Arcam FMJ A32, A32 P35-3, A32 P35 User Manual

Service Manual

ARCAM

FMJ A32, P35 & P35/3 Ampliﬁ ers

A32

Issue 1.0

ARCAM

Contents List

Phono board L870

o Circuit description o Component overlay o Parts list o Circuit diagrams

Amplifier & PSU board L882

o Circuit description o Component overlay o Parts list o Circuit diagrams

Display board L928

o Component overlay o Parts list o Circuit diagrams

Preamplifier board L937

o Circuit description o Component overlay o Parts list o Circuit diagrams

3rd Channel Amplifier board L911

o Component overlay o Parts list o Circuit diagrams

Transformer specifications

o L914TX o L915TX

General assembly

o A32 - Mechanical parts list o P35 - Mechanical parts list o P35/3 - Mechanical parts list

Phono Board L870

Contents

Circuit description

Component overlay

Parts list

Circuit diagrams

AV8 Phono Board

Circuit Description Refer to circuit diagram L870 sheet 1

The phono board is a simple single stage RIAA amplifier and consists of two channels of high gain amplification, and the ability to switch between moving magnet (MM) and moving coil (M C) settings.

PSU

The unit derives its + fitted into with only local decouplin g capacitors on board.

Interface

The unit connects to the host unit via a 8 way connector.

Amplification

The left channel h as designators beginnin g with 100, and the right with 200. For the purposes of this description the left channel will be described, as the right channel is the same in all respects.

The amplifier is a small signal class A voltage feedback amplifier with switch able gain. The input consists of an actively loaded differential pair of very low noise PNP transistors (TR106, TR107). These transistors are very specific and should only be replaced with identical parts with the E grade high gain. TR100 and TR101 form a current source for the pair, which sets the qu iescent current for the entire amplifier. The active load consists of TR110 and TR111, which forms part of the differential current mirror with TR112, TR113 & TR114. This differential stage also h as an active load (TR102 & TR10 3) to keep gain to a maximum.

Both of these di fferential stages are design ed to have as much gain as p ossible to enable the single stage desi gn . The RIAA respon se is achieved in th e feedback network: C101, C110, C111, C112, C119, C120 and R115, R112. C115 is used to correct between MM and MC gains as the amplifier is non-inverting.

SW100 switches between MM and MC. Two poles of the switch change between the different loading required for each type of the cartridge: R108 & C109 for MM and added in parallel for MC R104 & C108. The other two poles chan ge the feedbac k resister valu e to alter th e gain: R105 for MM and added in parallel for MC R123.

The DC offset is c ontrolled by a non-i nverting s ervo built around IC100. The amount of servo current is different for each gain setting via R111 (MM) and R124 (MC) so that the low frequency high pass point remains the same for both settings. However the high pass point for the circuit is set by C113. This gives a warp filter, stops DC start-up thumps from upsetting DC coupled circuitry and an approximation of the RIAA/IEC curve (-2d B @ 20Hz)

The output is class A buffered by a d ual mirror follower (TR104, TR105, TR108, TR109). The quiescent current is set by D100 and R118 and R119.

Closed loop st ability is achieved with C16, C117, gi ving symmetrical slewing capability.

15V regulated rails from the unit it is

L870 Phono Board Parts List Issue 2.0

Designator Part Description

C1 2A410 CERD 100N 63V -20% +80% RA C100 2N622 ELST 22U 63V C101 2D210 PPRO 1N0 5% 63V RA C102 2P710AS ELEC 100U 25V SILMIC C103 2P710AS ELEC 100U 25V SILMIC C104 2K410 PEST 100N 63V 10% C105 2K410 PEST 100N 63V 10% C106 2K410 PEST 100N 63V 10% C107 2K410 PEST 100N 63V 10% C108 2D147W PPRO W 470P 63V 5% RA C109 2D110W PPRO W 100P 63V 5% RA C110 2D247N PPRO 4N7 63V 5% RA C111 2D310 PPRO 10N 63V 1% AXIAL C112 2D310 PPRO 10N 63V 1% AXIAL C113 2K510 CAP MKS2 1U0 16V 10% C115 2D213N PPRO 1N3 63V 5% RA C116 2D147W PPRO W 470P 63V 5% RA C117 2D147W PPRO W 470P 63V 5% RA C118 2U610 ELST NON POLAR 10UF 35V C119 2D110N PPRO 100P 63V 5% RA C120 2D210 PPRO 1N0 5% 63V RA C200 2N622 ELST 22U 63V C201 2D210 PPRO 1N0 5% 63V RA C202 2P710AS ELEC 100U 25V SILMIC C203 2P710AS ELEC 100U 25V SILMIC C204 2K410 PEST 100N 63V 10% C205 2K410 PEST 100N 63V 10% C206 2K410 PEST 100N 63V 10% C207 2K410 PEST 100N 63V 10% C208 2D147W PPRO W 470P 63V 5% RA C209 2D110W PPRO W 100P 63V 5% RA C210 2D247N PPRO 4N7 63V 5% RA C211 2D310 PPRO 10N 63V 1% AXIAL C212 2D310 PPRO 10N 63V 1% AXIAL C213 2K510 CAP MKS2 1U0 16V 10% C215 2D213N PPRO 1N3 63V 5% RA C216 2D147W PPRO W 470P 63V 5% RA C217 2D147W PPRO W 470P 63V 5% RA C218 2U610 ELST NON POLAR 10UF 35V C219 2D110N PPRO 100P 63V 5% RA C220 2D210 PPRO 1N0 5% 63V RA D100 3A4148 SSDIODE 1N4148 75V D101 3A4148 SSDIODE 1N4148 75V D102 3A4148 SSDIODE 1N4148 75V D200 3A4148 SSDIODE 1N4148 75V D201 3A4148 SSDIODE 1N4148 75V D202 3A4148 SSDIODE 1N4148 75V IC100 5B071 IC FET OPAMP TL071 IC200 5B071 IC FET OPAMP TL071 PCB1 L870PB_2 PRINTED CIRCUIT BOARD R100 1H133 RES MF W4 1% 330R R101 1H110 RES MF W4 1% 100R R102 1H110 RES MF W4 1% 100R R104 1H110 RES MF W4 1% 100R R105 1H110 RES MF W4 1% 100R R106 1H110 RES MF W4 1% 100R R108 1H347 RES MF W4 1% 47K R109 1H312 RES MF W4 1% 12K R110 1H312 RES MF W4 1% 12K R111 1H356 RES MF W4 1% 56K R112 1H410 RES MF W4 1% 100K R113 1H410 RES MF W4 1% 100K R114 1H410 RES MF W4 1% 100K R115 1H315 RES MF W4 1% 15K R116 1H522 RES MF W4 1% 2M2

