In order get the output devices to tu rn fully on and achieve the
greatest efficiency, an auxiliary rail is formed from a charge pump
driven by the main AC outputs from the transformer. C304,305 &
D301,302 charge on alternate cycles. In th e other cycle the charge
on C304,305 is pu mped into C310,311 respectively. Thi s creates a
voltage much greater than the standard rails, but wi th low regulation
and high ripple. This is then regulated to 38V via emitter followers
Q300,302,303,305 from a zener stabilized voltage (D304,305)
The Pre-amp ±15V supply comes from linear regulators Z300,301.
The transformer is a dual voltage unit, the required voltage being
selected b y which fuse holder is fitted with the appropriat e fuse. If an
alternative to 115V or 230V is requ ired then a different transformer
is fitted. Th e secondary is centre tapped for the main rails, but a
separate winding is added for the uC supply.
The star ground p oint SP300 is not a component on the PCB and is
on the schematic only for layout purposes.
Pre-amp
The direct signal path (line in, tone controls defeated) consists of a
single op-amp gain stage, buffering the j-fet input multiplexers and
Tape Monitor and AV switches . The selected input is AC coupled to
the op-amp, the output is DC to the Pre-out, via a j- f et "pre-mute"
switch. This switch is controlled by the uC in such a way that when
ever the output relay changes state signal is removed from the power
amplifier (A65/75).
The MM phono stage (by passable A75 only, SW100) is opamp
based. The circuit is AC coupled to give RIAA/IEC low frequency
response. The c artridge is loaded by R100,101,C119,120. The 1kHz
gain is 56dB, and RIAA eq set by
R110,112,111,113,C127,129,128,130.
Each line input has static protection a ffor ded by the ba ck to back
diodes (e.g. D101,103), and RF filtering is set by R118,C134 etc.
The selected input is buffered for Tap e output by Z104 set as a
follower. The output resistor is inside the feedback loop so to keep
the output impedance low and short protection is catered for.
C121,122 maintain stabilit y i nto capacitive loads.
From the tape monitor switch (Z105) onwards the signal is AC
coupled (C151,152). From there the signal is fed to the tone controls.
These consist of two stages: Balance and buffer (Z160), and
Bass/Treble Z161. The output of this stage is again AC coupled.
Switch SW162 selects whether the tone adjusted signal or a direct
signal reaches the gain section. In the A75 Z164 can be used to
bypass the volu me control when AV input is selected. This fixes th e
gain to allow an external AV processor to control the volume setting.
The gain stage (Z163) has a gain of 2.2, and the same low impedance
output arrangement as the Tape output. The signal from this stage to
the Pre- amp outputs and the Power amp input is controlled by a
pre-mute (Z171). This removes thumps and clicks, and in the case of
the Power amp r elay changing state, removes signal so ar cing will
not take place.
Power Amplifier
The new topology is a high gain voltage feedback design, with a
bipolar output stage. Its signal path is designed to be fully
symmetrical, despite not having a symmetrical circuit (symmetrical
rail to rail). This is achieved with a new dual differential input gain
block, where the input differential sees a symmetrical active load
(Q600,601,700,701) before driving the second voltage amplification
stage (VAS) differential (Q603,604,703,704). This could be
described as two Wilson mirrors with their bases clamped together.
The second differential also has an active load of a more traditional
mirror (Q610,611,710,711), and this gives a push pull signal to the
input of the output stage, improving on the traditional single ended
current source of the often called "blameless" topology.
The input pair (Q60 6,607,618,619, 706,707,718,719 ) are in a
Darlington confi guration to lower input current offset which in turn
will lower DC offset at the output, as the impeda nces seen on the
input nodes are necessarily high for the wide bandwidth required.
The output stage is a simple bipola r darlington (Q614,615,714,715).
However, the devices used are Darlingt on devices with integral
diodes. This means the bias temperature compensation i s part of the
output device enabling smaller number of devices, no extra devices
on the heatsink (vital for this mechanical layout) and more consistent
temperature compensation of bias. Also the resi st ors are also
integral, saving a large amount of PCB real estate and hand insertion
cost, while also able to be used as bias set test points and protection
sensing.
Due to the dec reased efficiency of this type of output stage over the
previous bootstrapped Mosfet output, a separate set of rails is
required for the VAS stage in order to satu rate the output transistors.
The operation of this is outlined in the Power supply section.
However, these rails are individually regulated for each channel to
decrease power supply crosstalk.
The protecti on of the output stage is taken care of by a proprietary IC
(Z101). This takes care of the switch on delay (R150,C149), DC
offset (R102,103,C147), Thermal trip (TH100), and safe operating
area.
