge Output Topology

MA-2402 Service Manual
4-4 Circuit Theory
130445-1 Rev. A
©2000 Crown International, Inc.
for example, the power supply produced a total of 70V
fromrailtorail(±35VDCmeasured from ground with no
signal),theamplifieroutputwouldreachapositivepeak
of +70V.
Conversely, during a negative swing of the HS output
where HS PNP devices conduct, the op-amp would
output a positive voltage forcing LS NPN devices to
conduct. This would result in +Vcc swinging toward
ground potential and –Vcc further from ground poten-
tial. At the negative amplifier output peak, +Vcc = 0V
and –Vcc equals the total power supply potential with a
negative polarity. Using the same example as above, a
70V supply would allow a negative output peak of
–70V. In summary, a power supply which produces a
total of 70VDC rail to rail (or ±35VDC statically) is ca-
pable of producing 140V peak-to-peak at the amplifier
outputwhenthegroundedbridgetopologyisused. The
voltage used in this example are relatively close to the
voltages of the MA-602.
The total effect is to deliver a peak to peak voltage to
the speaker load which is twice the voltage produced
by the power supply. Benefits include full utilization of
thepowersupply(itconductscurrentduringbothhalves
of the output signal; conventional designs require two
powersuppliesperchannel,onepositiveandonenega-
tive), and never exposing any output device to more
thanhalfofthepeaktopeakoutputvoltage (whichdoes
occur in conventional designs).
Low side bias is established by a diode string which
alsoshuntsbuiltupchargesontheoutputdevices. Bias
isadjustableviapotentiometer.Flybackdiodesperform
the same function as the HS flybacks. The output of the
LS is tied directly to chassis ground via ground strap.
4.6 Output Device Emulation Protection
(ODEP)
To further protect the output stages, a specially devel-
opedODEPcircuit isused.Itproducesa complexana-
log output signal. This signal is proportional to the al-
ways changing safe-operating-area margin of the out-
put transistors. The ODEP signal controls the Voltage
Translator stage by removing drive that may exceed
the safe-operating-area of the output stage.
ODEP senses output current by measuring the voltage
dropped across LS emitter resistors. LS NPN current
(negative amplifier output) and +Vcc are sensed, then
multipliedtoobtainasignalproportionaltooutputpower.
Positive and negative ODEP voltages are adjustable
via two potentiometers. Across ±ODEP are a PTC and
a thermal sense (current source). The PTC is essen-
tially a cutoff switch that causes hard ODEP limiting if
heatsinktemperatureexceedsasafemaximum,regard-
less of signal level. The thermal sense causes the dif-
ferential between +ODEP and –ODEP to decrease as
heatsink temperature increases. An increase in posi-
tive output signal output into a load will result in –ODEP
voltage dropping; an increase in negative output volt-
age and current will cause +ODEP voltage to drop. A
complex RC network between the ±ODEP circuitry is
used to simulate the thermal barriers between the inte-
rior of the output device die (immeasurable by normal
means) and the time delay from heat generation at the
dieuntilheat dissipatestothethermalsensor. Thecom-
bined effects of thermal history and instantaneous dy-
namic power level result in an accurate simulation of
the actual thermal condition of the output transistors.

Circuit Theory 4-5
130445-1 Rev. A MA-2402 Service Manual
©2000 Crown International, Inc.
NEGATIVE
LOW SIDE
OUTPUT
PNP STAGE
POSITIVE
LOW SIDE
OUTPUT
NPN STAGE
NEGATIVE
HIGH SIDE
OUTPUT
PNP STAGE
POSITIVE
HIGH SIDE
OUTPUT
NPN STAGE
HIGH SIDE
BIAS
SERVO
+LVA
-1
-LVA
-1
+VOLTAGE
TRANSLATOR
-VOLTAGE
TRANSLATOR
OUTPUT
DEVICE
EMULATION
PROTECTION
ERROR
AMP
VGSBGS
BALANCED
INPUTS
INVERTING
BRIDGE
BALANCE
LOW SIDE
BIAS
DIODE
STRING
-1
-1
MAIN NEGATIVE FEEDBACK (NFb) LOOP
Figure 4.3 Typical Crown Grounded Bridge Amplifier Basic Block Diagram (One Channel Shown)

MA-2402 Service Manual
4-6 Circuit Theory
130445-1 Rev. A
©2000 Crown International, Inc.
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