Arcam Alpha 9 Service Manual

ALPHA 9/9P SERVICE MANUAL ALPHA 9/9P SERVICE MANUAL
ARCAM ALPHA 9/9P SERVICE MANUAL
Issue 1 Serial No. 001 - (Paul Newton July 96)
Circuit Description
Input/Output Selection
The Alpha 9 integrated amplifier has 6 line level
inputs and optional phono input selected by use of
the motorised 'listen' switch, SW201.
The use of the 'record' switch SW202 allows
recording of one source whilst listening to another.
The Processor/Tape 2 switch, SW207, allows tape
monitoring of a 3 head tape deck connected to the
Processor/Tape 2 sockets. It can also be used to
allow input signals to be routed to an external
surround decoder for processing before being
routed back to the Alpha 9.
Tone Control & Preamp Out/Power Amp In
The tone control stage on the Alpha 9 is based
around a dual op-amp Z2. The first half provides
a buffer stage for the tone controls and the second
one with its frequency dependent feedback is used
as the tone control itself.
The output of the tone control then passes through
the balance control RV201 and then onto the
power amp stages and the preamp out buffer
stage formed by Z203.
The tone controls and balance pot can be
bypassed by use of the 'direct' switch SW205.
The Alpha 9 can be driven from an external
preamplifier source by moving the internal switch
SW206 to disconnect the Alpha 9 preamp stages
from the power amp.
Power Amplifier
The signal from either the Alpha 9 input sockets or
an external preamp enters the power amp stage
via C12 and R35.
Q11, Q16 form a long, tailed pair driven by the
current source formed by Q18 and Q19.
Q6 is the Class A stage driving the quasi
complementary output stage of Q4, Q5, and
mosfet's Q1, 2.
The Alpha 9 and Alpha 9P utilize an auto bias
circuit to maintain the correct quiescent current
(IQ). The circuit works by monitoring the voltage
across the 0.22 ohm resistors R1, RA1 and
comparing it to a voltage derived from the - 44v rail
via R24. Both these voltage are fed into the inputs
of comparator Z3. If the voltage across the 0.22
ohm resistors increases
comparator goes high impedance and Q15 is
turned off. This causes capacitor C72 to discharge
and reduce the bias on the base of Q17 which
reduces the IQ. Conversely if the voltage across
the 0.22 ohm resistors is low then Q15 is turned on
more and C72 charges and increases the bias on
Q17 and so increases the IQ.
conditions a low frequency, variable mark/space
ratio, waveform can be seen on the collector of
transistor Q20 as the bias is being automatically
adjusted.
Z1 is a d.c. servo used to maintain a low d.c. offset
at the speaker outputs.
Power Supplies
The Alpha 9/9P is powered by a toroidal
transformer with 2 sets of secondary windings.
The main windings are used to provide +/-44V
D.C. after rectification and smoothing by diodes
D201-204 inc. reservoir capacitors C201, 202.
The second winding is rectified to provide an
unregulated supply voltage for the motor driver ics,
Z402 (for the volume control) & Z403 (for the listen
selector).This unregulated supply is also smoothed
and regulated with a simple stabilizer circuit (zener
diode D402 and associated components) to
provide a stable voltage for the micro controller
Z401.
Protection/ Mute circuit
The protection and mute functions are carried out
by Z501. This performs turn on delay, d.c. offset
detection and thermal cutout. It also displays the
status of the amplifier with the use of a bicolour
LED - 'Green' for ready and 'Amber' for when the
speakers are disconnected internally because of a
fault condition or on initial power up whilst the 5
second switch on delay mute is operating.
If the unit suffers a current overload the protection
circuit may latch and the unit will not reset until it
has been switched off and on again. If the
protection is activated by a thermal problem the
unit will begin to operate again when it has cooled
down.
The thermal cut out operating point is set by the
thermistor TH501, mounted close to the output
device heatsinks.
Disc Stage (optional)
the output of the
Under signal