Arcam-P85_3-int-sm(2)维修电路原理图.pdf

A85DiVA A85, P85 & P85/3 Amplifi ers Service Manual ARCAMIssue 2.0RadioFans.CN 收音机爱 好者资料库 Contents List ! Contents list ! Circuit description ! Service guide ! Circuit diagrams ! Component overlays ! Circuit board parts list ! General assembly parts list RadioFans.CN 收音机爱 好者资料库 Pre-amplifier circuit description The A85 preamplifier is a high-performance, DC coupled design with microprocessor control of input select, two independent tape loops, electronic volume control, tone bypass and electronic tone control. It features a discrete power supply and low-noise linear circuitry to obtain very good distortion and noise performance, suitable for high quality source material such as CD or DVD-A. Input switching Each of the inputs has a pair of diodes to the 15V rails to prevent static spikes from causing damage to the CMOS multiplexers. In addition, there is a simple resistor-capacitor filter with a corner frequency of approximately 340kHz to remove any unwanted high frequency interference from the signal. This uses high-quality polypropylene capacitors for best performance. Z104 and Z105 are the main input select multiplexers, which are configured in a virtual earth unity gain arrangement with Z115 and Z116. This arrangement is slightly lower distortion than the normal one, at the cost of a slightly higher noise floor. It is an inverting configuration, which is restored to correct polarity by the inverting electronic volume control which follows. Z115B and Z116B are integrating servos, which take out any DC from the input signal before the following stages. The servos are 2-pole, with a passive 2nd pole being formed by R180 and C147 (for the left channel) to remove broadband noise from the output of the servo and improve speed of response. Z100 thru Z103 are the input selectors for the 2 tape loops. These are normal non-inverting selectors which are buffered before being passed on to the phono sockets. Z109A output is decoupled by R108 which is included in the feedback path. Local high frequency feedback occurs around C108 to allow the tape loop output to be very low impedance, whilst being stable into a capacitive load such as may be presented by a screened interconnect cable. This is the same for all tape outputs. Z106 is configured as a double pole changeover switch, used to select the tone controls. The tone controls are bypassed when not required so that the noise and distortion can be minimised. Tone control circuit The tone control circuit is a non-inverting one, using a gyrated bell filter for the bass and a simple shelving filter for the treble. Left channel description The input is attenuated by 6dB and biased to a voltage of +2.5V DC by C111, R113, R112, R110, R111 and C110. This is so the signals fall within the 0 - 5VDC required by the digital potentiometer Z108. Z111B and its associated components form an active equivalent of a series resonant LCR circuit. This has an impedance minimum of 5.4k at around 80Hz with Q=0.7 The reason the bass is done as a band-boost filter rather than a shelving filter is so that you can boost the real bass without causing lots of sub-audio loudspeaker cone excursion which wastes power and may damage the drive units. The digital pots Z108D and Z108A control the bass and treble respectively. This is done by moving the wiper connected to the frequency-sensitive impedance between the non-inverting and inverting terminals of Z112A, effectively changing the ratio of feedback boost and feed-forward attenuation of the circuit at the desired frequencies, thus providing a EQ gain control that is symmetrical on a logarithmic scale, with the use of a linear pot. Z112B provides the 6dB of gain necessary to bring the nominal signal level back to unity. C116 and C117 remove the 2.5VDC offset from the output, to prevent clunks when the tone controls are activated. Z108 is controlled by a simple 3-wire serial interface from the microprocessor. Each of the digital lines has its own ground return to minimise electromagnetic interference. They are connected together only at the GND pin of the IC. Volume control Z107 is a VSDVC electronic volume control IC. It works, in conjunction with an external op-amp, by varying the feed-forward and feedback resistors in an inverting gain configuration. In this way, it can allow output signal swings of up to 22Vpp whilst operating from a single +5VDC power supply. Also, it allows the user the choice of external circuitry to fine-tune the performance. The gain is controlled from the microprocessor via a 3-wire serial interface. The analogue supply rail is derived from the local +5V via R185 and C156 / C157. Z117 is the output op-amp. Its outputs are decoupled via R186, R187, C158 and C159 so that it has a low output impedance but can drive cable capacitance without oscillation. R186 and R187 are included in the audio frequency feedback loop to reduce output impedance when driving difficult cables. RLY100 is a mute relay which shunts the preamp output to ground. This is to prevent thumps and squeals when the units is powered up or down. Power supply The transformer winding is connected to SK300. The voltage is rectified and smoothed by D300, D301, D306, D307 and C300, C310. The unregulated voltage should be around 27VDC. F300 and F301 are secondary fuses, as the low power preamp winding would not blow the primary fuses if short circuited. The voltage regulators are discrete compound emitter followers. I will describe the +15V supply as the negative is essentially an exact mirror image. Q300 and R300 act as a constant current source, supplying around 7mA into D310. C302 and C314 reduce ripple and broadband noise on the zener diode. Q305 and Q306 form a complementary Darlington NPN transistor which is configured as an emitter follower, producing the +15VDC at its output. C303 is to provide bulk charge storage and to reduce the AC output impedance of the power supply. D302 prevents reverse bias of the supply during power down. Z301 is a conventional LM317 type circuit to drop the +15V rail down to +5V for the tone and volume control circuits. Star point SP300 explicitly connects the differently named ground nets together at one point, to minimise hum. Amplifier & 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 display 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, 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 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 active 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 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 Darlington transistors which are optimised 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 switch SW100 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 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 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 (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 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 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 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 remains 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-voltage 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 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. Secondary 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 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 rectifier BR200 to produce the main positive and negative supplies for the power 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 where it is chopped and fed to IC200B to provide the output signal. 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 if the user has unplugged the mains cord, so that it 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. 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 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 which the output transistors 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 IC202A and IC202B respectively, to be cleaned up via the Schmitt trigger.