Into the power amp stages but from this point we can
configure the unit as a separate Pre amp/Power amp
by moving the jumpers to the pin 2 and 3 of Con 304
and Con 305, and then remove the jumpers at Con
302 and Con 303 this may also be of use when fault
finding as we can effectively isolate the and D.C
offset/distortion problems coming from the Pre amp
stage to the power amp stages and test these stages
as a separate entity.
The Pre amp power up mute and power down mute is
controlled by the relay at location RLY300 A/B the
relay also triggers to mute the switching noise when
switching between inputs (see micro control/protection
and display section).
Power amp stages.
The main power amplifiers are of a Class A/B design
which use SAP “audio” transistors in a asymmetrical
current feedback configuration, Input and feedback
paths are D.C coupled and there is a active integrating
servo to remove D.C offsets from the outputs.
The basic principle of operation is as follows: Left
channel description given only, read all references as
5xx for the right channel.
The input level of the power amp stage is clamped by
the 3V9 zener diodes at positions DZ402 and DZ403
this protects the power amp input stages from gross
overload and subsequent damage R410 and R402 and
C416 in parallel form a 340khz filter.
IC400A is a V-I converter with a gain of 2 it’s output
will be a accurate amplification of it’s input voltage (i.e
the output voltage at pin 1 will be identical to the input
at pin 3 but twice the amplitude) the output voltage is
driven unto a 44 ohm load formed by R445 and R446
this op-amp is used in a slightly unusual configuration
in that it’s power supply pins are used as a current
output and the output pin is used as a current
feedback. Transistors TR404 and TR408 supply the +/-
15v rails and act as cascades to pass it’s supply pin
currents through to the current mirrors.
The “feedback current” flows back from the power amp
output via R447-R450 to allow IC400A to swing it’s
output, this is why the term current feedback is used –
it’s is the current flowing in the feedback resistors that
sets the overall gain of the amplifier.
IC400B acts as an inverting integrator and it’s purpose
is to remove DC from the loudspeaker outputs. Any
positive D.C offset will cause the output of IC400B to
go negative thus increasing the current in it’s negative
supply pin and pulling the output voltage back to zero
R420 and C442 set the time constant of the integrator
so that audio frequencies are ignored
The transistors found at locations TR400 and
TR401/TR402 form the PNP Wilson mirrors and TR416
and TR411/TR417 form the Wilson NPN current mirrors,
emitter degeneration is provided by R405, R406 (+) and
R407, R408 (-).
R415 and R416 decouple the current mirror stages from
the main power supply stages to allow the bootstrap circuit
to operate this circuit is formed by C423 and C424 and the
Metal film 1 watt resistors at locations R452/R453, the
boots strap is provided to allow the output stage rails to go
up and down slightly with the output signal to the
loudspeaker, this enables the driver stage to fully saturate
the output drivers giving the greatest output and the best
thermal efficiency.
TR405 and TR407 are the Pre-drivers, TR412/TR414 and
R411/R434 provide a current limit of about 30mA under
fault conditions. R423 and R428 loosely couple the
outputs of the output drivers to the inputs of the SAP
output devices, this allows the output devices inbuilt
temperature sensing diodes to accurately control the
quiescent current of the output drivers as the temperature
varies, C425 and C405 ensure that both halves of the
output stage receive an equal A.C component.
The output transistors are TR413 and TR415 these are
specially designed for audio power use Sanken SAP15N
and SAP15P devices they provide a inbuilt 0W22 thick film
power resistor and temperature sensing diodes that track
the V
BE
versus temperature characteristics of the power
transistors allowing for accurate control of the quiescent
current.
R459 and C427 form the Zobel network which is provided
to ensure that the amplifier see a constant and resistive
load at high frequencies C421 locally couples the “High
frequency” and signal grounds together at the input stage
for the same reason. Any signs of burning or scorching of
R459 will indicate that the unit amplifier channel(s) have
been running unstable or oscillating.
The Bias should be set for minimum distortion (THD+N)
using a 0.5v rms 20 Khz sine wave into the CD input with
the amplifier set for full gain, the absolute maximum level
of Bias acceptable is 22mV measure across CON401 (L)
or CON 501 (R) the adjustment is made using RV400 and
RV500 we set the bias at the factory using an extremely
accurate Audio Precision-audio analyser.
If you do not have access to a distortion level meter
capable of 0.05% or better accuracy you may be able to
rough set the amplifier to a typical reading of 15mV (at
cold switch on).