L870 Phono Board Parts List Issue 2.0

Designator Part Description

R117 1H522 RES MF W4 1% 2M2 R118 1H022 RES MF W4 1% 22R R119 1H022 RES MF W4 1% 22R R120 1H022 RES MF W4 1% 22R R121 1H022 RES MF W4 1% 22R R122 1H339 RES MF W4 1% 39K R123 1H010 RES MF W4 1% 10R R124 1H256 RES MF W4 1% 5K6 R200 1H133 RES MF W4 1% 330R R201 1H110 RES MF W4 1% 100R R202 1H110 RES MF W4 1% 100R R204 1H110 RES MF W4 1% 100R R205 1H110 RES MF W4 1% 100R R206 1H110 RES MF W4 1% 100R R208 1H347 RES MF W4 1% 47K R209 1H312 RES MF W4 1% 12K R210 1H312 RES MF W4 1% 12K R211 1H356 RES MF W4 1% 56K R212 1H410 RES MF W4 1% 100K R213 1H410 RES MF W4 1% 100K R214 1H410 RES MF W4 1% 100K R215 1H315 RES MF W4 1% 15K R216 1H522 RES MF W4 1% 2M2 R217 1H522 RES MF W4 1% 2M2 R218 1H022 RES MF W4 1% 22R R219 1H022 RES MF W4 1% 22R R220 1H022 RES MF W4 1% 22R R221 1H022 RES MF W4 1% 22R R222 1H339 RES MF W4 1% 39K R223 1H010 RES MF W4 1% 10R R224 1H256 RES MF W4 1% 5K6 SK1 8D230 PHONO SKT 2-WAY HOR EMC GOLD SK2 8K2408 8-WAY AMP CT CONN SW100 A1013 SW PUSH 4PCO TR100 4A556 TRANS LF SS P BC556B TR101 4A556 TRANS LF SS P BC556B TR102 4A556 TRANS LF SS P BC556B TR103 4A556 TRANS LF SS P BC556B TR104 4A556 TRANS LF SS P BC556B TR105 4A556 TRANS LF SS P BC556B TR106 4A1085 TRANS LF SS P 2SA1085 TR107 4A1085 TRANS LF SS P 2SA1085 TR108 4A546 TRANS LF SS N BC546B TR109 4A546 TRANS LF SS N BC546B TR110 4A546 TRANS LF SS N BC546B TR111 4A546 TRANS LF SS N BC546B TR112 4A546 TRANS LF SS N BC546B TR113 4A546 TRANS LF SS N BC546B TR114 4A546 TRANS LF SS N BC546B TR200 4A556 TRANS LF SS P BC556B TR201 4A556 TRANS LF SS P BC556B TR202 4A556 TRANS LF SS P BC556B TR203 4A556 TRANS LF SS P BC556B TR204 4A556 TRANS LF SS P BC556B TR205 4A556 TRANS LF SS P BC556B TR206 4A1085 TRANS LF SS P 2SA1085 TR207 4A1085 TRANS LF SS P 2SA1085 TR208 4A546 TRANS LF SS N BC546B TR209 4A546 TRANS LF SS N BC546B TR210 4A546 TRANS LF SS N BC546B TR211 4A546 TRANS LF SS N BC546B TR212 4A546 TRANS LF SS N BC546B TR213 4A546 TRANS LF SS N BC546B TR214 4A546 TRANS LF SS N BC546B

87654321

LEFT CHANNEL

C1 100N CD

SK1 PHONO2HG

EMC

0V_SIG

Q_1

Q_2

L870C2_2.0.SCH

LEFT IN LEFT OUT

RIGHT CHANNEL L870C3_2.0.SCH

RIGHT IN RIGHT OUT

Q_3

Q_4

+15V

Q_6

-15V

SK2 1 2 3 4 5 6 7 8

AMPCT8

0V_HF0V_SIG

Q_5

Q_7 Q_8

PCB1

PCB

L870PB_2

1 2 3 4 5 6 7 8

EL1

Update Box

UPDATE_BOX

DRAWING TITLE

A85 PHONO STAGE - TOP LEVEL

23425

A & R Cambridge Ltd. Pembroke Avenue Denny Industrial Centre Waterbeach Cambridge CB5 9PB

Notes:

Filename

G:\DATA\ECO\ECO AGENDA\01_1070 l870 A85 PHONO ISSUE2\L870_2.0.ddb - L870c1_2.0.PRJ

Circuit Diagram

01_1070 JAG 17/4/01 updated pcb and scm 2

00_1051 JAG 22/3/01 PRODUCTION ISSUE 1

ECO No. DESCRIPTION OF CHANGE

INITIALS

Date Printed

DATE

Drawn by:

JBR23-Apr-2001

1 3Sheet of

DRAWING NO.

