Cambridge Audio 840A - Page 57

Clip detection.
When it is enabled, the clip-detect system turns down the volume when excess clipping of either
amplier output occurs. Very brief episodes of clipping are ignored. The volume stays reduced and does
not ramp back up in the absence of clipping.
The clip detect system is based on Q23, Q24 and associated components. Normally both transistors are
on, and the CLIP signal at Q24 collector is high.
Since the circuit operates by monitoring how far the output is from the supply rails, variations in supply
voltage are compensated for.
DC oset protection:
The output of the Left amplier is ltered by R35 and C3 to remove audio content. If a DC voltage of
either polarity exists Q21,Q22 turn on and the voltage on Q21 collector falls. This point is wire-ORed
with the DC oset detector voltage on the right power amplier PCB via CN1.
Detailed operation is as follows:
+ve DC oset: D4, Q21, Q22, D6 conduct. Other diodes reverse-biased.
-ve DC oset: D3, Q21, Q22, D5 conduct. Other diodes reverse-biased.
Overtemperature protection
Positive thermistor TH1 is mounted on the main amplier heat-sink. At approx 100degC, its resistance
has risen to the point that the voltage on pin 6 of comparator U1:B exceeds the reference voltage of
+850 mV set up by R69,R70, and the comparator output goes low to 0V. R71 provides a small amount
of hysteresis to give clean switching. The comparator has an open-collector output and R75 is the
external pull-up resistor. The output signal is passed to the main PIC.
Speaker short-circuit detection
When the amplier goes from standby to ON, the speaker outputs are checked for short-circuits before
the output relays are closed. This system is centred on comparator U1:A but also involves circuitry on
the Right Amplier PCB.
Short-circuits are checked for by passing a small current through the loudspeaker impedances and
monitoring the resulting voltage. A DC current of 5mA is injected through R101 on the Right Amplier
PCB, via the normally-closed contacts of RL1 and RL2. This places all connected loudspeakers, left and
right, A and B, in parallel so the resulting voltage is not large. It is sent to the left amplier PCB, where
comparator U1:A compares it with a reference voltage of 5mV set up by potential divider R85,R87. If
the combined loudspeaker resistance is below 1 Ohm, the output of U1:A goes high, signalling a short
condition to the main PIC.
Headphone output attenuator
The headphone output attenuator comprises R40 and R41. The headphone socket is on the preamp
PCB.
Power Amplier Right PCB
This description is subdivided as follows:
The right power amplier
The output relays
Output relay control system
Mains-fail detect system
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