Arcam FMJ P7 User manual

P7
FMJ P7
7-Channel Amplifier
Service Manual
ARCAM
Issue 1.0

Contents List
Section Issue
Technical specifications
! Technical specification -
! Rear panel silk screen -
Amplifier board L924
! Circuit description -
! Component overlay 1.0
! Parts list 1.1
! Circuit diagrams 1.1
Controller board L925
! Circuit description -
! Component overlay 1.0
! Parts list 1.2
! Circuit diagrams 1.2
Transformer specifications
! L911TX 1.0
! L912TX 1.0
! L920TX 1.0
! L921TX 1.0
Mechanical
! General assembly parts list -
! Assembly diagrams
Front panel -
TXtray - -
Rear panel -
Chassis -

Technical
Specifications
Contents
! Technical
specifications
! Rear panel silk screen

Technical Specifications
All measurements are with 230V/50Hz mains power
Continuous output power
All channels driven, 20Hz – 20Khz, 8 ohm 150W per channel 1.05kW total
All channels driven, 20Hz – 20kHz, 4 ohm 230W per channel 1.62kW total
One or two channels driven at 1kHz, 8 ohm 160W per channel
One or two channels driven at 1kHz, 4 ohm 250W per channel
One or two channels driven at 1kHz, 3.2 ohm 300W per channel
Peak output current capability 25A per channel
Total harmonic distortion
At any level up to rated power, into 4 or 8 ohms <0.05% (20Hz – 20kHz)
Typically <0.005% at 1kHz
Frequency response +-0.2dB (20Hz – 20 kHz)
-1dB at 1Hz and 100kHz
Residual hum and noise
Referenced to full power -122dB, 20Hz – 20kHz, unweighted
Voltage gain x 28.3 (1V input gives 100W/8 ohm output)
Input impedance 22k ohm in parallel with 470pF
Output impedance 50m Ohm at 20Hz, 1kHz
120m ohm at 20kHz
Power requirements 115V or 230VAC, 50/60Hz, 3kW maximum via heavy duty IEC mains inlet
A soft start system eliminates large inrush currents at switch on
Dimensions W430 x D450 x H180 mm
Weight 31kg (68 Ib) nett
35kg(77Ib)packed

~
50 – 60 Hz
3700 VA
MAX
SERIAL No. LABEL
CAUTION – SHOCK HAZARD, DO NOT OPEN.
ACHTUNG – VOR OEFFNEN DES GERAETES NETZSTECKER ZIEHEN.
ATTENTION – RISQUE DE CHOC, NE PAS ENLEVER.
PRECAUCION – PELIGRO DESCARGA, NO ABRIR.
RADIO INTERFERENCE (USA) THIS PRODUCT COMPLIES WITH PART 15 OF FCC RULES, OPERATION
IS SUBJECT TO THE FOLLOWING CONDITIONS; (1) THIS DEVICE MAY NOT CAUSE HARMFUL
INTERFERENCE (2) THIS DEVICE MUST ACCEPT ANY INTERFERENCE RECEIVED, INCLUDING
INTERFERENCE THAT MAY CAUSE UNDESIRED OPERATION.
THIS PRODUCT IS CERTIFIED BY THE MANUFACTURER TO COMPLY WITH DHHS RULE 21 CFR
SUBPART J APPLICABLE AT THE DATE OF MANUFACTURE.
DESIGNED & MADE IN THE UK BY:
A & R CAMBRIDGE LTD, WATERBEACH,
CAMBRIDGE, CB5 9PB.
WARNING: THIS APPLIANCE MUST BE EARTHED.
12V TRIGGER
IN
7 RS BACK 6 R SURR 5 RIGHT 4 CENTRE 3 LEFT 2 L SURR 1LS BACK
POWER INLET
230V

Amplifier
Board
L924
Contents
! Circuit description
! Component overlay
! Parts list
! Circuit diagrams

P7 Amplifier Module
Circuit Description
Refer to L924 circuit diagrams
Introduction
L924 is the power amplifier module for the P7 multichannel
amplifier. There are 7 identical modules in the P7. The circuit
design is based on the A85 / A32 output stage topology.
The main features of the amplifier module are as follows:
• Preset ‘THX’ gain (29dB closed loop gain). 0dBV input
signal corresponds to 100 watts into 8Ωoutput power
• Capable of producing 150 watts of sinusoidal output
power into an 8Ωresistive load (with greater than
250W into 3.2Ωsubject to thermal dissipation limits)
• Relay coupled output for silent power on / off and load
protection
• Opto-isolated fault and control lines to the
microprocessor PCB (to avoid hum loops and
instability, to improve EMC performance and crosstalk)
• DC coupled signal path with integrating servo to
remove residual DC errors
• Instantaneous load protection
• Mono block design (each channel is electrically isolated
from all others and has independent power supply
windings on the mains transformer)
• Integrated modular heatsink for good thermal
performance and ease of assembly / servicing
• Low harmonic and intermodulation distortion
• Flat frequency response
• Fast (and symmetrical) slew rate
• High damping factor
• Unconditionally stable into loads of up to ±90°phase
angle
Sheet 1
The input to the amplifier is connected via SK103. The 2
phono sockets are connected in parallel to allow ‘daisy-
chaining’ of amplifier modules. R104 provides a DC leakage
path to the chassis (i.e. mains power earth) to prevent small
transformer leakage currents causing the electrical 0V of the
amplifier to rise significantly above mains earth potential.
C104 provides an EMC coupling between the local input
ground and the chassis to reduce common mode RF noise.
Star point SP101 connects the differently named electrical 0V
nets at a single point. This is to ensure the correct wiring
topology of the ground connections on the printed circuit
board. SP101 provides a good common ground reference
point when making voltage measurements on the PCB. Note
that 0V_DIG is not connected to SP101, as this is the
microprocessor ground.
Relay RLY101 connects the output of the amplifier to the
load via socket SK105.
L101 and R103 form part of a ‘Zobel’ network to decouple
the load at high frequencies to ensure amplifier stability into
capacitive loads.
Note that signals 6 through 9 are open collector outputs,
active low, referred to 0V_DIG with no pull-up resistors.
This is because they are wire OR’d on the microprocessor
PCB (L925), where the pull-up resistors to +5V digital are
located.
The line ‘NFB’ provides for a portion of the negative feedback of the
amplifier to be taken on the load side of RLY101. The components
that allow for this (R236 thru R239) are not presently fitted, meaning
that RLY101 is not included in the feedback loop.
SK104 connects to the microcontroller PCB. Note that all signals on
this connector are electrically isolated from the amplifier circuit
itself, via either opto isolators or the relay coil of RLY101. The 10-
pin connector has the following signals:
SK104
Pin Type Name Description
1 GND 0V DIG Microprocessor ground return
2 PSU +24V_DIG +24 volt digital power supply
(referred to 0V_DIG only) for
relay coil RLY101
3 MUTE Not used
4 I/P OUT_RLY Relay drive for the output relay
RLY101 (LOW = output relay
ON)
5 Not used
6 O/P THERMPR
OT Open collector thermal fault signal (LOW = FAULT)
7 O/P VIPROT Open collector short circuit fault
signal (LOW = FAULT)
8 O/P DCPROT Open collector DC fault signal
(LOW = FAULT)
9 O/P FAULT Open collector overall fault signal
(LOW = FAULT)
10 Not used
Sheet 2
Port INPUT connects the input of the amplifier, referred to 0V_SIG,
which is the precision signal ground reference.
Zener diodes DZ202 and DZ203 limit the input signal amplitude to
approximately 5.3Vpk. This is to prevent damage to the input of op-
amp IC200, due to a leaky source signal or electrostatic discharge.
R223, R228 and C210 form a passive 1
st
order low pass filter with a –
3dB corner frequency of roughly 330kHz to prevent ultrasonic
signals from entering the circuit and possibly causing damage.
The main amplifier circuit is a ‘classic’ current feedback design.
IC200A is configured as a non-inverting amplifier with a gain of 2.
Its purpose is to provide current outputs (via its power supply pins)
and a current input (via its output pin). This forms the voltage to
current (transimpedance) conversion and phase splitting necessary to
drive the voltage gain stage. The ‘current feedback’ occurs because
when IC200 drives its 44Ωload to ground, the power supply pin
currents are half-wave rectified versions of the drive current of the
amplifier. This causes voltage gain, which is buffered and passed on
to the outputs. The feedback from the output to pin 1 of IC200 acts to
reduce the gain of the amplifier; when this current is roughly equal to
the current required to drive the input signal into 44Ω, equilibrium is
reached and the closed loop gain is defined. The output stage
provides the vast majority of the current required to drive the 44Ω
signals to ground. The op-amp only provides a very small error
current sufficient to give the required voltage magnification.
Transistors TR204 and TR203 are wired as cascodes (common base
amplifiers). Their purpose is to provide IC200 with ±15V power
supply rails, whilst allowing IC200’s power supply pin currents to
pass through them to the NPN and PNP current mirrors.
The resistor, zener diode and capacitor circuits on the bases of TR204
and TR203 are to provide a controlled ramp up during power on, a
stable power supply voltage and good local HF decoupling.