L870C1

ISSUE

87654321

C102 100U SILMIC

R111 56K MF

R124

5K6 MF

R123 10R MF

C119

100P PP

C101

1N0 PP

C110

4N7 PP C120

1N0 PP

C112

10N PP AX C111

10N PP AX

R112 100K MF

C105 100N PE

Q_101

R113 100K MF

R114 100K MF

R115 15K MF

TL071CD

R101 100R MF

TR102 BC556B

TR108 BC546B

R116 2M2 MF

IC100

C118

10U NP

R117 2M2 MF

TR104 BC556B

TR100 BC556B

C100 22U EL

R109 12K MF

R110 12K MF

LEFT IN

R104 100R MF

SW100B 4PCO

C108 470P PPW

R108 47K MF

C109

100P PPW

TR106 2SA1085

R100 330R MF

TR101 BC556B

TR107 2SA1085

C115 1N3 PP

SW100D 4PCO

R105 100R MF

101112

Q_102

R102 100R MF

TR103 BC556B

D100 1N4148

C104

100N PE

0V_SIG0V_HF

C106 100N PE

+15V

TR109 BC546B

R118 22R MF

R119 22R MF

TR105 BC556B

C113

1U0 PE

0V_SIG

R122 39K MF

LEFT OUT

0V_SIG0V_SIG

TR110 BC546B

D101 1N4148

R120 22R MF

1 2 3 4 5 6 7 8

TR111 BC546B

D102 1N4148

R121 22R MF

DRAWING TITLE

A85 PHONO STAGE - LEFT CHANNEL

A & R Cambridge Ltd. Pembroke Avenue Denny Industrial Centre Waterbeach Cambridge CB5 9PB

C116

470P PPW

23425

TR112 BC546B

C103 100U SILMIC

Notes:

Filename

G:\DATA\ECO\ECO AGENDA\01_1070 l870 A85 PHONO ISSUE2\L870_2.0.ddb - L870C2_2.0.SCH

TR113 BC546B

TR114 BC546B

R106 100R MF

0V_SIG0V_HF

Circuit Diagram

C117 470P PPW

C107 100N PE

Q_103

-15V

01_1070 JAG 17/4/01 updated pcb and scm 2

00_1051 JAG 22/3/01 PRODUCTION ISSUE 1

ECO No. DESCRIPTION OF CHANGE

INITIALS

Date Printed

DATE

Drawn by:

JBR23-Apr-2001

2 3Sheet of

DRAWING NO.

L870C2

ISSUE

87654321

C202 100U SILMIC

R211 56K MF

R224

5K6 MF

R223 10R MF

C219

100P PP C201

1N0 PP C210

4N7 PP

C220

1N0 PP

C212

10N PP AX C211

10N PP AX

R212 100K MF

C205 100N PE

Q_201

R213 100K MF

R214 100K MF

R215 15K MF

TL071CD

R201 100R MF

TR202 BC556B

TR208 BC546B

R216 2M2 MF

IC200

3 2

C218 10U NP

R217 2M2 MF

TR204 BC556B

TR200 BC556B

C200 22U EL

R209 12K MF

R210 12K MF

RIGHT IN

R204 100R MF

SW100A 4PCO

C208

470P PPW

R208 47K MF

C209

100P PPW

TR206 2SA1085

R200 330R MF

TR201 BC556B

TR207 2SA1085

R205 100R MF

C215 1N3 PP

SW100C 4PCO

Q_202

R202 100R MF

TR203 BC556B

D200 1N4148

C204

100N PE

0V_SIG0V_HF

C206 100N PE

+15V

TR209 BC546B

R218 22R MF

R219 22R MF

TR205 BC556B

C213

1U0 PE

0V_SIG

R222 39K MF

RIGHT OUT

C217

0V_SIG0V_SIG

TR210 BC546B

D201 1N4148

R220 22R MF

TR211 BC546B

D202 1N4148

R221 22R MF

DRAWING TITLE

A85 PHONO STAGE - RIGHT CHANNEL

23425

A & R Cambridge Ltd. Pembroke Avenue Denny Industrial Centre Waterbeach

1 2 3 4 5 6 7 8

Cambridge CB5 9PB

470P PPW

TR212 BC546B

C203 100U SILMIC

Notes:

Filename

G:\DATA\ECO\ECO AGENDA\01_1070 l870 A85 PHONO ISSUE2\L870_2.0.ddb - L870C3_2.0.SCH

TR213 BC546B

TR214 BC546B

R206 100R MF

0V_SIG0V_HF

Circuit Diagram

470P PPWC216

C207 100N PE

Q_203

-15V

01_1070 JAG 17/4/01 updated pcb and scm 2

00_1051 JAG 22/3/01 PRODUCTION ISSUE 1

ECO No. DESCRIPTION OF CHANGE

INITIALS

Date Printed

DATE

Drawn by:

JBR23-Apr-2001

3 3Sheet of

DRAWING NO.

