Cambridge Audio 840A Service Manual page 56

The XD control circuit is a differential pair of transistors with one input grounded and the other driven
by the main amplifier output voltage, scaled down appropriately by R63, R64. The drive to the displacer
is taken from collector load R58, to give the required phase inversion. R59 is present simply to equalise
the dissipation in the differential pair transistors to maintain their balance.
The tail of the differential pair is fed by constant-current source Q27. Since half of the standing current
through the differential pair flows through R58, the value of the tail current-source sets the quiescent
displacement current. Q27 is a simple current-source biased by silicon diodes D12, D13.
The displacement current itself is handled by the voltage-controlled current source Q18, Q17. The
Q18, Q17 structure acts as a unity-gain voltage stage with 100% voltage feedback from Q17 collector
to Q18 emitter, and it sets up the voltage across R58 as a voltage across the low-value resistor R31,
minus the Vbe drop of Q18.
There is provision for turning off the XD system under PIC control, to make servicing easier. This is
done by Q26. Normally the XD ON/OFF signal coming in on Pin 9 of CN2 is high, (+5V) so Q31 is
held off and Q26 is off. When XD is disabled, XD ON/OFF goes low and Q31 is turned on via R83. Q26
is then turned on via R82. The equivalent to Q26 on theright power amp PCB is also turned on by the
connection through Pin 10 of CN1.
Power supplies
The power supply for the power amplifier consists of three rails. The voltages given here are nominal as
they depend on the exact value of the mains supply voltage
The +48V rail supplies all the positive power to the amplifier.
The -48V rail powers only the amplifier output stage, including the XD Crossover Displacement system.
The -51V rail provides negative power for the small-signal circuitry. This permits greater output as
without it the negative-going voltage-swing of the voltage-amplifier stage is limited, and negative clipping
occurs before positive clipping.
Overload protection.
The amplifier is protected against short-term overloads by dual slope VI limiting. The upper half of the
output stage is protected by Q19 and associated components.
When overload occurs Q19 conducts and shunts current away from the base of Q1, limiting the
amplifier output. The current so shunted is limited in magnitude by the current-source Q4.
D2 prevents Q19 conducting backwards on negative output excursions.
The protection of the lower half of the output stage is very similar, Q20 being the VI limiter. However,
in this case the current shunted away when Q20 conducts is not inherently limited; this is done by the
extra current-limiting circuit Q11, R15, R16. When the voltage across R9 becomes excessive, Q11 turns
on and shunts the base drive away from Q9.
The amplifier is protected against long-term overloads by the PIC opening the output relay. Operation
of the VI limiter Q20 draws extra current through R9 and turns on Q25 via R49; this happens at a lower
current than that at which Q11 limits the VAS current.
Q25 collector is wire-ORed with the equivalent transistor on the right-hand amplifier PCB, and the
OVERLOAD SIGNAL goes to the preamp PCB via CN2, pin6.
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