Transistors TR200, TR201 and TR202 form a PNP Wilson
current mirror. Likewise TR205, TR207 and TR206 form an
NPN Wilson current mirror. The outputs of these two current
mirrors are connected together via the bias network around
TR212.
The two current mirrors combine to provide a very high-gain
current to voltage (transresistance) gain stage, which provides
all the voltage gain of the amplifier (roughly 80dB at low
frequency).
C205, C207, R221 and R222 provide the loop compensation
for the amplifier. They combine to produce an open-loop pole
at roughly 10kHz and a corresponding open-loop zero around
500kHz. This allows for good time domain performance and
clean square wave reproduction. The amplifier is designed to
be critically damped. There should be no ringing or overshoot
apparent on the output signal when a (small) step function is
applied to the input.
Diodes D200 and D202 act to limit the current through
TR202 and TR206 in the event of a fault condition. When the
input current exceeds 14mA the diodes conduct and the
transresistance stage becomes a constant current source,
killing the open loop gain and preventing damage to the
transistors.
Resistors R219 and R220 decouple the supplies for the
amplifier gain stages from the main power rails. This is to
permit the bootstrap circuit to modulate these supplies,
increasing efficiency. The bootstrap will be described in more
detail later.
TR212 provides a 4.7V bias voltage to allow the following
pre-driver stage to operate in class ‘A’. It also acts as a V
BE
multiplier for TR209 and TR214 to maintain an
approximately constant current as the ambient temperature
inside the box changes.
TR209 and TR214 form a class ‘A’ pre-driver emitter
follower stage to boost the current gain and isolate the
transresistance stage from the output transistors. This is
important to keep the loop gain of the amplifier high and thus
minimise distortion. TR208 and TR213 act as a current limit
(roughly 30mA) to prevent the destruction of TR209 and
TR214 in a fault condition.
R247, R248, R249 and R250 are to loosely decouple the
emitters of TR209 and TR214 from the output stage. This is
very important. The output devices (Sanken power
Darlingtons) have inbuilt temperature compensating diodes
which control the bias voltage to their bases. Each output
device has a 150Ωresistor so that the inbuilt diodes can
accurately control quiescent V
BE
and hence collector current
as the output power and device temperature varies. Preset
potentiometer RV200 adjusts the quiescent current. NB
Ensure that the amplifier has fully warmed up before
adjusting the quiescent current. D201 protects the output
devices from destruction in the event of the preset
potentiometer going open circuit. PL200 allows the test
engineer to measure the bias voltage (and thus collector
current).
C217, C218, C220 and C221 provide local HF stability
around the output transistors to prevent parasitic oscillation.
D204 and D205 are catch diodes to reduce the effects of
induced back-EMF in the loudspeaker load.
R254 and C223 form part of the ‘Zobel’ network that ensures
the amplifier sees a constant load of roughly 4.7Ωat very
high frequencies. This helps to improve stability and reduce
HF output noise.
C208 and C209 provide local high frequency decoupling for
the output devices.
IC200B forms the DC integrating servo. Its purpose is to remove
residual DC errors due to slight device mismatch and component
tolerances. It is configured as an inverting integrator with a time
constant of 0.47 seconds. Any positive DC offset at the output of the
amplifier will cause the output of the op-amp to go negative,
increasing the current in the negative supply pin and thus ‘pulling’
the output down to ground (and vice versa). D203 protects the
inverting input of IC200B in a fault condition.
The bootstrap circuit consists of C213, C214, R241, R242, R219 and
R220. The purpose of the bootstrap is to allow the output voltage
swing to modulate the power supply rails of the input and voltage
gain stages. This allows this circuit’s power supply voltage to exceed
the main power rails connected to the output devices, allowing the
driver stage to fully drive the output and thus give the best thermal
efficiency. The ‘bottom’ of R219 sees a peak-to-peak voltage swing
of approximately 15 volts at full output power (i.e. it goes 7.5 volts
above the rail at the peak of the cycle). The ‘top’ of R220 should see
the same voltage swing.
Sheet 3
This sheet contains the protection circuits and interface to the
microprocessor signals.
TR309, TR305 and their associated components form the
instantaneous load protection circuit for the output transistors. They
sense the voltage across the 0.22Ωemitter resistors (hence emitter
current) and the collector-emitter voltage, cutting off the base drive to
the output transistors when the collector current or device power
dissipation exceeds a preset limit.
The protection circuit is designed to allow large (unrestricted)
currents into loads of 3Ωand above but limit the current into a short
circuit or very low impedance load. C318, C319, R335 and R336
form a 2.2ms time constant, which will allow larger transients of
current delivery for a few milliseconds, to ensure that the amplifier
has a sufficiently large transient capability to drive ‘difficult’
loudspeaker loads with a music signal.
TR311 also turns on when the protection circuit activates. This
switches on optocoupler IC300B causing a fault signal to be
transmitted to the microcontroller. The microcontroller will then
switch off the output relay to protect the amplifier.
TR310, TR302 and their associated components form the DC offset
detection circuit. A positive DC offset at the output will turn on
TR310. A negative DC offset at the output will turn on TR302, thus
causing TR313 to turn on. In either case optocoupler IC300A is
switched on causing a fault signal to be transmitted to the
microcontroller. The microcontroller will then switch off the output
relay to protect the loudspeaker voice coils from overheating.
Thermistor TH300 is connected to the positive supply rail, adjacent to
the collector leg of one of the power output devices. This allows it to
sense the collector temperature of the output device. Its impedance
when cool is low, typically a few hundred ohms. In the event of a
thermal overload (above 110°C), TH300 will go to a high impedance
state. This will turn on TR301, which then turns on TR300, causing
optocoupler IC300D to switch on, sending a fault signal to the
microcontroller. The microcontroller will then switch off the output
relay until such time as the unit has cooled down to an acceptable
level (80°C or so). TR301 is configured with a small amount of
hysterisis (positive feedback) to ensure a clean signal is transmitted to
the microprocessor via IC300D.
Optocoupler IC300C is connected in series with the 3 optocouplers
mentioned above, producing an overall fault signal. This is so that the
microcontroller can determine in which module the fault has
occurred, permitting selective control of the output relay for each
module in the amplifier.