L870C3

ISSUE

Amplifier + PSU

Board L882

Contents

Circuit description

Component overlay

Parts list

Circuit diagrams

Amplifier & PSU Circuit Description

Refer to L882 circuit diagrams

This is the printed circuit board that provides the power supply and output s tage amp lifiers for the A32 integrat ed and P35 power amplifiers.

Its function is to:

! Drive the loudspeakers ! Provide an (always on) a uxiliary 5VDC supply for the

micro controller and display interface

! Receive logic signals from the micro controller to turn

on the main amplifier supply relay (mains) and connect either pair of speaker output sockets

! Send logic signa ls to the micro cont roller pertainin g to

the state of th e amplifiers (sh ort circuit protect ion, DC offset protection, thermal protection)

! Receive and demodulate RC5 remote style control

codes via th e rear panel jack and transmit them t o the micro controller

! Send a 12V trigger output via the rear panel jack for

control of an auxiliary power amp when the unit is on

! Receive a 12V trigger input from t he rear jack (for use

in the power amp only version)

! Drive a pair of headphones via attenuating resistor

networks The power amplifier is a symmetrical, class B, bipolar junction transistor output, current-feedback design (of which more later) with DC-coupled signal and feedback paths, featuring an active integrating voltage servo to control DC offsets.

It features ‘instantaneous’ safe operating area protection in addition to sendi ng a sign a l to th e micro to turn off th e out put relays in the even t of user or thermal ove rload. Since it i s a DC-coupled design, the unit senses DC at the output and triggers the micro to turn off the loudspeaker relays in the event of excessive levels (possibly due to a faulty source component or short circuit output transistor).

The output stage uses Sanken specialised ‘audio amplifier’ power bipolar Darlin gton transist ors which are optimi sed for use with this type of topology. Consequently the unit has excellent measured performance in terms of noise, slew rate, output impedance and distortion (harmonic and intermodulated) and is essentially load invariant (to a first order the measured performance is independent of the load impedance).

L882 Circuit Sheet 1

The audio input to the amplifier is connected to SK102 (which connects to the output of the preamp PCB). This signal is passed on via SK104A which forms the preamp out connection to the outside world.

SK104B provides the power amp input connection, with switchSW100 selecting between pre / power and integrated modes. The unit is wired as a preamp / power amp combination with the switch depressed, allowing the user to insert a processor or other function (e.g. graphic EQ) between the output of the preamp and the input of the power amp. With the switch in the ‘out’ position the power amp input socket is ignored and the input to the power amp is conn ected internally to the output of the preamp. PL100 and PL101 are ‘handbag’ links fitted to the power amp only version to connect both pairs of phono sockets in parallel for daisy chaining (as there is no preamp output on a power amp).

Relays RLY100 and RLY101 switch the two pairs of loudspeaker output sockets and are controlled by the micro lines describes above. Transistors TR100 and TR101 operate in ‘constant current sink ’ mode which allow relay current to be approximately constant although the main power supply

rails will vary with mains input and load conditions. The current is around 20mA per relay.

Star point SP100 is the ground ‘mecca’ for the entire amplifier (comp rising all three PCB s within the uni t). All of the separately named grounds are joined explicitly at this point. Different named grounds are used to ensure that no two ‘different’ grounds share copper, which could compromise the noise, distortion or crosstalk performance of the amplifier.

The loudspeaker output signals are passed to socket SK106 which connects to SK107 and onto the headphone output via the attenuation resistors R103 thru R106.

The hierarchy containing the other sheets is self explanatory. Each of the port names shown on the top sheet connec ts to the port of th e same name on th e lower sheets.

L882 Circuit Sheet 2

This sheet contains the power supplies, the rear panel jack socket trigger circuits, the standby relay control and the ‘interface’ci rcuits between t he output signa ls of the power amplifiers and the inputs expected by the micro processor.

The mains input enters the unit at SK203, with capacitors C205 and C206 acting as conducted RF suppression. The earth connection is passed on to the chassis (for safety reasons th e cha ssi s met alwork remai ns c onn ected to mains power earth at all times). Switch SW200 is the voltage selector switch, allowing the unit to be operated in 230V or 115V mains countries by switching the dual-primary mains transformers between series and parallel winding. Varistors VR200 and VR201 act to prevent over-voltag e surges from damaging the unit. If the user selects 115V operation and then connects the unit to a 230V supply, the varistors will go to a low impedance and blow the primary fuses. Any very high voltage line transients will also be suppressed, helping to eliminate transformer isolation breakdown.

Relay RLY200 switches the primary side of the mains transformer, allowing the micro to control the on / off status of the amplifier. Its contacts are snubbed by capacitors C207 and C208 (to eliminate switching spark transients and prolong relay lifespan). The primary windings of the toroidal mains transformer connect to SK204.

PCB mounted transformer TX200 is powered all the time that mains i s presen t on SK203 , irres pect ive of the on / off status of the amplifier. This is to ensure that the micro processor is always operational and can thus control the mains switching for the main amplifier. Secondary fuse F202 limits th e current in the event of a failure mode, as the short circuit primary current of TX200 would be insufficient to blow the mains fuses.

Diodes D200 thru D203, C227 and IC201 provide the 5VDC supply which powers the micro and display PCB and the rela y coils. C224 is to reduce diode noise being transmitted back through the leakage capacitance of TX200.

