Arcam Alpha 8 Manuel d'entretien

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ALPHA 7/8/9 CD SERVICE MANUAL

Arcam Alpha 7/8/9 CD Service Manual

Issue 1 Serial No. 0001 - (Paul Newton July 96)

Circuit Description

The following notes describe the operation of the

Alpha 7/8/9 CD players. They include the circuitry on

the main, DAC and display PCBs but not that on the

servo PCB attached to the mechanism.

Power Supplies

The mains input is fitted with inductors L2,3,6,7 to

filter electromagnetic interference coming out of the

player.

Mains transformer T201 has three secondary

windings supplying two separate power supplies and

an AC supply for the display filament:

Analogue Supply

Bridge rectifier D4,5,6,7 and reservoir capacitors

C209-C212 produce unregulated split rails of about

+/-22V.

Z204 and Z205 provide regulated outputs of +18V

and -18V respectively that power the opamps in the

audio output stage.

Since there is no power switch on this secondary, the

analogue supplies are always connected.

Digital Supply

Switch SW201-A disconnects the digital secondary

winding when the 'Power' switch is out.

An unregulated supply of about 11V is produced by

the bridge rectifier D8,9,10,11 and reservoir

capacitor C36.

This is then regulated to provide three +5 volt

supplies (Z208 general logic supply, Z207 DAC

supply, Z203 system clock generator supply) and a

+7.3v supply for themechanism and motor drive

circuits(Z206).

A -30v supply is formed by Z209 and associated

components to power the display grid.

Muting

When the unit is turned off SW201-A discharges

capacitor C29 quickly. This along with the AMUTE

signal from the microprocessor, controls the output

mute transistors Q1,101 via level shifter Q3.

Clock Generator

The system clock is generated by a discrete oscillator

based around Q2 and 16.9344MHz oscillator X1. The

output of Q2 is amplified and buffered by inverters

Z202-E & D and sent to the clock input of the DAC

Z201.

DAC & Audio Outputs (Alpha 7 only)

The DAC is a Delta-Sigma design from Burr Brown,

the PCM1710U.

The serial digital data from the decoder Z206 is input

to pins 1 to 3. The system clock is input to pin 5.

The DAC is powered entirely from one 5V supply,

although it is split and decoupled between the

analogue and digital sections of the device.

The analogue outputs from each channel are

processed identically.Three poles of filtering are

provided by the active filter based around Z1-A. Z1-B

buffers the audio output and sets the output level.

Muting is provided by turning on Q1 and Q101.

DAC & Audio Outputs (Alpha 8 only)

This is a similar circuit as the one used on the Arcam

Alpha 6 DAC PCB but designed for Sony format

input data and a better digital filter, the SM5843AP1,

Z14.The digital filter is used with a SM5864AP 20-bit

DAC chip, Z12, (described here as a PWM noise

shaper to avoid confusion with the 1-bit DAC Latch

which follows it). The output from this is re-latched by

Z7, Z8.

Digital FilterA PIC micro on the mother board

controls the filter with the signals RB0 to RB7. The

SM5843 filter has internal pull-up resistors on its

inputs resulting in RB4 & RB5 being logic 1.

The SM5843APT requires several control lines from

the PIC...

RB7 FSEL2, Selects sampling rate for de-emphasis.

Static low for 44.1K.

RB6 FSEL1, Selects sampling rate for de-emphasis.

Static low for 44.1K.

RB5 IW2N, Selects input word length. Static high for

16 bits.

RB4 MDT, Serial data input for volume control. Burst

at switch-on for setting to full.

RB3 RSTN, Reset line. Short pulse low at switch on,

then static high.

RB2 MLEN, Serial data latch input for volume

control.

RB1 MCK, Serial clock input for volume control.

RB0 Not connected.

No Dither On Silence

The digital filter adds 'dither' to the output signal.

This improves the low-level performance of the DAC.

The PWM filter normally mutes when it detects

digital silence to prevent any unwanted noise and

idle-tones on the audio outputs when no music is

playing. A slight click is audible when this happens.

Unfortunately, the added dither prevents this mute

from operating because it holds it open. Q11 detects

digital silence on the data input to the filter. When

silence is detected, the dither function is switched off