L924 Amplifier Module Parts List Issue 1.1
Designator Part Description
BR300 3BGBU8D Diode Bridge Rectifier GBU8D Plastic Package 8A 200V
BR301 3BGBU8D Diode Bridge Rectifier GBU8D Plastic Package 8A 200V
C103 2C410 Capacitor SM 1206 X7R Ceramic 50V 10% 100N
C104 2C210 Capacitor SM 1206 NPO Ceramic 50V 5% 1N0
C200 2C410 Capacitor SM 1206 X7R Ceramic 50V 10% 100N
C201 2C410 Capacitor SM 1206 X7R Ceramic 50V 10% 100N
C202 2C410 Capacitor SM 1206 X7R Ceramic 50V 10% 100N
C203 2N710 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 100UF 25V
C204 2N710 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 100UF 25V
C205 2D110W2 Capacitor Boxed Polyprop 5mm Pitch 250V 5% 100P
C207 2D110W2 Capacitor Boxed Polyprop 5mm Pitch 250V 5% 100P
C208 2C410A Capacitor SM 1812 X7R Ceramic 500V 10% 100N
C209 2C410A Capacitor SM 1812 X7R Ceramic 500V 10% 100N
C210 2D147W Capacitor Boxed Polyprop 5mm Pitch 100V 5% 470P
C212 2K447 Capacitor Boxed Polyester 5mm Pitch 10% 63V 470N
C213 2N710B Capacitor Radial Electrolytic Dia 10mm Pitch 5mm 100UF 100V
C214 2N710B Capacitor Radial Electrolytic Dia 10mm Pitch 5mm 100UF 100V
C215 2N610 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 10UF 50V
C216 2N610 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 10UF 50V
C217 2C047B Capacitor SM 0805 NPO Ceramic 200V 5% 47PF
C218 2C047B Capacitor SM 0805 NPO Ceramic 200V 5% 47PF
C220 2C047B Capacitor SM 0805 NPO Ceramic 200V 5% 47PF
C221 2C047B Capacitor SM 0805 NPO Ceramic 200V 5% 47PF
C223 2K347 Capacitor Boxed Polyester 5mm Pitch 10% 63V 47N
C225 2N610 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 10UF 50V
C300 2C410 Capacitor SM 1206 X7R Ceramic 50V 10% 100N
C301 2N710 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 100UF 25V
C305 2C410 Capacitor SM 1206 X7R Ceramic 50V 10% 100N
C306 2C410 Capacitor SM 1206 X7R Ceramic 50V 10% 100N
C318 2N710 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 100UF 25V
C319 2N710 Capacitor Radial Electrolytic Dia 5mm Pitch 5mm 100UF 25V
C320 2V710 Capacitor Non-Polar Radial Electrolytic 100UF 16V
C321 2V710 Capacitor Non-Polar Radial Electrolytic 100UF 16V
C322 2P910AM Capacitor Radial Electrolytic Dia 40mm PCB Mount 10000uF 71V
C323 2P910AM Capacitor Radial Electrolytic Dia 40mm PCB Mount 10000uF 71V
D101 3AS16W Diode Surface Mount Small Signal BAS16W SOT-23 Package
D200 3AS16W Diode Surface Mount Small Signal BAS16W SOT-23 Package
D201 3AS16W Diode Surface Mount Small Signal BAS16W SOT-23 Package
D202 3AS16W Diode Surface Mount Small Signal BAS16W SOT-23 Package
D203 3AV99W Diode Dual Surface Mount Small Signal BAV99 SOT-23 Package
D204 3B4003 Diode 1N4003 DO-41 Package
D205 3B4003 Diode 1N4003 DO-41 Package
D300 3AS16W Diode Surface Mount Small Signal BAS16W SOT-23 Package
D301 3CW315V Zener Diode 0.25W Surface Mount BZX84C15V SOT-23 Package
D303 3AS16W Diode Surface Mount Small Signal BAS16W SOT-23 Package
D307 3AS16W Diode Surface Mount Small Signal BAS16W SOT-23 Package
D308 3CW322V
D309 3CW322V
DZ200 3CW315V Zener Diode 0.25W Surface Mount BZX84C15V SOT-23 Package
DZ201 3CW315V Zener Diode 0.25W Surface Mount BZX84C15V SOT-23 Package
DZ202 3CW34V7 Zener Diode 0.25W Surface Mount BZX84C4V7 SOT-23 Package
DZ203 3CW34V7 Zener Diode 0.25W Surface Mount BZX84C4V7 SOT-23 Package
IC200 5B072D Opamp TL072CD SO-8 Package
IC300 5T3Q66 Opto Isolator Quad PC3Q66Q
L101 7D002C Inductor AC 2u2 SELF BONDED
PL100 8K6201 CON SINGLE ROW HDR 0.1IN VERTICAL 2WAY
PL200 8K6201 CON SINGLE ROW HDR 0.1IN VERTICAL 2WAY
R103 1H822 Resistor Metal Film 0.25W 1% 2R2
R104 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R207 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R208 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R210 1A327 Resistor 1206 Surface Mount 0.25W 1% 27K