The mains transformer secondary winding is connected to SK200. This is a centre tapped winding, and is used with full bridge rect ifier BR200 to produce the ma in positive and negative su pplies for the p ower amp. C209 and C210 are the large reservoir capacitors, with C211 and C212 acting as high frequency decouplers. The main power supply rails and ground are accessible on SK205 for future module expansion.

The circuitry around SK201A and IC200 is to receive and demodulate remote control commands sent in via the rear panel jack socket. This is for multi-room applications. L200 and C200 form a parallel resonant circuit at approximately 37kHz. The output from this bandpass filter is passed into IC200A wh ere it is ‘chopped’ and fed to IC200B to provide the output sign al.

SK201B is a 13VDC signal trigger output which is active whenever the amplifier is powered up. R218 and DZ207 / C223 provide a reference voltage which is buffered by TR200. TR201 and R217 act as a current limit and prevent damage due to a short circuit on the output of SK201B. The maximum current is approximately 65mA.

TR203 and TR202 are a complementary Darlington pair which turn on mains relay RLY200 when activated by a signal from the microprocessor.

TR204 and its associated components are to detect whenever AC mains is present at the IEC socket. This is to notify the microprocessor i f the user has unplugged the mains cord , so that it can take the necessary a ction (muting all t he outputs and switching off the mains rela y). The reservoir capacitors should last at least 4 mains cycles which gives the microprocessor plenty of time for a cont rolled shutdown.

TR204 forms a monostable circuit. Each cycle of AC turns on TR204 via R211. TR204 then ‘shunts’ C229 ensuring that it is kept at a low potential. If more than one mains cycle is missing, then R 219 charges up C229 sufficiently to t rigger Schmitt inverter IC202E thus passing on a logic signal to the microprocessor. The use of a Schmitt inverter for IC202 is to ensure that the micro receives ‘clean’ logic levels - the hysteresis voltage (about 0.5V) is sufficient to prevent circuit noise from producing a string of ‘ghost’ signals when analogue levels are near the threshold point.

TH200 is a positive tempco thermistor placed adjacent to the heatsink on wh ich the outp ut transistors are mounted. Wh en the tempera ture of th e thermis tor exceeds 90 degrees Celsius the thermistor goes to a high impedance and so the input to IC202F goes low. This triggers a HIGH output to the micro indicating thermal overload.

The VI protection signals from the left and right channels pass into IC202A and IC202B respectively, to be ‘cleaned up’ via the Schmitt trigger. They are then NOR’d using TR205 which send s a HIGH signal to t he micro in the even t of either chann el sufferi ng a s hort c ircuit or cu rrent overloa d. Exactly the same approach is used for the DC fault lines using IC202C and IC202D.

L882 Circuit Sheet 3

This is the main audio power amplifier circuit. The amplifier is a class B design, which uses SAP ‘audio’ transistors in a symmetrical current feedback configuration. Input and feedback paths are DC coupled and there is an active integrating servo to remove DC offsets from the output.

The basic principle of operation is follows: The input signal is amplified by a factor of 2 in IC300A. This drives a 44˜ impedance to ground causing the supply pin currents to change with the signal level. These changing supply pin currents are then ‘reflected’ by a pair of complementary Wilson mirrors and passed on to a series of buffer transistors before being connected to the load. The ‘feedback current’ flows back from the output terminal via R331 and R332 and attempts to provide the current necessary to allow IC300A to swing its output without drawing excessive current from its supply pins, thus making the change in supply current very small indeed. This is why the term ‘current feedback’ is used - it is the current flowing in

the feedback resistors that sets the overall gain of the amplifier.

IC300B acts as an in verti n g in t egrat or an d its purp ose i s t o remove DC from the loudspeaker output. Any positive DC offset will caus e the outpu t of IC300B to go negat ive, thus increasing the current in its negative supply pin and pulling the output voltage back towards zero. R330 and C317 set the time constant of this integrator (0.47 seconds) so that audio frequency components are ignored and only DC and subsonic frequencies are removed.

The input to the amplifier is limited to ±5.4V via back-toback zener di odes DZ302 and DZ303. This is to prevent the user from grossly overdriving the input to the amplifier and possibly causing damage. The diodes appear before series resist or R324 so that their va riable c apaci tance d oes not introduce high frequency harmonic distortion.

R324, R327 and C316 act as an input filter - this is a first order low pass filter with a corner frequency of around 340kHz to prevent RF signa ls from being in ject ed into the front end of the amplifier. The corner frequency was chosen such that the ph ase shift int roduced is less than 5˜ at 20kHz (considered by the AES to be the minimum perceptib le relative amou nt by the hu man ear). The in put impedance of the amplifier is 23kW at DC, falling to around 14kW at 20kHz.

Operational amplifier IC300A is acting as a non-inverting gain of 2, driving the input signal into a 44W impedance to ground via R322 and R33 7. Its output voltage will b e an accurate amp lification of its input volta ge (i.e. the signal on pin 1 should look id entic al to that on pin 3 but at twic e the amplitude). The op-amp is used in a slightly unusual configuration here, in that its power supply pins are used as a (current) output, and its output pin is used as a (current) feedback.

Transistors TR311 and TR303 supply the ±15V rails to the op-amp, and act as cascades to pass its supply pin currents through to the current mirrors, whi ch sit at a potential too high for the op-amp to be c onnected directly.

