Arcam AVR400 User Manual 2

AVR400 Service Manual Issue 1

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FAN 80 x 80 x 25mm

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AVR400 Power Amplier Circuit Description

Apart from the power transformer, the power amplier electronics is fully contained on the large double sided PTH PCB and heatsink located at the bottom of the AVR400. This is called the Main Board on the schematic diagrams (pages 11 and 12). The Main Board also contains the mains input circuitry, in-

cluding the safety fuses and standby power transformer, so great care should be exercised when probing this area of the board.

Note that some small surface mount components are soldered to the underside of this PCB.

The 7 power ampliers are identical in terms of circuitry, although necessary compromises in the physical layout may give rise to slight dierences in measured noise, crosstalk and distortion performance. The two channels at the extreme ends of the heatsink have less radiating area available to them and will run hotter under load – this is not normally an issue as these are assigned to the SBL and SBR channels.

Looking from the rear of the AVR, facing the at face of the heatsink, the channel order is SBR, SR, FR, C, FL, SL and SBL, the same order as the loudspeaker terminals. The SBL and SBR channels can also be assigned to zone 2 or as duplicates of the FL and FR channels for when passively biamping the main stereo loudspeakers. In this latter condition we recommend assigning the SBL and SBR outputs to the tweeters of the FL and FR speakers, in order to minimize the power ampliers’ heat dissipation.

Note that the 8 pre-amplier outputs are also on this PCB – apart from SUB their phono sockets are eectively in parallel with the power amplier inputs, which are fed from the Input Board via a ribbon cable and CON103. This connector also carries 5 power supply and power amplier control signals to and from the system microprocessor (μP) IC151 situated on the Input Board above the Main Board.

The ampliers’ power supply is provided from a centre-tapped secondary winding on the toroidal power transformer, via the connector BN508, to the bridge rectier D5830. This is mounted on a small PCB near the top of the heatsink. The rectied AC is then sent to the main PCB via connectors BN581/582. To avoid induced hum and distortion it is important to keep these cables twisted tightly together and well away from the actual power amplier circuitry. The main 15,000μF 80V reservoir capacitors, C835 and C836, are positioned on the main PCB well away from the power amplier input traces and close to the system star ground. The smoothed DC is fed to the power ampliers’ Vcc and Vee lines near the centre of the heatsink via a twisted-pair cable, again to minimize induction into the power ampliers. Vcc and Vee are typically +/- 52V at 234VAC with no signal. Q5845 sends a fraction of Vcc to the muting control on the Input Board.

The FL (front left) channel will now be described in detail.

The input stage is a long tailed pair Q5101 and Q5102, with local degeneration provided by R5105 and R5107. The tail is fed from the negative rail via an approx 3mA ring-of-two constant current source, Q5109 and Q5110. R5101 and C5101 at the input provide high frequency rollo and help keep residual DAC ultrasonic noise above 100kHz out of the power amplier. DC blocking is provided by C5102 at the input and C5107 in the feedback loop so that the whole power amplier has unity gain at DC. The midband AC gain is 22000/680 = 32.35 after allowing for the attenuation provided by R5101 and R5102. Thus 875mV at the input produces 100W into 8 ohms at the output.

The long tailed pair’s collectors are loaded by a current mirror, Q5103 and Q5104. The resistors R5103/4 and R5105/6 are 1% tolerance to minimize even order distortion. The collector of Q5101 also feeds the Darlington class A voltage amplier stage (VAS) made up of Q5115 and Q5116. Q5113 is loaded by the output stage and the 8mA constant current source made up by Q5125 and Q5126. The amplier’s main frequency compensation network (for stability) comprises C5115 plus the combination of C5116 and R 5115. This adds gain inside the loop (two pole compensation) at high audio frequencies so that the

additional feedback further reduces high frequency distortion and crossover distortion within the audio band.

These stages are partially decoupled from the Vcc and Vee power supplies by D5130/R5130/C5130 and D5129/R5129/C5129 respectively. They are also bootstrapped to the amplier output via the networks R5118/C5128/R5128 and R5117/C5127/R5127. This raises the supply lines by approximately 3V at full output to avoid clipping the driver stage prematurely.

