Arcam AVR400 service manual Spdif inputs, ADCs, DACs and Audio DSPs

IC101 has one 8 channel variable level output bus labelled VOL01 through to VOL08. Capacitors C295-8 and C231-4 float the ground ends of the associated internal potentiometers to minimize clicks This bus goes via specially selected 100μF/25V capacitors to the dual op amps IC121-124, wired as voltage follow- ers and running from the +/- 15V supplies. The C, SL, SR, SBL and SBR outputs are shunt muted when required via the 560R resistors R383-388 and the dual transistors Q303-305.

The subwoofer output SW has two shunt mute circuits. One using Q311 works in parallel with the rest of the channels. The other, using Q308 and half of Q303 allows muting of the SW channel alone.

The FL and FR channels take off the stereo headphones amplifier feed from IC121 (it goes via the con- nector WF101 to the Front Panel Board), then add an extra dual voltage follower IC125 and a double pole shunt mute switch using 4 x 270R resistors and Q301-302. All the above ICs are JRC NJM2068s or equivalent.

All 8 of these outputs go to via the connector WF103 to the preamplifier output sockets and (except for the subwoofer output) to the 7 power amplifier inputs located on the Main Board.

The ADC input amplifiers use a JRC NJM2068 for each channel, operating as a single-ended input to differential output converter, with HF filtering above about 400kHz. This is sμFficient for anti-aliasing purposes as the analogue modulator of the ADC samples at 6.144 MHz and only needs to keep out frequencies above 6MHz.

The array of 12 switching transistors in the bottom right hand corner of the schematic is arranged in 4 blocks of 3 devices (two npn and one pnp per block). The array is used to control the muting of 4 specific groups of audio outputs, taking into account the presence of AC mains via P_U (the pull up line from the Power Supply board) and the MUTE_POWER line derived from the power amplifiers’ Vcc rail

–this is normally high when Vcc is high. The output lines are SB_MUTE2 (for the surround back chan- nels SBL and SBR), ZONE2_MUTE2, HP_MUTE2 (for the headphones relay on the Front Panel board) and FUNC_MUTE2 (for the 6 main audio channels excluding SBL and SBR).

Zone 2 volume is independently adjustable in 1dB steps via IC131 (rohm BD3812F). Its outputs are bμFfered and amplified 15dB by the dual op amp IC132 and can be muted when required by Q402. The output goes directly to two phono sockets forming half of JK11 on the back panel.

SPDIF inputs, ADCs, DACs and Audio DSPs

The AVR400 has 4 coaxial 75 ohm SPDIF (Sony/Philips Digital InterFace) inputs and 2 TOSLINK optical inputs on its back panel. A further optical input is located on the front panel as part of JK92 (this socket also includes the AUX and MIC inputs). There are 6 further external SPDIF inputs - one per HDMI input socket and one on the HDMI output socket (the Audio Return Channel or ARC).

Each of the 4 coaxial inputs is bμFfered by two NOR gates, contained in IC159 and IC160, before being switched by one half of the dual 4 input multiplexer IC147. The output on pin 7 is then sent to the 8 input multiplexer IC140. Its other 6 inputs comprise the 3 optical receivers already mentioned, the HDMI input (multiplexed down from 5:1 on the HDMI board), the HDMI ARC and the output of the Venice 6 DAB/Ethernet module. The 8th input is unused. The output MUX_SPDIF is sent to the SPDIF receiver IC153.

IC153 is a Wolfson WM8804 run in hardware mode. It uses X707 to generate its own 12MHz internal clock. IC153 automatically identifies and dejitters incoming SPDIF signals with sample rates of 32kHz, 44.1kHz, 48kHz, 88.2kHz, 96kHz and 192kHz. Its output, still in SPDIF format, WM_SPDIF, is sent to pin 43 of the system codec IC143.