TR300, TR301 and TR321 form a PNP Wilson current mirror, which reflects the current sunk by the positive supply pin of IC300. Likewise TR314, TR315 and TR320 form an NPN Wilson current mirror, which reflects the current sourced by the negative supply pin of IC300.

R315 thru R318 provide emitter degeneration of approximately 300mV for the current mirrors (as they pass about 3mA DC in quiescent conditions), to ensure accurate operation independent of the small variations between the transistors in the current mirrors. They also ensu re that the current passing down the next stage is reasonably constant as the internal temperature of the amplifier changes,

VBE

swamping out small thermal variations in the

of the

mirror transistors.

R319 and R320 slightly decouple the rails to the current mirrors from the main power rails of the amplifier, to allow the bootstrap circuit to operate. The bootstrap consists of C302 and C3 06 with met al film power r es is tors R352 and R353. The bootstrap is provided to allow the power supply rails of the current mirrors to go up and down slightly with the output signal into the loudspeaker. This enables the driver stage to fully sat urate the output transistors and thus give the greatest power output and best thermal efficiency for any gi ven power rail voltage. The voltage on the ‘i nsid e’ end of R3 19 and R3 20 will va ry by about 12 volts peak to peak at full output power, rising above the main power rails during signa l peaks.

C307 and C308 with R333 and R335 provide the compensation necessary to ensure stability when the loop is closed. The y are Mill er cap acitor s which dramat icall y reduce the transim pedance (i.e. current to volt age gain) of the current mirrors at high frequencies. The present value of 47pF provides for a unity gain open loop bandwidth of around 75MHz, whilst ensuri ng a closed loop ga in margin of around 6dB (note that gain margin in a current feedback design is not dependent on system bandwidth to a first order approximation ). R333 and R335 provide a ‘zero’ in the open loop frequency response which is tailored to give the best time domain performance (i.e. to make high frequency square waves look square with minimal ringing or overshoot).

DZ304 and C311 provide a fixed 4.7V bias voltage to allow the following stages to operate correctly. C311 is there to ensure that both halves of the following stage receive an equal AC signal component at high frequency.

TR310 and TR307 are the ‘pre-driver’ t ransistors, which act to buffer th e outputs from th e preceding stage and drive the Darlington out put power transistors. TR30 9 an d R3 21 act as a current limit, to ensure tha t the emi tt er curren t of TR310 d oes not exceed 30mA in a fault condition. TR306 and R323 provide the same function for TR307.

R338 and R339 are to loosely c ouple the ou tputs of th e predriver stage to the inputs of the Darlington power output devices. This is so that the inbuilt temperature sensing diodes of the outpu t transist ors can accur ately control t he quiesc ent current of th e outpu t stage a s the junct ion temp erature of t he power devices va ries . C 3 12 and C318 en su re t h at bot h halves of the output stage receive an equal AC signal component.

The output transistors are TR318 and TR319. These are Sanken SAP15N and SAP15P devices respectively. They are specially designed for audio power amplifier use. In addition

hFE

to high current gain (Darlington with a typi cal

of 20,000) they provide an inbuilt emitter resistor (thick film power resistor of 0W22) and temperature sensing diodes which

VBE

closely and rapidly track the

versus temperature characteristic of the power transi stors, allo wing for easy, fastresponding and reasonably accurate control of quiescent current.

RV300 is for fine trimming of the quiescent current. PL300 provides a convenient measuring point for this, which is short-circuit protected in the event of a slip with the multimeter probe! All of the remaining circuitry to the right of TR318 and TR319 is essentially for output stage protection...

Transistors TR312 and TR304, along with the network of resistors and capacitors to which they are connected, provide instantaneou s overload p rotection of the ou tput sta ge. This is a conventional single slope VI protection scheme, which allows much greater curren t to be delivered into a rated load than into a short circuit. The values allow for 18A peak delivery (at clip) into a purely resistive load, 7A peak (at clip) into a purely capacitive load and around 4A peak into a short circuit. R345, C303, R346 and C304 allow these values to be doubled for short transient bursts (approximately 2.7 milliseconds) so that impulsive musical transients can be delivered cleanly with minimal risk of damaging the output transistors.

TR313, TR302 and their associated components send a signal to the microprocessor when the instantaneous protection circuits are having to work ‘hard’ to prevent amplifier overload. Th is instructs the micro that th e user is severely abusing the amplifier and will switch off the loudspeaker relays to prevent possible permanent damage. In reality, if you short circui t the outpu ts at any ap precia ble volum e level, this circuit will trigger a nd the microprocessor will turn off the loudspeaker relays and send a signal t o the user.

R308, R314 and C320 form a low pa ss filter from which the DC detection circuits can sense excessive DC at the loudspeaker outputs. If there is any po sitive DC present, then TR316 will turn on, which turns on TR305 and thus activates th e DC protection lin e to the micro, turnin g off the loudspeaker relays.

If there is a ny negati ve DC presen t, then TR 308 will tu rn on, which turns on TR317 which then turns on TR305 in turn, causin g the same effect.

R350 and C319 are the Zobel network which is provided to ensure the amplifier ‘sees’ a consta nt and resistive load at very high frequencies, to aid stability, although the amplifier will be stable without the Zobel fitted.

C313 locally couples the ‘high frequency’ and loudspeaker ground returns together at the output to overcome the effects of track inductance back to the star point. C309 couples the ‘h igh frequency’ and signal grounds together at the input for the same reason.