The output stage comprises classic complementary emitter followers Q5150/Q5170 (NPN) and Q5160/ Q5180 (PNP). The On Semiconductor output transistors have a current gain that is sustained to about 10 amps and a very large safe operating area, which allows the ampliers to drive low impedances well. They also have built-in thermal compensation diodes which helps stabilize the quiescent current both statically (when hot) and dynamically (when playing music at high level) – this minimizes crossover distortion and improves sound quality.

The output stage biasing is performed in the network around the amplied diode Q5120, which is mounted in intimate thermal contact with the driver transistor Q5130, plus the two built-in diodes associated with the output transistors. The thermistor R5122 is positioned on the PCB close to the heatsink and provides extra downward compensation at very high temperatures. Bias is set by VR51 and is largely independent of temperature – it should be set to16-20mV when measured across the outer terminals of the compound emitter resistor R5175, using the 2 pin connector CN51, 5 minutes or more after the AVR400 is powered up.

The power amplier output is routed across the PCB to the back panel. It includes a Zobel network (sometimes called a Boucherot cell) R5183/C5183 and a series inductor L5185 damped by a 4.7R 2W resistor R5184. These components help isolate the amplier from reactive loads to ensure high frequency stability. One half of the normally-o relay RL52 is used to switch the load in and out.

Each power amplier is protected against overload in a number of ways. The complementary transistors Q5130 and Q5131 protect the NPN half of the output stage and Q5140 and Q5141 the PNP half. They operate as Sziklai pairs, passing negligible current until a threshold voltage of approx 600mV is reached across R5132 and R5142. Between 600 and 700mV the pairs then ramp up current smoothly, diverting it away from the bases of Q5150 and Q5160 to limit the output stage drive to a safe level, within the power transistors’ SOA (safe operating area). The 600mV threshold voltage depends upon both the instantaneous current and voltage across the output transistors, set by the networks R5132/R5136/R5137/R5138/ R5175 for the top half and R5142/R5146/R5147/R5148/R5175 for the bottom half. R5135/R5145 and the zener diodes D5135/D5145 change the slope of the protection locus at high Vce voltages. R5134/R5135 plus C5134/C5145 prevent fast transients and brief overloads from prematurely triggering the protection.

The above dual slope SOA protection is self resetting but if a gross overload persists for more than a second or two (such as when a channel’s output is short circuited with music playing at a moderate to loud level) then the open collector transistor Q5181 sinks current for long enough to initiate the amplier’s full shutdown procedure via the line SOA_PROTECT. This can also be triggered by a total output stage failure (which passes enough current through R5175 to turn on Q5188) via OVERLOAD or by an excessive DC oset at the output terminals (via R5185) via V_DET. All these signals, and others, feed into the protection module, described below.

The protection module comprises 8 transistors and associated parts positioned at the back of the PCB near the preamplier output sockets. It has a single output line named PROTECT which, when pulled down from Vcc to ground, instructs the system μP IC151 to shut down the whole amplier. This occurs when any of the following events happen:

1) Any amplier channel pulls current through the SOA_PROTECT line for long enough to charge up capacitors C5871 and then C5872 so that Q5874 turns on.

2) Any amplier channel pulls current through the OVERLOAD line for long enough to charge up C5882 and turn on Q5882 and thus Q5884.

3) Any amplier channel has a large long term (DC) oset (typically greater than +/-3V) sμFcient to charge up C5861 enough to turn on either Q5862 (positive oset) or Q 5864 (negative oset). These then turn on Q5863. N.B. This circuit is also used to detect imbalances in both the Vcc/Vee and +/-15V power supplies (the latter is generated on the Power Supply board).

4) When the PTC thermistor TH585 mounted at the top centre of the heatsink gets sμFciently hot (around 100C) and thus high resistance enough to cause Q5855 to turn on via the +12V supply. Intermediate temperatures will not activate PROTECT but will provide signals to the level detectors associated with the FAN_1 and FAN_2 lines, to run the cooling fans at high or low speeds respectively.