L924 Amplifier Module Parts List Issue 1.1
Designator Part Description
R211 1A327 Resistor 1206 Surface Mount 0.25W 1% 27K
R212 1A327 Resistor 1206 Surface Mount 0.25W 1% 27K
R213 1A327 Resistor 1206 Surface Mount 0.25W 1% 27K
R214 1A047 Resistor 1206 Surface Mount 0.25W 1% 47R
R216 1A047 Resistor 1206 Surface Mount 0.25W 1% 47R
R217 1A047 Resistor 1206 Surface Mount 0.25W 1% 47R
R218 1A047 Resistor 1206 Surface Mount 0.25W 1% 47R
R219 1H122 Resistor Metal Film 0.25W 1% 220R
R220 1H122 Resistor Metal Film 0.25W 1% 220R
R221 1A233 Resistor 1206 Surface Mount 0.25W 1% 3K3
R222 1A233 Resistor 1206 Surface Mount 0.25W 1% 3K3
R223 1A210 Resistor 1206 Surface Mount 0.25W 1% 1K0
R224 1A210 Resistor 1206 Surface Mount 0.25W 1% 1K0
R225 1A210 Resistor 1206 Surface Mount 0.25W 1% 1K0
R226 1A210 Resistor 1206 Surface Mount 0.25W 1% 1K0
R227 1A210 Resistor 1206 Surface Mount 0.25W 1% 1K0
R228 1A322 Resistor 1206 Surface Mount 0.25W 1% 22K
R229 1H022 Resistor Metal Film 0.25W 1% 22R
R230 1H022 Resistor Metal Film 0.25W 1% 22R
R231 1A247 Resistor 1206 Surface Mount 0.25W 1% 4K7
R232 1H156 Resistor Metal Film 0.25W 1% 560R
R233 1H156 Resistor Metal Film 0.25W 1% 560R
R234 1H168 Resistor Metal Film 0.25W 1% 680R
R235 1H168 Resistor Metal Film 0.25W 1% 680R
R236 1H239 Resistor Metal Film 0.25W 1% 3K9
R237 1H239 Resistor Metal Film 0.25W 1% 3K9
R238 1H239 Resistor Metal Film 0.25W 1% 3K9
R239 1H239 Resistor Metal Film 0.25W 1% 3K9
R240 1A510 Resistor 1206 Surface Mount 0.25W 1% 1M0
R241 1C210 Resistor Carbon Film 2W 5% 1K0
R242 1C210 Resistor Carbon Film 2W 5% 1K0
R245 1A022 Resistor 1206 Surface Mount 0.25W 1% 22R
R246 1A022 Resistor 1206 Surface Mount 0.25W 1% 22R
R247 1A115 Resistor 1206 Surface Mount 0.25W 1% 150R
R248 1A115 Resistor 1206 Surface Mount 0.25W 1% 150R
R249 1A115 Resistor 1206 Surface Mount 0.25W 1% 150R
R250 1A115 Resistor 1206 Surface Mount 0.25W 1% 150R
R254 1D847 Resistor Carbon Film 0W5 5% 4R7
R255 1A268 Resistor 1206 Surface Mount 0.25W 1% 6K8
R257 1A210 Resistor 1206 Surface Mount 0.25W 1% 1K0
R300 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R301 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R302 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R303 1A322 Resistor 1206 Surface Mount 0.25W 1% 22K
R304 1A127 Resistor 1206 Surface Mount 0.25W 1% 270R
R305 1A310 Resistor 1206 Surface Mount 0.25W 1% 10K
R306 1A310 Resistor 1206 Surface Mount 0.25W 1% 10K
R307 1A339 Resistor 1206 Surface Mount 0.25W 1% 39K
R310 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R311 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R314 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R315 1A110 Resistor 1206 Surface Mount 0.25W 1% 100R
R316 1A322 Resistor 1206 Surface Mount 0.25W 1% 22K
R320 1A322 Resistor 1206 Surface Mount 0.25W 1% 22K
R321 1A322 Resistor 1206 Surface Mount 0.25W 1% 22K
R332 1A322 Resistor 1206 Surface Mount 0.25W 1% 22K
R333 1A322 Resistor 1206 Surface Mount 0.25W 1% 22K
R334 1A247 Resistor 1206 Surface Mount 0.25W 1% 4K7
R335 1A022 Resistor 1206 Surface Mount 0.25W 1% 22R
R336 1A022 Resistor 1206 Surface Mount 0.25W 1% 22R
R337 1A210 Resistor 1206 Surface Mount 0.25W 1% 1K0
R338 1A133 Resistor 1206 Surface Mount 0.25W 1% 330R

