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1、A85 DiVA A85, P85 & P85/3 Amplifi ers Service Manual ARCAMARCAM Issue 2.0 RadioFans.CN Contents List ! Contents list ! Circuit description ! Service guide ! Circuit diagrams ! Component overlays ! Circuit board parts list ! General assembly parts list RadioFans.CN Pre-amplifier circuit description T
2、he A85 preamplifier is a high-performance, DC coupled design with microprocessor control of input select, two independent tape loops, electronic volume con5.1(e)16.3.1(n5? u7.1(e)16)15.1(umb10.62(ey)281(o)a)13.1()13.1( an.1(e)1d )15.1(u)6.9(,)2.2( el)15.1(e)16.8(c)1.8(tr)10.5(o)13u7.1(e)16ee Amplifi
3、er & PSU Circuit Description L882PB is the printed circuit board that provides the power supply and output stage amplifiers for the A85 integrated and power amplifiers. Its function is to: 1. Drive the loudspeakers(!) 2. Provide an (always on) auxiliary 5VDC supply for the micro controller and displ
4、ay interface 3. Receive logic signals from the micro controller to turn on the main amplifier supply relay (mains) and connect either pair of speaker output sockets 4. Send logic signals to the micro controller pertaining to the state of the amplifiers (short circuit protection, DC offset protection
5、, thermal protection) 5. Receive and demodulate RC5 remote style control codes via the rear panel jack and transmit them to the micro controller 6. Send a 12V trigger output via the rear panel jack for control of an auxiliary power amp when the unit is on 7. Receive a 12V trigger input from the rear
6、 jack (for use in the power amp only version) 8. 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 activ
7、e integrating voltage servo to control DC offsets. It features instantaneous safe operating area protection in addition to sending a signal to the micro to turn off the output relays in the event of user or thermal overload. Since it is a DC- coupled design, the unit senses DC at the output and trig
8、gers 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 Darlington transistors which are optimised for use with this type
9、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 T
10、he 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 switch SW100 selecting between pre / power and
11、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 sock
12、et is ignored and the input to the power amp is connected 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
13、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 inpu
14、t and load conditions. The current is around 20mA per relay. Star point SP100 is the ground mecca for the entire amplifier (comprising all three PCBs within the unit). All of the separately named grounds are joined explicitly at this point. Different named grounds are used to ensure that no two diff
15、erent 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 t
16、he other sheets is self explanatory. Each of the port names shown on the top sheet connects to the port of the same name on the 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 circuits
17、 between the output signals 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 the chassis metalwork r
18、emains connected 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-volta
19、ge 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.
20、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 t
21、ransformer connect to SK204. PCB mounted transformer TX200 is powered all the time that mains is present on SK203, irrespective 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. S
22、econdary fuse F202 limits the 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 relay coils. C224 is to redu
23、ce 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 rectifier BR200 to produce the main positive and negative supplies for the power amp. C209 and
24、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
25、 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 where it is chopped and fed to IC200B to provide the output signal. SK201B is a 13VDC signal trigger output which
26、 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 com
27、plementary 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 if the user has unplugged the mains cord, so that it
28、can take the necessary action (muting all the outputs and switching off the mains relay). The reservoir capacitors should last at least 4 mains cycles which gives the microprocessor plenty of time for a controlled shutdown. TR204 forms a monostable circuit. Each cycle of AC turns on TR204 via R211.
29、TR204 then shunts C229 ensuring that it is kept at a low potential. If more than one mains cycle is missing, then R219 charges up C229 sufficiently to trigger 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 m
30、icro 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 which the output transist
31、ors are mounted. When the temperature of the thermistor 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 I
32、C202A and IC202B respectively, to be cleaned up via the Schmitt trigger. They are then NORd using TR205 which sends a HIGH signal to the micro in the event of either channel suffering a short circuit or current overload. Exactly the same approach is used for the DC fault lines using IC202C and IC202
33、D. 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
34、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 Wilso
35、n 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
36、 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 inverting integrator and its purpose is to remove DC from the loudspea
37、ker output. Any positive DC offset will cause the output of IC300B to go negative, 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 ign
38、ored and only DC and subsonic frequencies are removed. The input to the amplifier is limited to 5.4V via back-to-back zener diodes 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 resistor R3
39、24 so that their variable capacitance does 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 signals from being injected into the front end of the amplifier. The
40、corner frequency was chosen such that the phase shift introduced is less than 5? at 20kHz (considered by the AES to be the minimum perceptible relative amount by the human ear). The input impedance of the amplifier is 23kW at DC, falling to around 14kW at 20kHz. Operational amplifier IC300A is actin
41、g as a non-inverting gain of 2, driving the input signal into a 44W impedance to ground via R322 and R337. Its output voltage will be an accurate amplification of its input voltage (i.e. the signal on pin 1 should look identical to that on pin 3 but at twice the amplitude). The op- amp is used in a
42、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,
43、which sit at a potential too high for the op-amp to be connected 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 sourc
44、ed 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. The
45、y also ensure that the current passing down the next stage is reasonably constant as the internal temperature of the amplifier changes, swamping out small thermal variations in the VBE of the mirror transistors. R319 and R320 slightly decouple the rails to the current mirrors from the main power rai
46、ls of the amplifier, to allow the bootstrap circuit to operate. The bootstrap consists of C302 and C306 with metal film power resistors 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 loudspeak
47、er. This enables the driver stage to fully saturate the output transistors and thus give the greatest power output and best thermal efficiency for any given power rail voltage. The voltage on the inside end of R319 and R320 will vary by about 12 volts peak to peak at full output power, rising above
48、the main power rails during signal peaks. C307 and C308 with R333 and R335 provide the compensation necessary to ensure stability when the loop is closed. They are Miller capacitors which dramatically reduce the transimpedance (i.e. current to voltage gain) of the current mirrors at high frequencies
49、. The present value of 47pF provides for a unity gain open loop bandwidth of around 75MHz, whilst ensuring a closed loop gain 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 pro
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