Note that the fans’ 12V supply is gated via Q5909 and Q5911. This means the fans will not run when no signal is present on the FL, C or SR channels, so that during quiet passages no fan noise should be audible.

AVR400 Main Power Supplies Circuit Description

The main DC power supplies are located on the Power Supply Board - a double sided PTH PCB adjacent to the Main Board. Note that the mains input switching, mains fuses, standby power transformer with its associated unregulated DC supply and the relay for enabling the power amplier supply rails are located on the Main Board. The two boards communicate via CON501.

Considering the Main Board rst, the mains voltage switching uses a double pole double throw slide switch accessible from the back panel. One pole addresses the standby transformer and the other the toroidal power transformer. Ensure the switch setting matches the supply voltage before switching on the AVR400. Nominal settings are 115 and 230V +/- 15%. The 20mm 115V fuse in line with the toroidal transformer is rated at 15A T (Time delay) and the 20mm 230V fuse is rated at 8A T. Always replace these fuses with the same type and value. The standby transformer T5941 is not fused but is designed to go open circuit in case of overheating (e.g. if left connected to a 230V supply for longer than a few minutes when the mains voltage selector is set to 115V).

The standby transformer generates approximately +9V DC via the bridge rectier diodes D5495/6/7/8 and the 1,000μF reservoir capacitor C5947.This is sent via pin 7 of CN501 to the Power Supply PCB (confusingly marked as 5V). The rail voltage is also routed to the system microprocessor (μP) via D5965/ R5965 and pin 6 of CN103 as POWER_MUTE.

The 5V relay RY594 is normally open. When the mains switch is closed then the SUB_POWER rail (approx +4.3V) is activated from the Power Supply PCB via the standby transformer. When the system has booted correctly, without any shutdown signals, then the POWER_RELAY signal from the system μP also goes high, pulling down Q5947 hard. Only then does the relay close and switch on the main toroidal power transformer, enabling the rest of the system to boot up.

Now consider the Power Supply Board, found on page 14 of the schematic. This generates all the main DC supplies for the AVR400 except +Vcc and -Vee for the power ampliers and the non-logic part of the VFD display requirements of the Front Panel Board. Note that additional local regulation also takes place on the other PCBs, e.g. for large digital ICs.

Two secondary windings from the toroidal power transformer are fed in via CN63. Pins 1, 2 and 3 connect to a centre-tapped secondary winding used to generate approx +/- 20VDC via the bridge rectier diodes D603/3/4/5 and the 2,200μF 35V reservoir capacitors C609 and C610. R603 and R604 are 0.47R 1W

fusible resistors for circuit protection – if they fail replace only with the same type and value.

The 3 terminal regulators IC63 and IC62 are mounted on two of the larger heatsinks near the back of the amplier. These provide +/-15V to the op amps on the amplier’s Input Board via the 11-way connector BN62. The Input Board also then routes the +/-15V onwards to the Front Panel Board.

Pins 4 and 5 of CN63 receive AC from another transformer secondary to generate approx +15VDC via the heatsink mounted bridge rectier D601 and the 18,000μF 25V reservoir capacitor C631. F601 and F602

are hard wired 6.3Amp T (time delay) fuses for circuit protection – if they fail replace only with the same type and value.

This +15V unregulated supply has 4 main outputs:-

1) It is regulated down to +12V with the low dropout (LDO) 3 terminal regulator IC64. This is situated on the heatsink closest to the Main Board, near the back panel. Its output goes to BN62 for the +12V triggers and also to CN501, to drive the power ampliers’ cooling fans.

2) It supplies the HDMI Board via CN61. To minimize ripple currents in the ground return, Q643 is wired as a low dropout voltage follower with low pass ltering via R650 and C569.

3) It feeds the switched mode buck regulator IC61 via L631 and C615. The tank circuit comprises L632, C640 and C641, discharged via D631. R636 and C643 make up a snubber to reduce overshoot. This provides a high current +5V supply to the Input Board via pins 4 and 5 of BN62. This 5V supply also feeds two 3V3 linear regulators, IC67 and IC68. These in turn supply the audio DSP ICs on the Input Board via pins 10 and 11 of BN62.