L924 Amplifier Module Parts List Issue 1.1
Designator Part Description
R339 1A133 Resistor 1206 Surface Mount 0.25W 1% 330R
R340 1A310 Resistor 1206 Surface Mount 0.25W 1% 10K
R341 1A310 Resistor 1206 Surface Mount 0.25W 1% 10K
R342 1A412 Resistor 1206 Surface Mount 0.25W 1% 120K
R343 1A410 Resistor 1206 Surface Mount 0.25W 1% 100K
RLY101 A220 Relay 1P2T 24V
RV200 6F110V Preset Vertical Mount 100R linear
SK103 8D226 Phono socket 2 way vertical gold
SK104 8K2810 CON HORIZ PCB SKT 10WAY
SK105 8D421 Connector 4mm 2 way horizontal
SK300 8K2302 CON MINIFIT HCS V 2WAY
SK301 8K2302 CON MINIFIT HCS V 2WAY
TH300 1T007 Thermistor PTC SM 110 degrees C
TR200 4AFMMT597 Transistor FMMT597 SOT23 Package
TR201 4AFMMT597 Transistor FMMT597 SOT23 Package
TR202 4B1740 Transistor 2SA1740 SOT-89 Package
TR203 4AFMMT597 Transistor FMMT597 SOT23 Package
TR204 4AFMMT497 Transistor FMMT497 SOT23 Package
TR205 4AFMMT497 Transistor FMMT497 SOT23 Package
TR206 4B4548 Transistor 2SC4548 SOT-89
TR207 4AFMMT497 Transistor FMMT497 SOT23 Package
TR208 4AFMMT497 Transistor FMMT497 SOT23 Package
TR209 4B4548 Transistor 2SC4548 SOT-89
TR210 4CSAP15N Transistor SAP15NY
TR211 4CSAP15N Transistor SAP15NY
TR212 4AFMMT497 Transistor FMMT497 SOT23 Package
TR213 4AFMMT597 Transistor FMMT597 SOT23 Package
TR214 4B1740 Transistor 2SA1740 SOT-89 Package
TR215 4CSAP15P Transistor SAP15PY
TR216 4CSAP15P Transistor SAP15PY
TR300 4AFMMT497 Transistor FMMT497 SOT23 Package
TR301 4AFMMT597 Transistor FMMT597 SOT23 Package
TR302 4AFMMT597 Transistor FMMT597 SOT23 Package
TR305 4AFMMT597 Transistor FMMT597 SOT23 Package
TR309 4AFMMT497 Transistor FMMT497 SOT23 Package
TR310 4AFMMT497 Transistor FMMT497 SOT23 Package
TR311 4AFMMT497 Transistor FMMT497 SOT23 Package
TR312 4AFMMT497 Transistor FMMT497 SOT23 Package
TR313 4AFMMT497 Transistor FMMT497 SOT23 Package
Z100 L924PB BLANK PCB 7 CHANNEL AMP MODULE
Z101 E107AY ASSEMBLY OF REAR PANEL MODULE AND SILK SCREEN
Z102 E915HK Finished Heatsink for single channel amp module
Z103 HF4V09B SCREW SELF-TAPPING-SEMS NO.4 X 9MM PAN TORX-SLOT STEEL ZINC-PLATE BLK
Z104 HF4V09B SCREW SELF-TAPPING-SEMS NO.4 X 9MM PAN TORX-SLOT STEEL ZINC-PLATE BLK
Z105 HF4V09B SCREW SELF-TAPPING-SEMS NO.4 X 9MM PAN TORX-SLOT STEEL ZINC-PLATE BLK
Z106 E950MC INSULATOR P7 TRANSISTOR INSULATING PAD
Z107 HB3B12A SCREW TAPTITE HEXAGON WASHER FACE M3X12MM
Z108 HB3B12A SCREW TAPTITE HEXAGON WASHER FACE M3X12MM
Z109 HB3B12A SCREW TAPTITE HEXAGON WASHER FACE M3X12MM
Z110 HB3B12A SCREW TAPTITE HEXAGON WASHER FACE M3X12MM

ISSUE
DRAWING NO.
DRAWING TITLE
DATE
Filename:
ECO No. DESCRIPTION OF CHANGE
L924C1 1.1.sch
P7 Amplifier Module
Contact Engineer: L924CT 7 Mar 2002
INITIALS
Printed: 1 3Sheet of
Notes:
Contact Tel: [phone]Jonny Reckless
A & R Cambridge Ltd.
Pembroke Avenue
Cambridge CB5 9PB
Waterbeach
EMC
1
N
F
SK103
PHONO2G
C104
1N0 SM
0V_SIG
0V_LS
THERMPROT
VIPROT
DCPROT
RLY101A
RLY G2R-1E-24DC 16A SPDT
RLY101B
RLY G2R-1E-24DC 16A SPDT
+24V_DIG
MUTE
OUT RLY
1
2
3
4
5
6
7
8
9
10
SK104
10WAY-H-0.1-F
FAULT
D101
BAS16W SM
C103
100N SM
L101
2U2 AIR CORED
R103
2R2 MF
R104
100R SM
0V_DIG
+24V DIG
OUT RLY
INPUT
NFB
OUTPUT
EMITTER1
EMITTER2
BASE1
BASE2
L924C2
L924C2_1.1.sch VIPROT
DC
EMITTER1
EMITTER2
OUTPUT
BASE1
BASE2
THERM
FAULT
L924C3
L924C3_1.1.sch VIPROT
DCPROT
FAULT
SP101
STAR_8
0V_PSU
0V_LS0V_ZOB
0V_SIG
0V_HF
THERMPROT
SK105
4MM 2W HOR
TP100
TP101
TP112
TP111
TP103
TP102
TP104
TP105
TP106
TP107
TP108
TP109
TP110
1
2
PL100
2WVERTJUMPER
NB: Nets +24V_DIG and 0V_DIG are isolated from amplifier circuit
Z100
L924PB
PCB HEATSINK INSULATOR M3 TAPTITE M3 TAPTITE M3 TAPTITE M3 TAPTITE
SUB PANEL SCREW SCREW SCREW SUB PANEL SUB PANEL SUB PANEL
Z102
E915HK
Z107
HB3B12A
Z108
HB3B12A
Z109
HB3B12A
Z110
HB3B12A
Z101
E107AY
Z103
HF4V09B
Z104
HF4V09B
Z105
HF4V09B
Z106
E950MC
02_E055 JR 4/3/2002 FIXED DC FAULT AT POWER ON SEQUENCE BY FEEDBACK 1.1
Electrical ground star point
Inputs
Amplifier circuit Protection and supplies
To microprocessor PCB
NB: Feedback around relay not fitted
Microprocessor GND
Microprocessor +24V
MUTE not used
Output relay control
Not used
Thermal protection
SOA protection
DC offset protection
Overall fault signal
NB: Signals THERMPROT, VIPROT,
DCPROT and FAULT are open
collector outputs, active low,
referenced to 0V_DIG with no pull up
resistors
Not used
TRANSISTOR

