Arcam AV8 service manual Power supply Refer to circuit diagram L921 sheet

The surround and sub channels (IC205/206/208/209) are very similar to the left channel described above except for the op-amp ground references being tied directly to 0V_SIG. External multichannel inputs (FROM EXT MAIN L, FROM EXT SURR L, etc) have 100pF NP0 capacitors to ground as an EMC preventative measure.

Volume control and headphones Refer to circuit diagram L921 sheet 5,6

Volume control is performed by a Burr-Brown PGA2310 volume control chip. This chip is pin and software compatible with the Crystal Semiconductor CS3310 volume control chip. The only difference is that the internal op-amps run off ± 15 volts rather than ± 5 volts. The gain is under micro control with steps of 0.5dB. All volume controls share data, clock and mute control lines but have individual chip select lines. The volume of each channel can be controlled individually. “Zero-crossing” detect is permanently enabled on each volume chip to give click-free volume changes. If, however, clicking can be heard when changing volume, especially at higher levels, it is likely that there is a dc offset into the volume control chip. This points to an error in the previous stage which should have removed all dc. The DCA XMUTE line is a power-on mute with a time constant set by R703 and C701 (sheet7). The output op-amps (IC 502, 505 etc) are configured to block dc and are inverting amplifiers to restore the polarity of the signal as the volume control chips are non-inverting.

Since all outputs are identical only the main left output around IC502A will be described. IC501 is a self contained volume control with non-inverting op-amp. C501 prevents oscillation of IC502A. C507 blocks dc to subsequent stages of the circuit but is in the feedback path. This provides some compensation for capacitor non- linearities. R527 provides a local dc path to prevent the op-amp latching into rail. R502 provides output protection, limiting current in the event of the output being shorted. R503 provides a weak 0V reference to prevent the output floating. IC503 is a DG413 analogue switch which acts as a mute under micro control via demultiplexer IC708. The five DG413s play a vital role in preventing thumps at turn on/off.

At power on, the MAIN OUTS ENABLE and ZONE 2 OUTS ENABLE lines are forced low in hardware, pulling the output connectors to ground. The hardware control of these lines is determined by an RC time constant (see ‘control lines from micro’ below for details). This allows sufficient time for the micro to boot up and take control of these lines.

At power off, the micro detects power loss and forces the outputs into the mute (grounded) state. This prevents any pops. However, if the DG413s were run directly off the +/- 15V rails, the outputs were found to drift to a dc level that tripped power amplifiers into dc protect. This was due to the way the power rails collapsed. (The -15V rail actually went slightly positive for a time causing the dc output drift.) D506 and D507 isolate the output mutes from the main +/-15V supply. C543 keeps the supplies to the output mutes up until long after the main +/-15V rails have completely collapsed. R537 and R538 are actually capacitors to decouple both local supplies to ground.

D503 protects the output stage by shunting over-voltage spikes to the power rails. (It should be noted that the diodes are not power devices, therefore they are likely to fail if a high energy pulse is fired up the outputs of the unit.) C502 shunts any high frequency signals at the output to ground to help EMC.

The source for the headphone can be from the MAIN output (IC502) or ZONE2 (IC505). Selection is by 74HCT4053 multiplexers IC507 and IC508 under micro control via control line demultiplexer IC708. IC509 is an LM4880 headphone driver IC which runs off the +5V (analogue) rail. Its supply is drawn through a 2.2ohm resistor to try to provide some crosstalk isolation to the input multiplexers +5V power rail.

Zone2 and record loop outputs Refer to circuit diagram L921 sheet 4

The input switches on schematic page 1 mix onto a virtual earth bus (ZONE2, TAPE OUT or VCR OUT). The associated op-amps are found here. Zone2 (IC401) has the same type of dc blocking circuit as described before for sheet 2 of the schematics. Note however that the feed forward resistor is 100kΩ . To maintain unity gain, the feedback resistors R402 and R404 have been chosen such that their value in parallel with 1MΩ equates to 100kΩ .

TAPE and VCR record outputs (IC402 and IC403) have 100kΩ as the feedback resistor in series with a 47Ω output protection resistor. The outputs are capacitively coupled without compensation with a weak 100kΩ pull-down to 0V_SIG to prevent the outputs floating off.

Control lines from the micro

Refer to circuit diagram L921 sheet 7

Control lines for individual switches etc are derived from 16-bit words sent as serial data to three sets of serial to parallel latches (port expanders). In this way, many local control lines can be provided whilst only using a few micro I/O ports. The same data and clock lines also drive the PGA2310 volume control chip. IC702 decodes chip selects for the volume control and latch devices. On power up, the contents of the latches is cleared using a buffered R-C circuit (R703, C701, IC701E/F). This also mutes the volume control chips. In practice however, this latch clear and volume mute line is of limited use as the +5V analogue supply collapses slowly. If the unit is switched off and back on within a period of 5 seconds, C701 will not have discharged via D701 sufficiently for the input of Schmitt trigger IC701E to interpret a LOW condition. The default power-on state for the volume control chips mute in any case.

IC708 is a special case as it provides the control of the MAIN and ZONE 2 OUTS ENABLE. These must be low at power up to prevent turn-on thumps, due to the initialisation time period of the micro this must be done in hardware. C710 and R729 generate a line that is HIGH at switch-on, which sets the output of the latch to tristate (high impedance). R535 and R536 pull these lines low so that the output mute ICs switch the outputs to ground, theoretically keeping the outputs silent during power-up. In practice a small tick may be heard because the logic devices don’t begin to operate properly until they have a few volts across them. C710 charges via R729 over a period of approximately one second. The falling OE line to IC708 enables the latch outputs, which have by now been written to by the uC.

Power supply

Refer to circuit diagram L921 sheet 8

The board is powered from the +/-18V and +21V supplies which are regulated using standard LM317T and