The safe op erating area is monitored by a network which
compensates for the voltage across the output devices , current
through them, and duration of power (R627,606,625,605,C617 &
symmetrical associated components) and this is sent to the protection
IC via (Q605,612,613,616,617,712,713,716,717).
The output of the protection IC is sent to the micro controller (uC) to
signal the output relay (RLY1) status. Also, for output muti ng
purposes, the uC can switch the relay via Q100.
Input Selection
The uC cont r o ls the operation of the unit. IR commands are received
from RX900 on a break-off PCB found on the front panel and fed via
LK900/LK903 3-way ribbon. On power on/off the current state of
the pre-amp switching stored in an EEPR O M Z906. The inp ut
switching is 5V logic level and is direct from the uC to the DPDT
analogue switches in the case of Tape Monito r ( Z105), AV
(Z164,A75 on ly), Pre-mute (Z171) and Mute Q100. The source
selector is achieved by 8 way analogue multiplexors (Z102,103) and
the control signals are decoded from the uC from MUX1,2,3 by
Z907 which also drives the appropriate front panel LED. The input
selection is set by the remote control or the rotary encoder on the
front panel display PCB. Tape monitor and AV mode(A75 only) is
set by SW900.
Volume Control
The volume control is a motorized manual type, where the signals
are routed through the 8-way ribbon LK189. The motor drive comes
from Z905, which is a logic level controllable motor driver. It has its
own unregulated PSU (D907,C920),an d the speed of the volume
knob is dictated by the REF pin. The circuit (Q900,R905,901,C901)
allows the speed to vary from just ab ove stall to full as the volume
control button is depressed on the remote.
SIM Module
The connector PL161 (P75 only) allows the fitting of the ARCAM
SIM module. In order to allow this to operate LK172 & LK173 must
be removed. This will allow 12V trigger signals to operate the P75.
Headphones
The Speaker outputs (SK600,700) are connected to the headphone
socket (SK162) via an 8-way ribbon LK170,171. The output is
attenuated by a resistor network R161,162,236,237. The Relay drive
is fed through the headphone socket's switch (HPSW1,2) so that
when headphones are inserted the speakers are disconnected. Switch
SW163 allows the separate switching of the second speaker
terminals on SK600,700.
A65, A75 & P75 Service Guide
Fault diagnostics
Fault Action
No power Check mains fuse
Stuck in protection Check power supply rails
Intermittent audio on one channel Check for dry joints around o/p
Fails to respond to commands Check supply to remote circuit
Volume control rubs on front panel Pull knob out from fascia
Audio crackles on o/p Check for component break down
Power supply test points
Position Voltage
LK125 +37 volts DC
LK126 -37 volts DC
LK271 or black speaker terminal 0 volts
LK303 +15 volts DC
LK301 - 15 volts DC
Q300 emitter +38 volts DC
Q304 emitter +38 volts DC
Q302 emitter -38 volts DC
Q305 emitter -38 volts DC
D910 emitter +5 volts DC remote supply
IQ settings
Model IQ setting Comments - (22/01/02)
A65 37mV
A75 37mV
P75 37mV
A85 12mV Set for 8.5mV cold, reset for 12mV when warm
P85 12mV Set for 8.5mV cold, reset for 12mV when warm
A65 plus 12 to 15mV
A75 plus 12 to 15mV
P75 plus 12 to 15mV
!
To isolate a fault between pre and power stage remove C602 (left channel) or C702 (right
channel).
!
Always check and re-solder o/p device SAP15N and SAP15P pin connections.
!
Replace Z900 IC if software version is below version 3 ( note version 82 is also ok ).
!
Make sure R127 is not touching LK130.
!
When replacing the top cover with a new top it may be necessary to re-drill the fixing holes.
Check transformer connections
Check power supply rails
Check protection circuit voltages
Check for dry joints on o/p devices
devices
Check IC Z900
Check remote flex foil cable
Check for dry joints on selector pcb
using hot air gun & freezer spray
Check transformer connections
Hints & tips
PHONO & INPUTS
L942C3_1.0.SCH
ACSENSE
PROT_OP_L
PROT_OP_R
PROTECTIONMUTE
TAPECONTROL
MUX2
MUX3
PROT SENSE
SELOPL
SELOPR
HPSW2
TAPOL
TAPOR
MUX1
HPSW1
REMOTE CONTROL
L942C7_1.0.SCH
MUTE
MUX1
MUX2
MUX3
PROT SENSE
TONE CONTROL
L942C4_1.0.SCH
AVCONTROL
PREMUTE
TAPECONTROL
SELOPL
SELOPR
HPSW2
HPSW1
PREMUTE
ACLT1
PROT_OP_R
PROT_OP_L
PWRLEFT
PWRRIGHT
TAPOL
TAPOR
SP2_SEND_L
SP2_RETN_L
OP_L
OP_R
SP2_SEND_R
SP2_RETN_R
AVCONTROL
LEFT CHANNEL
L942C5_1.0.SCH
SP2_SEND_L
SP2_RETN_L
PWRLEFT
RIGHT CHANNEL
L942C6_1.0.SCH
PWRRIGHT
SP2_SEND_R
SP2_RETN_R
PROTECTION
OP_L
A1
A1
OP_R
A1
A1
POWER SUPPLY
L942C2_1.0.SCH
ACLT1
ACSENSE
USE 'exlude A65' FILTER FOR BOM
DRAWING TITLE
A & R Cambridge Ltd.