D303 and D304 a re ‘flyback’ di odes to protect the output transistors from reverse bias when the amplifier is heavily clipped into an indu ct i ve loa d (such as a loud s p eaker voi ce coil!)

Sheet 4 is an identical copy of sheet 3 so I will not describe it separ ately.

L882 Amplifier and PSU Board Parts List Issue 3.0

Designator Part Description

BR200 3BGBU8D BRIDGE RECTIFIER 8A 200V C100 2C210 MLC 1N0 50V X7R 10% SM C200 2D168 PPRO 680P 5% 63V RA C201 2C410 MLC 100N 50V X7R 10% SM C202 2C410 MLC 100N 50V X7R 10% SM C203 2C410 MLC 100N 50V X7R 10% SM C204 2C410 MLC 100N 50V X7R 10% SM C205 2K233 SUPPR CAP 3N3 250V C206 2K233 SUPPR CAP 3N3 250V C207 2K233 SUPPR CAP 3N3 250V C208 2K233 SUPPR CAP 3N3 250V C209 2N910A ELST 10m 63V RA 35mm C210 2N910A ELST 10m 63V RA 35mm C211 2H410 PCRB 100N 100V 10% RA 5mm C212 2H410 PCRB 100N 100V 10% RA 5mm C213 2C310 MLC 10N 50V X7R 10% SM C214 2C310 MLC 10N 50V X7R 10% SM C215 2C310 MLC 10N 50V X7R 10% SM C216 2C310 MLC 10N 50V X7R 10% SM C217 2C310 MLC 10N 50V X7R 10% SM C218 2C310 MLC 10N 50V X7R 10% SM C219 2C310 MLC 10N 50V X7R 10% SM C220 2C410 MLC 100N 50V X7R 10% SM C221 2C310 MLC 10N 50V X7R 10% SM C222 2N610 ELST 10U 50V C223 2N610 ELST 10U 50V C224 2A410 CERD 100N 63V 20% RA C225 2A410 CERD 100N 63V 20% RA C226 2A410 CERD 100N 63V 20% RA C227 2N833 ELST 3M3 25V C228 2N810A ELST 1M0 10V C229 2N622 ELST 22U 63V C230 2C310 MLC 10N 50V X7R 10% SM C231 2C210 MLC 1N0 50V X7R 10% SM C232 2C410 MLC 100N 50V X7R 10% SM C233 2N610 ELST 10U 50V C234 2D422 220NF CLASS X2 CAP 275VRMS C235 2C210 MLC 1N0 50V X7R 10% SM C300 2N710 ELST 100U 25V C301 2N710 ELST 100U 25V C302 2N710B ELST 100U 100V C303 2N710 ELST 100U 25V C304 2N710 ELST 100U 25V C305 2N610 ELST 10U 50V C306 2N710B ELST 100U 100V C307 2C110 MLC 100P 50V NPO 5% SM C308 2C110 MLC 100P 50V NPO 5% SM C309 2C410 MLC 100N 50V X7R 10% SM C310 2C410 MLC 100N 50V X7R 10% SM C311 2H410 PCRB 100N 100V 10% RA 5mm C312 2C310 MLC 10N 50V X7R 10% SM C313 2C410 MLC 100N 50V X7R 10% SM C314 2C410 MLC 100N 50V X7R 10% SM C315 2C410 MLC 100N 50V X7R 10% SM C316 2D147W PPRO W 470P 63V 5% RA C317 2K447 PEST 470N 63V 10% C318 2N610 ELST 10U 50V C319 2K410 PEST 100N 63V 10% C320 2V710 ELST NON POLAR 100UF 16V C400 2N710 ELST 100U 25V C401 2N710 ELST 100U 25V C402 2N710B ELST 100U 100V

L882 Amplifier and PSU Board Parts List Issue 3.0

Designator Part Description

C403 2N710 ELST 100U 25V C404 2N710 ELST 100U 25V C405 2N610 ELST 10U 50V C406 2N710B ELST 100U 100V C407 2C110 MLC 100P 50V NPO 5% SM C408 2C110 MLC 100P 50V NPO 5% SM C409 2C410 MLC 100N 50V X7R 10% SM C410 2C410 MLC 100N 50V X7R 10% SM C411 2H410 PCRB 100N 100V 10% RA 5mm C412 2C310 MLC 10N 50V X7R 10% SM C413 2C410 MLC 100N 50V X7R 10% SM C414 2C410 MLC 100N 50V X7R 10% SM C415 2C410 MLC 100N 50V X7R 10% SM C416 2D147W PPRO W 470P 63V 5% RA C417 2K447 PEST 470N 63V 10% C418 2N610 ELST 10U 50V C419 2K410 PEST 100N 63V 10% C420 2V710 ELST NON POLAR 100UF 16V D100 3B4003 RECTIFIER 1N4003F 1A 200V D101 3B4003 RECTIFIER 1N4003F 1A 200V D200 3B4003 RECTIFIER 1N4003F 1A 200V D201 3B4003 RECTIFIER 1N4003F 1A 200V D202 3B4003 RECTIFIER 1N4003F 1A 200V D203 3B4003 RECTIFIER 1N4003F 1A 200V D205 3AS16W DIODE SS SM BAS16W D206 3AS16W DIODE SS SM BAS16W D300 3AS16W DIODE SS SM BAS16W D301 3AS16W DIODE SS SM BAS16W D302 3AS16W DIODE SS SM BAS16W D303 3B4003 RECTIFIER 1N4003F 1A 200V D304 3B4003 RECTIFIER 1N4003F 1A 200V D400 3AS16W DIODE SS SM BAS16W D401 3AS16W DIODE SS SM BAS16W D402 3AS16W DIODE SS SM BAS16W D403 3B4003 RECTIFIER 1N4003F 1A 200V D404 3B4003 RECTIFIER 1N4003F 1A 200V DZ204 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ207 3CW315V ZENER 15V OW35 SM SOT23 DZ300 3CW315V ZENER 15V OW35 SM SOT23 DZ301 3CW315V ZENER 15V OW35 SM SOT23 DZ302 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ303 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ305 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ306 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ400 3CW315V ZENER 15V OW35 SM SOT23 DZ401 3CW315V ZENER 15V OW35 SM SOT23 DZ402 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ403 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ405 3CW34V7 ZENER 4V7 OW 35 SM SOT23 DZ406 3CW34V7 ZENER 4V7 OW 35 SM SOT23 EL 8M101 EARTH LEAD F200 C12207 FUSE 20mm 2A AS F200 F022 INS COVER PCB FUSEHOLDER F200 8S004 FUSEHOLDER 20mm PCB F201 F022 INS COVER PCB FUSEHOLDER F201 8S004 FUSEHOLDER 20mm PCB F201 C12207 FUSE 20mm 2A AS F202 C3501 FUSE R452 T500mA HS200 F008 HEATSINK TO220 8.6 DEGC/W HS200 F006 HEATSINK CLIP TO220 13/8.6 DC/W IC200 5M393AD IC COMPARATOR SM DUAL LM393A IC201 5D7805 IC VREG POS 7805