ISSUE
DRAWING NO.
DRAWING TITLE
DATE
Filename:
ECO No. DESCRIPTION OF CHANGE
L924C2 1.1.sch
P7 Amplifier Module
Contact Engineer: L924CT 7 Mar 2002
INITIALS
Printed: 2 3Sheet of
Notes:
Contact Tel: [phone]Jonny Reckless
A & R Cambridge Ltd.
Pembroke Avenue
Cambridge CB5 9PB
Waterbeach
R223
1K0 SM
TR200
FMMT597 TR201 FMMT597
R214
47R SM R216 47R SM
TR205
FMMT497 TR207
FMMT497
R217
47R SM R218 47R SM
C212
470N PE
0V_SIG
INPUT
0V_HF
0V_HF
DZ200
15V 350MW SM
DZ201
15V 350MW SM
C201
100N SM
C200
100N SM
C202
100N SM
0V_SIG
0V_SIG
D205
1N4003F
D204
1N4003F
R228
22K SM
RV200
100R PSET 1
2
PL200
BIAS
R225
1K0 SM
R227
1K0 SM
Trim output stage Iq
D201
BAS16W SM
R211
27K SM
R213
27K SM
TR210
SAP15N
TR215
SAP15P
DZ202
4V7 350MW SM
DZ203
4V7 350MW SM
R231
4K7 SM
0V_SIG
TR204
FMMT497
TR203
FMMT597
+
C203
100U EL
+
C204
100U EL
R247
150R SM
R249
150R SM
R240
1M0 SM
R221
3K3 SM
R222
3K3 SM
C210
470P PPW
+
C215
10U EL
+C213
100U EL 100V
+C214
100U EL 100V
TR211
SAP15N
TR216
SAP15P
R250
150R SM
R248
150R SM
TR212
FMMT497
R257
1K SM
R255
6K8 SM
R241
1K CF 2W
R242
1K CF 2W
3
21
84
IC200A
TL072CD
5
6
7
IC200B
TL072CD
NFB
R229
22R MF
R230
22R MF
R212
27K SM
R210
27K SM
0V_HF
C217
47P/200V C218 47P/200V
C220
47P/200V C221
47P/200V
C205
100P/250V
C207
100P/250V
C209
100N X7R 1812 500V
C208
100N X7R 1812 500V
D203
BAV99W
R219
220R MF
R220
220R MF
R232
560R MF
R234
680R MF
R233
560R MF
R235
680R MF
R236
R237
R238
R239
+C216
10U EL
D200
BAS16W SM
D202
BAS16W SM
R224
1K0 SM
R226
1K0 SM
OUTPUT
+55V
-55V
+C225
10U EL
EMITTER1
EMITTER2
BASE1
BASE2
R245
22R SM
R246
22R SM
R207
100R SM
R208
100R SM
TR208
FMMT497
TR213
FMMT597
0V_ZOB
R254
4R7 W2 CF
C223
47N PE
TR214
TR PNP 2SA1740
TR209
TR NPN 2SC4548
TR202
TR PNP 2SA1740
TR206
TR NPN 2SC4548
TP200
TP201
TP202
TP204
TP205
TP203
R236 THRU R239 NOT FITTED
02_E055 JR 4/3/2002 FIXED DC FAULT AT POWER ON SEQUENCE BY FEEDBACK 1.1
NF
NF
NF
NF
Input Filter
V to I converter
Integrating DC servo
PNP current mirror
NPN current mirror
Pre driver
Pre driver
Bootstrap
Output stage
Output stage
Pre driver bias
Measure Iq

ISSUE
DRAWING NO.
DRAWING TITLE
DATE
Filename:
ECO No. DESCRIPTION OF CHANGE
L924C3 1.1.sch
P7 Amplifier Module
Contact Engineer: L924CT 7 Mar 2002
INITIALS
Printed: 3 3Sheet of
Notes:
Contact Tel: [phone]Jonny Reckless
A & R Cambridge Ltd.
Pembroke Avenue
Cambridge CB5 9PB
Waterbeach
R338
330R SM
R339
330R SM
R300
100R SM
R301
100R SM
TR309
FMMT497
TR305
FMMT597
TR311
FMMT497
VIPROTDC
R332
22K SM
R321
22K SM
R302
100R SM
R310
100R SM
R311
100R SM
TR302
FMMT597
TR310
FMMT497
R320
22K SM
R316
22K SM
D308
22V 350MW SM
D309
22V 350MW SM
R341
10K SM
R340
10K SM
0V_DIG
R337
1K SM
IC300A
OPTO-PC3Q66-QUAD-SM
IC300B
OPTO-PC3Q66-QUAD-SM
D307
BAS16W SM
D300
BAS16W SM
C300
100N SM
TR313
FMMT497
TR312
FMMT497
R314
100R SM R315 100R SM
R333
22K SM
D303
BAS16W SM
0V_HF
0V_HF
C321
100U NP R334 4K7 SM
0V_DIG
EMITTER1
EMITTER2
OUTPUT
+55V
-55V
BASE1
BASE2
R335
22R SM
R336
22R SM
+1
~2
4
~
3
BR300
BRGBU8D
0V_PSU
+55V
+1
~2
4
~
3
BR301
BRGBU8D
1
2
SK300
MOLEXPWR2WAY_R/A
1
2
SK301
MOLEXPWR2WAY_R/A
-55V
+C323
C AM 71V 10000U
+C322
C AM 71V 10000U
R343
100K SM
IC300D
OPTO-PC3Q66-QUAD-SM
0V_DIG
R342
120K SM
TH300
PTH 110DEG SM
0V_HF
THERM
IC300C
OPTO-PC3Q66-QUAD-SM FAULT
C305
100N SM
C306
100N SM
C320
100U NP
+C318
100U EL
+C319
100U EL
TP300
TP302
TP303
TP304
TP305
TP306
TP307
R307
39K SM
R304
270R SM
R305
10K SM
R303
22K SM
TR301
FMMT597
TR300
FMMT497
R306
10K SM
D301
15V 350MW SM
+C301
100U EL
02_E055 JR 4/3/2002 FIXED DC FAULT AT POWER ON SEQUENCE BY FEEDBACK 1.1
Instant SOA limiting
Instant SOA limiting
DC offset detection
Over temperature sensing
Power supplies
AC from transformer
AC from transformer

Controller
Board
L925
Contents
! Circuit description
! Component overlay
! Parts list
! Circuit diagrams