4) It feeds the +5V 3 terminal LDO regulator IC66 via the diode D616, which itself is preceded by the smoothing network comprising R615 (2W 15R) and C615. Note IC66’s output is actually approx +5.7V because of D644 in its ground line; this is brought back to +5V after D643, to feed the relay

IC66 is noteworthy because it continues working when the amplier is in standby.

It takes a second input from the +9V standby power supply on the Power Amplier PCB – D616 acts as a gate to prevent this from feeding back to the +15V supply when the system is in standby. It also allows the +15V supply to override and remove the load from the +9V supply when the amplier is fully booted up.

D606 and D607 form half of a second bridge rectier (with the other two diodes coming from the full bridge rectier D601). These diodes charge up the 1μF/50V capacitor C604 to +15V, generating a “mains power present” signal. The 10K resistor R614 in parallel with C604 continually discharges it so that this signal eectively disappears within about 50 milliseconds of the mains being switched o. This +15V goes to the Main Board via pin 8 of BN62, where it is used as a “pull up” signal to help control the various audio muting circuits.

AVR400 Front Panel Board Circuit Description

The Front Panel Board is a double-sided PTH PCB. It contains the VFD (Vacuum Fluorescent Display) and its associated electronics, plus the keyboard, power status LED, IR remote receiver and headphones amplier. A daughter board carries the front panel I/O socketry. See page 1 of the schematic diagram.

It communicates with the Input Board via CN101 and a 31 way ribbon cable. Other connectors comprise CN94 which connects to two secondaries of the main power transformer and the hard wired BN93 which connects up the daughter board.

There are two associated break o boards. One houses the front panel mounted single pole mains switch plus its suppression capacitor C901 and a connector BN502. The second is mounted on the power ampliers’ heatsink and is used to route power to the cooling fans and also to guide the 31 way ribbon cable.

The VFD draws AC lament power from a centre tapped winding of the power transformer connected to pins 1, 2, and 3 of CN94. The centre tap connects to ground via the zener diodes D901/2 and C903 to provide the lament with its required DC oset. Pins 4 and 5 connect to a relatively high voltage transformer secondary which is half wave rectied by D916 and smoothed by C907 and C960. The zener diodes D903 and D904 in series with R906 generate +40V which is then coupled to the emitter follower Q901 to provide a nominal regulated 40V HT power rail for the VFD.

The VFD’s internal driver IC and external data bμFfer IC901 run from the main +5V supply generated in the Power Supply board and routed onwards through the Input Board. The drive signals (data, clock, chip select and reset) come directly from the system microprocessor via CN101.

The 12 front panel switches are arranged in 2 blocks of 6 with resistive divider chains connected to two ADC inputs on the system micro. These have 10K pull-up resistors at the system μP end to complete the potential divider chains. The pnp switching transistors Q906 and Q907 turn on the 3V3 supply from the system μP via another 10K pull-up resistor at the μP end to provide interrupt control.

The power status LED D905 is a tri-colour type. The green side indicates power on and the red side standby. A high signal on the LED net turns on Q902 and Q903. This turns o the npn Q913 to disable the red LED and turns on the pnp Q912 to enable power to the green LED. The reverse is true when the LED line is low. Pulling the STB(LED) line low when the LED line is also low powers both LEDs and gives a yellow light to show when the unit is booting up. Note that the power supply is STBY+5V to allow the red LED to operate in standby mode.

RC901 is a Kodenshi KSM603TH5B encapsulated infra red receiver for processing commands from an external IR remote control. It is designed to work with the 36-38kHz carrier frequencies associated with the Philips RC5 protocol. Note that it operates from the ST+5V rail to enable the AVR400 to be woken up. It does not demodulate the IR – this is done by the system microprocessor.