L882 Amplifier and PSU Board Parts List Issue 3.0

Designator Part Description

IC202 5J7414D SURFACE MOUNT HEX SCHMITT TRIGGER IC300 5B072D IC AUDIO SM DUAL TL072 IC400 5B072D IC AUDIO SM DUAL TL072 L200 7D327 27mH INDUCTOR L300 7D002C INDUCT 2U2 12x20mm L400 7D002C INDUCT 2U2 12x20mm PCB L882PB_3 PRINTED CIRCUIT BOARD PL100 8K6201 2WAY MOLEX VERT MALE CONNECTOR PL101 8K6201 2WAY MOLEX VERT MALE CONNECTOR PL300 8K6201 2WAY MOLEX VERT MALE CONNECTOR PL400 8K6201 2WAY MOLEX VERT MALE CONNECTOR R100 1H010 RES MF W4 1% 10R R101 1A110 RES SM W4 1% 100R 1206 R102 1A110 RES SM W4 1% 100R 1206 R103 1.00E+133 RES CF 1W 330R 5% R104 1.00E+133 RES CF 1W 330R 5% R105 1.00E+110 RES CF 1W 100R 5% R106 1.00E+110 RES CF 1W 100R 5% R107 1A122 RES SM W4 1% 220R 1206 R108 1A122 RES SM W4 1% 220R 1206 R200 1A210 RES SM W4 1% 1K0 1206 R201 1A210 RES SM W4 1% 1K0 1206 R202 1A210 RES SM W4 1% 1K0 1206 R203 1A210 RES SM W4 1% 1K0 1206 R204 1A210 RES SM W4 1% 1K0 1206 R205 1A210 RES SM W4 1% 1K0 1206 R206 1A210 RES SM W4 1% 1K0 1206 R207 1A210 RES SM W4 1% 1K0 1206 R208 1A310 RES SM W4 1% 10K 1206 R209 1A310 RES SM W4 1% 10K 1206 R210 1A310 RES SM W4 1% 10K 1206 R211 1A310 RES SM W4 1% 10K 1206 R212 1A247 RES SM W4 1% 4K7 1206 R213 1A247 RES SM W4 1% 4K7 1206 R214 1A247 RES SM W4 1% 4K7 1206 R215 1A247 RES SM W4 1% 4K7 1206 R216 1A247 RES SM W4 1% 4K7 1206 R217 1A010 RES SM W4 1% 10R 1206 R218 1A310 RES SM W4 1% 10K 1206 R219 1A218 RES SM W4 1% 1K8 1206 R220 1A010 RES SM W4 1% 10R 1206 R221 1A010 RES SM W4 1% 10R 1206 R222 1A310 RES SM W4 1% 10K 1206 R223 1A310 RES SM W4 1% 10K 1206 R224 1A218 RES SM W4 1% 1K8 1206 R225 1A218 RES SM W4 1% 1K8 1206 R226 1A147 RES SM W4 1% 470R 1206 R227 1A210 RES SM W4 1% 1K0 1206 R228 1A410 RES SM W4 1% 100K 1206 R229 1A410 RES SM W4 1% 100K 1206 R230 1A215 RES SM W4 1% 1K5 1206 R231 1K515 RES W25 5% VR25 1M5 R300 1A310 RES SM W4 1% 10K 1206 R301 1A310 RES SM W4 1% 10K 1206 R302 1A310 RES SM W4 1% 10K 1206 R303 1A310 RES SM W4 1% 10K 1206 R304 1A310 RES SM W4 1% 10K 1206 R305 1A310 RES SM W4 1% 10K 1206 R306 1A310 RES SM W4 1% 10K 1206 R307 1A310 RES SM W4 1% 10K 1206 R308 1A310 RES SM W4 1% 10K 1206 R309 1A310 RES SM W4 1% 10K 1206

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