P7 Controller Circuit Description
The Amp controller PCB Panel consists of 6 PCBs.
! Controller PCB
! Display PCB
! Connector PCB
! Mains Switch PCB
! 2x Wire Clamp PCBs
Overview
The controller PCB contains the microprocessor and most
of the circuitry for controlling the P7 amplifier. The
display PCB provides the LEDs and resistors for the
simple 7 LED display for the front panel of the unit. The
mains switch PCB provides a means of supporting the
front panel mounted mains switch. The connector PCB fits
on the base of the chassis near the rear panel and provides
a means of connecting the 7 amplifier modules to a
connector which is then linked to the controller PCB. The
connector PCB also provides a means of mounting the rear
panel mounted trigger input connector and connecting its
signals to the mains controller PCB.
The cable clamp PCBs are used to hold the transformer
power cables neatly to the side of the chassis base.
The controller PCB
The controller PCB provides the following functionality
! Mains power distribution, switching and soft-start
! Voltage selection for 115/230V operation.
! +5V (for logic) supplies
! +22V (for relay and fan) supplies
! Variable speed fan drive
! Relay control for soft starts and sequenced power up/
power down for switch on, switch off and fault
conditions.
! Heatsink temperature measurement for fan speed
control
! Transformer Thermal trip monitoring
! Mains supply monitoring for output muting
! Soft start resistor monitoring to prevent soft start
resistor overdissipation
! Amplifier module fault status monitoring for module
over-temperature, module DC offset, module VI
limit
! Output muting relays drive circuit
! Amplifier status display drive circuitry via the front
panel mounted LEDs.
! Watch-dog failsafe mechanism which activates in the
case of microprocessor failure
Mains power distribution, switching and
soft-start
The mains input supply is connected directly to the control
PCB via SK100. The mains supply is routed through the
power relay RLY100 and through the 3 parallel resistors
R100, R101, R102. These form a high power rated 50-
Ohm resistor that is used to limit the inrush current into the
2 large power toroids. These toroids are mounted beneath
the transformer tray at the front of the unit. The 50-Ohm
series resistor limits the inrush current to 5A and is
allowed to remain in circuit for a few hundred
milliseconds after power on. After the power amplifier
module electrolytic capacitors have charged to full supply
voltage the current through the resistor falls to a low value
(which is sensed by the circuitry around IC105 as
described below) and the soft start resistor is then switched
out of circuit by relay RLY101. This arrangement of
power relays ensures that relay RLY101 never switches
high voltages and its contacts remain clean.
Voltage selection for 115/230V operation
The control PCB provides a means of configuring the unit
for 115V or 230V operation. This is achieved by having 2
connectors marked with the appropriate voltage for each of
the units 3 transformers (1 auxiliary transformer which
powers the control PCB and 2 large toroids each of which
powers 3 ½ amplifier modules). The connectors SK101,
SK102 for the aux TX and SK103, SK104 for the Right
hand power toroid and SK105, SK106 for the left hand
power torroid are configured to place the two primary
windings of each transformer in parallel for 115V
operation and in series for 230V operation. The
arrangement of fuses (6 in total which are all fitted)
ensures that each winding has a series fuse when
configured in parallel for 115V operation and 1 of each
pair of fuses is used to fuse the series configured windings
when set for 230V operation. In this way there are 2
parallel fuses for each transformer when set for 115V (to
allow twice the current as required). The fuses are time
delay types.
C121 is X rated and provides suppression for rectifier
switching transients for EMC conducted noise compliance.
RC100 is a series RC network to prolong relay life.
+5V supply (for logic)
The auxiliary transformer has 2 secondary windings.
These are rectified by full wave rectifiers to form the DC
supplies.
The +5V supply is rectified by D101, D102, D105, D106
and smoothed by C102. The supply is then regulated by
the 3 terminal 5V regulator REG100. The supply is
designed to provide 9.5V DC to th input of the regulator
when fed with a nominal 230V. This allows 8V at the
input to the regulator (so that it remains within regulation)
when fed from 195V mains supply. The regulator is fitted
with a heatsink. C103 is provided to eliminate input
transients. C107 ensures regulator stability.
+22V supply (for relay and fan)
The +22V supply is formed in a similar manner by D103,
D104, D107, D108 and C101. The supply voltage is
chosen to give 22V when driven from a mains voltage of
230V. This gives around 24V when the mains supply is at
its maximum value (265V). This is due to the relays
having a maximum specified input voltage of 24V.
Variable fan speed drive
The fan for the amplifier is required to be very quiet. This
is achieved by having the fan speed proportional to
temperature. The temperature is measured by the
microprocessor (as described below) and when the
temperature exceeds a programmed threshold then the fan
is switched on at its lowest speed (approx 7V). As the
temperature of the unit increases the fan speed is increased
until it is operating at full speed (12V). This is achieved by
having the microprocessor output a PWM 5V pk to pk
square wave on line PWM_OUT. The square wave is
filtered by R112, C108 to form a DC level. The amplitude
of the DC level is thus proportional to the duty cycle that
is under microprocessor control. The filtered DC signal is
then amplified by the simple 4 transistor amplifier formed
around TR100, TR113, TR114 and TR115. The gain of the
amplifier is set by R117, R118 to around 6. This ensures

that the PWM_OUT signal duty cycle can move the output
of the amplifier over the required range. C104, C113
provide suppression for EMC and C112 provides amplifier
loop compensation. The supply to the fan is fused by F101
this fuse limits current in the event of the fan stalling. If
the fuse blows the watchdog timer cct immediately
switches off the power relays to ensure failsafe condition.
Relay control
The power relay and soft-start relay (RLY100, RLY101)
respectively are driven by microprocessor signals
MPOWER and MSOFT_ST*. Transistors TR108, TR111
buffer the microprocessor outputs. Diodes D115 and D119
prevent damage to the collectors of the transistors by the
inductance of the relay coils at coil switch off.
The power for the relay coils (+22V_SW) is provided
through TR116. This transistor is off (hence relays off)
when the watchdog circuit detects no microprocessor
activity (as described below) or the fan fuse F101 is blown
(as described above).
Heatsink temperature measurement
The microprocessor monitors the temperature of the
heatsink using the circuitry around TR112. Capacitor
C114 is alternately charged through fixed resistor R122
(2K2) and thermistor TH100. The time taken to charge the
capacitor is measured by the microprocessor. The ratio of
the time taken to charge the capacitor through the 2K2
fixed resistor compared with the time taken to charge the
capacitor through the thermistor allows the microprocessor
to calculate the resistance of the thermistor. The software
is then able to establish the temperature of the thermistor
from its resistance.
Three microprocessor lines are involved in this
measurement. TREF, TTEMP, C_DISCH.
A measurement cycle proceeds as follows.
C_DISCH goes high to discharge C114. After a delay to
ensure C114 is fully discharged C_DISCH then goes Low
and TREF is set as an output and goes high. TTEMP is set
as an input. This allows C114 to charge through R122.
The time taken for C114 to charge to the input high
threshold of the micro is measured by timing through input
TTEMP.
When the threshold is reached, C_DISCH is again taken
high to discharge the capacitor.
After a delay to ensure C114 is fully discharged C_DISCH
then goes Low and TTEMP is set as an output and goes
high. TREF is set as an input. This allows C114 to charge
through TH100. The time taken for C114 to charge to the
input high threshold of the micro is measured by timing
through input TREF.
When the threshold is reached, C_DISCH is again taken
high to discharge the capacitor.
This cycle is repeated continuously and the ratio of
capacitor charge times allows the ratio of resistance R122
to resistance TH100 to be measured. The measurement is
immune to variation in value of C114, saturation voltage
of TR112 and average supply voltage on the 5V rail. The
variation of input voltage 1 threshold of the TREF and
TTEMP inputs is not compensated for but in practice this
is usually found to be minimal. The method should give a
measurement accuracy of a few degrees C that is all that is
needed for fan speed control.
Transformer Thermal trip monitoring
The main power transformers have thermal trips built into
them. These are normally closed and go open when the trip
temperature is exceeded. The trips are put in series and
pulled to 5V through R131. In the event of 1 (or both)
transformer trips going open circuit then line
TX_OVTEMP goes high and the microprocessor is then
able to power the amplifier down as required.
Mains supply monitoring for output muting
In order to prevent thumps through the loudspeaker when
the amplifier is switched off it is necessary to detect the
removal of the mains supply so that the amplifier modules
can be muted.
This is affected by the circuitry around TR101 and the
microprocessor. The AC supply for the 22V rail is sensed
by R109. At the positive peak of the mains supply TR101
is turned on and saturates thereby discharging C109. In the
event of the AC supply disappearing then C109 will not be
discharged and will instead charge to 5V through R132.
The time constant R132 . C109 = 47mS sets the time taken
for line PWROK* to go high. So if the mains supply
disappears for around 5 capacitor charge cycles then
PWROK* will notify the microprocessor which will then
mute the amplifier outputs.
Soft start resistor monitoring
The soft start resistor is used to limit the inrush current
into the large power transformers. The resistor is designed
to be in-circuit only for the duration of the inrush current
at switch on. The resistor must be shorted out by the relay
before the amplifier is configured to deliver output current.
Also in the event of a fault (e.g. a short across one of the
amplifier module reservoir capacitors) then the resistor
might be exposed to a situation which might cause over
dissipation.
This is avoided by having the microprocessor monitor the
voltage across the resistor so that in the event of a fault the
amplifier can be switched off before damage to the resistor
occurs.
The voltage across the soft-start resistor is monitored by
the circuitry around IC105. When voltage is present across
the soft-start resistor (only the positive half cycle is
sensed) then the opto-transistor is turned on. This
discharges C100.
The RC time constant R104, C100 = 220mS dictates that
line SSPROT* will be low until around ¼ of a second after
the voltage across the soft-start resistor has fallen to zero.
Amplifier module fault status monitoring
! Module over-temperature
! Module DC offset
! Module VI limit
The amplifier modules contain circuitry that senses the
above fault conditions. The fault status is indicated to the
control PCB by means of open collector transistors on
each module that are ON when the fault exists.
The means of connecting the fault lines to the control PCB
requires explanation.
There are 7 amplifier modules, each of which has 3 fault
lines. If these were tracked individually then this would
require 21 lines to the control PCB.
In order to reduce the number of lines required then the
connections are changed so that the 3 fault lines from each
of the are 7 modules are connected in parallel. So that e.g.
the VI limit fault line is pulled low when any one of the 7
modules exhibits a VI limit fault. In order for the
controller to establish which module has the fault (so that

