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Amplifier & PSU Circuit Description
Refer to L882 circuit diagrams
This is the printed circuit board that provides the power
supply and output stage amplifiers for the A32 integrated and
P35 power amplifiers.
Its function is to:
! Drive the loudspeakers
! Provide an (always on) auxiliary 5VDC supply for the
micro controller and display interface
! Receive logic signals from the micro controller to turn
on the main amplifier supply relay (mains) and connect
either pair of speaker output sockets
! Send logic signals to the micro controller pertaining to
the state of the amplifiers (short circuit protection, DC
offset protection, thermal protection)
! Receive and demodulate RC5 remote style control
codes via the rear panel jack and transmit them to the
micro controller
! Send a 12V trigger output via the rear panel jack for
control of an auxiliary power amp when the unit is on
! Receive a 12V trigger input from the rear jack (for use
in the power amp only version)
! Drive a pair of headphones via attenuating resistor
networks The power amplifier is a symmetrical, class B,
bipolar junction transistor output, current-feedback
design (of which more later) with DC-coupled signal
and feedback paths, featuring an active integrating
voltage servo to control DC offsets.
It features ‘instantaneous’ safe operating area protection in
addition to sending a signal to the micro to turn off the output
relays in the event of user or thermal overload. Since it is a
DC-coupled design, the unit senses DC at the output and
triggers the micro to turn off the loudspeaker relays in the
event of excessive levels (possibly due to a faulty source
component or short circuit output transistor).
The output stage uses Sanken specialised ‘audio amplifier’
power bipolar Darlington transistors which are optimised for
use with this type of topology. Consequently the unit has
excellent measured performance in terms of noise, slew rate,
output impedance and distortion (harmonic and
intermodulated) and is essentially load invariant (to a first
order the measured performance is independent of the load
impedance).
L882 Circuit Sheet 1
The audio input to the amplifier is connected to SK102
(which connects to the output of the preamp PCB). This
signal is passed on via SK104A which forms the preamp out
connection to the outside world.
SK104B provides the power amp input connection, with
switchSW100 selecting between pre / power and integrated
modes. The unit is wired as a preamp / power amp
combination with the switch depressed, allowing the user to
insert a processor or other function (e.g. graphic EQ) between
the output of the preamp and the input of the power amp.
With the switch in the ‘out’ position the power amp input
socket is ignored and the input to the power amp is connected
internally to the output of the preamp. PL100 and PL101 are
‘handbag’ links fitted to the power amp only version to
connect both pairs of phono sockets in parallel for daisy
chaining (as there is no preamp output on a power amp).
Relays RLY100 and RLY101 switch the two pairs of
loudspeaker output sockets and are controlled by the micro
lines describes above. Transistors TR100 and TR101 operate
in ‘constant current sink’ mode which allow relay current to
be approximately constant although the main power supply
rails will vary with mains input and load conditions. The
current is around 20mA per relay.
Star point SP100 is the ground ‘mecca’ for the entire
amplifier (comprising all three PCBs within the unit). All
of the separately named grounds are joined explicitly at
this point. Different named grounds are used to ensure that
no two ‘different’ grounds share copper, which could
compromise the noise, distortion or crosstalk performance
of the amplifier.
The loudspeaker output signals are passed to socket
SK106 which connects to SK107 and onto the headphone
output via the attenuation resistors R103 thru R106.
The hierarchy containing the other sheets is self
explanatory. Each of the port names shown on the top
sheet connects to the port of the same name on the lower
sheets.
L882 Circuit Sheet 2
This sheet contains the power supplies, the rear panel jack
socket trigger circuits, the standby relay control and the
‘interface’circuits between the output signals of the power
amplifiers and the inputs expected by the micro processor.
The mains input enters the unit at SK203, with capacitors
C205 and C206 acting as conducted RF suppression. The
earth connection is passed on to the chassis (for safety
reasons the chassis metalwork remains connected to mains
power earth at all times). Switch SW200 is the voltage
selector switch, allowing the unit to be operated in 230V
or 115V mains countries by switching the dual-primary
mains transformers between series and parallel winding.
Varistors VR200 and VR201 act to prevent over-voltage
surges from damaging the unit. If the user selects 115V
operation and then connects the unit to a 230V supply, the
varistors will go to a low impedance and blow the primary
fuses. Any very high voltage line transients will also be
suppressed, helping to eliminate transformer isolation
breakdown.
Relay RLY200 switches the primary side of the mains
transformer, allowing the micro to control the on / off
status of the amplifier. Its contacts are snubbed by
capacitors C207 and C208 (to eliminate switching spark
transients and prolong relay lifespan). The primary
windings of the toroidal mains transformer connect to
SK204.
PCB mounted transformer TX200 is powered all the time
that mains is present on SK203, irrespective of the on / off
status of the amplifier. This is to ensure that the micro
processor is always operational and can thus control the
mains switching for the main amplifier. Secondary fuse
F202 limits the current in the event of a failure mode, as
the short circuit primary current of TX200 would be
insufficient to blow the mains fuses.
Diodes D200 thru D203, C227 and IC201 provide the
5VDC supply which powers the micro and display PCB
and the relay coils. C224 is to reduce diode noise being
transmitted back through the leakage capacitance of
TX200.
The mains transformer secondary winding is connected to
SK200. This is a centre tapped winding, and is used with
full bridge rectifier BR200 to produce the main positive
and negative supplies for the power amp. C209 and C210
are the large reservoir capacitors, with C211 and C212
acting as high frequency decouplers. The main power
supply rails and ground are accessible on SK205 for future
module expansion.