Pembroke Avenue
Waterbeach
Cambridge CB5 9PB
A65 PLUS AMP
Filename:
L942C1_1.0.PRJ
Notes:
TOP SHEET FOR A65 L942AY
Contact Engineer:
01_E020
Contact Tel: (01223) 203 200Travis Pierce
ECO No.DESCRIPTION OF CHANGE
TGP12/12/01PRODUCTION RELEASE1.0
DATE
INITIALS
11-Dec-2001
Printed:
17Sheetof
DRAWING NO.
L942C1
ISSUE
C600
100U EL 100V
0V_HF
+38V_L
+
R601
100R MF
R602
100R MF
R615
47R MF
0V_HF
C613
100N PE 100V
C614
100N PE 100V
0V_HF
+37V
+
C603
100U 50V YXF LOW Z
D601
ZENER 400MW 27V
PWRLEFT
C623
100N PE 100V
C602
+
22U SILMIC 2
R600
1K0 MF
C603
100P PPW
0V_SIG
R612
100K MF
0V_HF
R632
12K MF
Q600
BC556B
Q619
BC546B
+
C601
100U EL 100V
Q606
BC546B
R613
47R MF
Q607
BC546B
R617
680R MF
Q608
BC546B
Q601
BC556B
R614
47R MF
0V_HF
Q618
BC546B
R633
12K MF
C605
100U SILMIC 2
0V_SIG
R618
10K MF
R619
10K MF
Q609
BC546B
D600
4V7 400MW
C604
100P PPW
C610
100N PE 100V
R616
2K7 MF
+
R611
100K MF
+
C624
100U 25V ZA EL
Q611
2SC2240GR
+
C616
10U EL
Q604
2SA970GR
R620
10K MF
R603
100R MF
R634
470K MF
C625
10U EL
C607 100P PPW
Q602
BC556B
Q603
2SA970GR
+
C611
100N PE 100V
Q610
2SC2240GR
R604
100R MF
C612
100N PE 100V
RV600
220R PSET
0V_HF
C608
100P PPW
-38V_L
Q614
SAP15N
Q615
SAP15P
C626
100U 50V YXF LOW Z
Bias-L +
SET BIAS TO 12mV
Bias-L -
C615
+
100N PE 100V
0V_HF
-37V
D603
1N4003F
D604
1N4003F
R627
430R MF
R628
430R MF
R606
100R MF
R607
100R MF
R625
6K8 MF
R605
100R MF
C617
+
10U EL
+
C618
10U EL
R608
100R MF
R626
6K8 MF
D602
ZENER 400MW 27V
Q613
BC546B
R621
680R MF
R622
10K MF
D605
1N4148
Q612
2SC2240GR
0V_LS_L
Q616
2SA970GR
R624
10K MF
R609
100R MF
C609
22N CD
A1
R630
4R7 W2 CF
C619
100N PE 100V
Q605
BC556B
R623
10K MF
D606
1N4148
Q617
ZTX653
R610
100R MF
R629
47K MF
PROTECTION
L600
2U2H LAC SMALL
R631
2R2 CF
A1
SP2_RETN_L
470P CD
OP_L
RLY1A
RLY G5Z-2A-E
SPLE600
PD6
C620
0V_LS_L
0V_LS_L
C622
1N0 CD
SPLD600
C621
470P CD
SPLS600
Bot+
SK600B
SPKSKT_G
Bot-
SK600C
CHASS
SPKSKT_G
SP2_SEND_L
Top+
SK600A
SPKSKT_G
Top-
CL600
E856MC
HS600
E81801-75
CL601
E856MC
HS601
E81801-75
SP600
Q2-PAD-SAP
HS602
E81801-75
21mm x 32mm
SELF ADHESIVE
Q2 PAD
E852MC
HS603
E81801-75
SP601
21mm x 32mm
SELF ADHESIVE
Q2 PAD
E852MC
Q2-PAD-SAP
USE 'exlude A65' FILTER FOR BOM
DRAWING TITLE
A & R Cambridge Ltd.
Pembroke Avenue
Waterbeach
Cambridge CB5 9PB