it can be indicated on the front panel display) each module
also provides an address line. A fault occurring on a
particular module will cause both the fault line to be low
and also the address line. In this way the amplifiers can be
monitored by the controller through a total of 10 lines.
The address lines are called:
FAULTn* (where n = 1 to 7 as per amplifier module
number)
The fault lines are:
FLT_DC* to indicate a DC offset error
FLT_VI* to indicate VI limit protection
FLT_TEMP* to indicate that the temperature of the output
transistors has exceeded the 110C trip temperature.
Output muting relays
The muting relays are on the amplifier modules. The
relays allow the amplifier to be disconnected from the
loudspeaker load. This is required for muting switch on
and switch off thumps and in the event of fault conditions.
The relays are driven from the control PCB by the simple
open collector transistor drive arrangement formed around
TR102, TR104- TR110. The diodes D111-D118 protect
the transistor collectors against over voltage spikes
occurring when the inductive relay coils are switched off.
The open collector transistors are driven through the
74HC259 type addressable latch. This configuration
allows 24 output lines to be provided from 7
microprocessor lines. The outputs of one of the 259s
(IC103) are used to drive the muting relay transistors. The
remaining packages (IC101, IC102) are used to drive the
display LEDs as described below.
Control of the 259s is as follows. Each has 3 address bits
S0, S1, S2 which allows one of 8 outputs to be selected, a
DATA input which allows a 0 or a 1 to be latched to the
output and a GATE input which transfers the signal on
DATA to the selected (addressed) output of the package.
The GATE input is also used as a chip select to select
which of the 3 packages is being controlled. This allows
the S and DATA inputs to be connected in parallel on
multiple packages.
Amplifier status display
The amplifier status is indicated via the front panel
mounted LEDs. IC101, IC102 are used to directly drive
the tricolour LEDs which are mounted on the LED PCB.
Each channel has a single Tricolour LED. The LED has a
red and green LED built in and the third colour (yellow) is
produced by having both Red and Green LEDs ON at the
same time.
IC101 is connected to the red LEDs and IC102 is
connected to the green LEDs.
Watch-dog
The watch dog circuit provides a failsafe mechanism
which places the amplifier in a safe (OFF) state in the
event of the microprocessor failing or crashing. The
circuitry monitors the average voltage on the
WATCHDOG line and if it falls outside its required
voltage then it removes the power supply to the relays
which has the effect of muting the amplifier outputs and
removing the power to the power amplifier toroids.
The line WATCHDOG is toggled by the software in the
micro. Every time it completes a program loop it toggles
the state of the line. This results in a 50% duty cycle signal
that is averaged to 2.5V by the filter R114, C110. The
voltage is then fed to the window comparator formed
around IC104. The network R105, R106 and R107 sets the
upper and low thresholds at 75% of 5V and 25% of 5V
respectively.
The outputs are open collector and if the filtered
WATCHDOG signal exceeds the upper threshold or falls
below the lower threshold then the b-e junction of TR103
is pulled low and TR116 is then turned off – which
removes the power to the relays.
The filtered WATCHDOG signal can only move outside
the window if the WATCHDOG line sits high (or low)
continuously. This can only occur if the microprocessor
breaks or the software crashes.
This protection scheme also includes the fan fuse (F101).
If the fan fuse blows then TR103 gets no base current that
also immediately removes the power supply to the relays.
LED PCB
The LED PCB contains the 7 bicolour LEDs and their
current limiting resistors. The LED PCB also provides a
means of routing the mains switch wiring to the control
PCB via the LED PCB to control PCB flexfoil cable.
Connector PCB
The connector PCB provides a means of joining the
amplifier modules to the control PCB via the 22way
flexfoil cable. The PCB also includes circuitry for the 12V
trigger circuit.
The trigger circuit is formed around SK201. The 12V
trigger signal is converted to a 5V logic level compatible
signal by R200, D200. C201 and C202 provide EMC
suppression (since the signal is ultimately routed to the
Control PCB which contains a microprocessor). R202
isolates the ground pin of the jack socket from the
amplifier ground. This prevents possible ground loop
problems with other hifi components connected in the
system.
The zener diode clamped signal is limited to 4.7V pk
amplitude. This is fed to the microprocessor via line
TRIG/RC5
Fault conditions
Fault D.C
When a D.C fault is created on an amplifier all the LEDS
turn red accept the module with the fault that flashes
green.
Fault V.I
When a V.I fault is created all the LEDS turn red accept
the module with the fault which flash red.
Therm fault (amplifier overheating)
When a therm fault is created all the LEDS stay green and
the amp module with the fault flashes amber.
Thermal trip (TX overheating)
When a thermal trip is created all the LEDS flash amber,
stating that it is a transformer problem.