The headphones amplier IC902 drives external headphones directly via the 330μF series capacitors C935 and C936. IC902 has a gain of about 4, meaning that the headphones output will be about 4V rms when the volume control is set to clip the L and R main power ampliers (equivalent to about 120WPC into 8 ohms). IC902 is a JRC NJM5556AL capable of driving 7V rms into150 ohm loads and about +/100mA peak current into lower impedances. Mute transistors Q904/905 in series with 100R resistors are tted in front of its input to minimize switching transients and it is powered from the +/-15V supplies generated on the Power Supply Board.

The L and R headphones outputs go via BN93 to the Headphones Board, after passing through relay RL902 which is normally o. A positive voltage from the μP at the emitter of pnp Q911 turns on Q911 and generates the same positive voltage at the base of the npn transistor Q909. Because its emitter is connected to the -15V rail this pulls the relay on.

The FRONT AUX and MIC inputs come from the Headphones Board and through the microphone relay RL901. When the relay is o the AUX line level L and R signals are switched through to the unity gain bμFfer IC904 and then on to the Input Board via pins 3 and 1 of CON101. When the relay is closed (in

the same manner as described above for RL902) then the L front input is routed to the low noise microphone amplier IC903. This operates as two cascaded virtual earth ampliers, each with a voltage gain of approximately 21 (R961/960 and then R965/962). Note the MIC line is biased at +6V via resistors R956, R957 and R958. The amplied signal is output to the Input Board on pin 5 of CON101.

AVR400 Input Board Circuit Description

The Input Board comprises a 4 layer PCB; this is attached directly to the back panel via its various sockets and to the heatsink by two steel brackets. It is positioned underneath the HDMI Board and above the Main Board. 5 ribbon cable sockets connect it to the other 4 boards. CN71 connects it to a daughter board containing two 9 Pin D socket connectors (an RS232 serial port and an iPod dock interface), plus two 12V trigger sockets and 2 IR receiver sockets. Note some passive components are mounted on the underside of the PCB.

The Input Board circuitry is shown on pages 2 – 5 of the schematic diagram.

Page 2 covers the analogue inputs, volume control and line level outputs.

Page 3 covers the SPDIF (digital) inputs, clock recovery, audio DSPs and the codec (stereo ADC plus 8-channel DAC).

Page 4 covers the system microprocessor and a slaved support microprocessor

Page 5 covers interfaces to the DAB/Ethernet module, and the boot loader microprocessor used to update the system SW via USB.

Analogue inputs, volume control and outputs

IC101 is a Renesas R2A15218FP analogue multiplexer and volume control, with a gain range of +42 to

-95dB in 0.5dB steps. It is digitally controlled from the system microprocessor via the I2C bus on pins 49 and 50. A high logic signal on pin 51 enables the system mute. IC101 has +/- 7V supplies generated from the +/-15V rails with the regulators IC102 and IC103.

All stereo external line level inputs using phono sockets are routed to IC101 via 100R/220pF low pass CR lters. IC101 also handles the AUX-L, AUX-R and the (mono) MIC_SIGNAL setup microphone inputs coming from the front panel, plus the internal stereo outputs from AM/FM tuners (TUN-L and TUN-R) and the DAB/ethernet receiver (VENICE_L and VENICE_R). IC101 additionally switches two multichannel signals - the 8 channel direct input and the outputs from the 8 post-DAC lters. Note that the +/- 7V power supply limits the input signals to approximately 4V rms before overload occurs.

The post-DAC lters comprise 4 low noise NJM2068 dual op amps, running from the +/- 7V supplies. One op amp is assigned to each channel and performs the dual functions of converting a dierential input from the DAC to a single ended output, whilst simultaneously functioning as a three pole 50kHz active lter.

The AM/FM tuner module’s outputs pass through the inductors L310 and L302 (providing 19kHz notch and 38 kHz low pass ltering) and the shunt mute circuits formed by the 330R resistors R354/R369 and the two halves of Q306 plus Q307.

IC101 has a xed level stereo output for Zone 2 (SUB_L and SUB_R) and a second one, adjustable from 0dB to -18dB in 6dB steps, for the AVR400’s analogue to digital converter (ADC_L and ADC_R).

+ 23 hidden pages