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additional feedback further reduces high frequency distortion and crossover distortion within the audio
band.
These stages are partially decoupled from the Vcc and Vee power supplies by D5130/R5130/C5130 and
D5129/R5129/C5129 respectively. They are also bootstrapped to the amplier output via the networks
R5118/C5128/R5128 and R5117/C5127/R5127. This raises the supply lines by approximately 3V at full
output to avoid clipping the driver stage prematurely.
The output stage comprises classic complementary emitter followers Q5150/Q5170 (NPN) and Q5160/
Q5180 (PNP). The On Semiconductor output transistors have a current gain that is sustained to about
10 amps and a very large safe operating area, which allows the ampliers to drive low impedances well.
They also have built-in thermal compensation diodes which helps stabilize the quiescent current both
statically (when hot) and dynamically (when playing music at high level) – this minimizes crossover
distortion and improves sound quality.
The output stage biasing is performed in the network around the amplied diode Q5120, which is
mounted in intimate thermal contact with the driver transistor Q5130, plus the two built-in diodes
associated with the output transistors. The thermistor R5122 is positioned on the PCB close to the
heatsink and provides extra downward compensation at very high temperatures. Bias is set by VR51 and
is largely independent of temperature – it should be set to16-20mV when measured across the outer
terminals of the compound emitter resistor R5175, using the 2 pin connector CN51, 5 minutes or more
after the AVR400 is powered up.
The power amplier output is routed across the PCB to the back panel. It includes a Zobel network
(sometimes called a Boucherot cell) R5183/C5183 and a series inductor L5185 damped by a 4.7R 2W re-
sistor R5184. These components help isolate the amplier from reactive loads to ensure high frequency
stability. One half of the normally-o relay RL52 is used to switch the load in and out.
Each power amplier is protected against overload in a number of ways. The complementary transistors
Q5130 and Q5131 protect the NPN half of the output stage and Q5140 and Q5141 the PNP half. They
operate as Sziklai pairs, passing negligible current until a threshold voltage of approx 600mV is reached
across R5132 and R5142. Between 600 and 700mV the pairs then ramp up current smoothly, diverting it
away from the bases of Q5150 and Q5160 to limit the output stage drive to a safe level, within the power
transistors’ SOA (safe operating area). The 600mV threshold voltage depends upon both the instantane-
ous current and voltage across the output transistors, set by the networks R5132/R5136/R5137/R5138/
R5175 for the top half and R5142/R5146/R5147/R5148/R5175 for the bottom half. R5135/R5145 and the
zener diodes D5135/D5145 change the slope of the protection locus at high Vce voltages. R5134/R5135
plus C5134/C5145 prevent fast transients and brief overloads from prematurely triggering the protec-
tion.
The above dual slope SOA protection is self resetting but if a gross overload persists for more than a
second or two (such as when a channel’s output is short circuited with music playing at a moderate to
loud level) then the open collector transistor Q5181 sinks current for long enough to initiate the ampli-
er’s full shutdown procedure via the line SOA_PROTECT. This can also be triggered by a total output
stage failure (which passes enough current through R5175 to turn on Q5188) via OVERLOAD or by an
excessive DC oset at the output terminals (via R5185) via V_DET. All these signals, and others, feed into
the protection module, described below.
The protection module comprises 8 transistors and associated parts positioned at the back of the PCB
near the preamplier output sockets. It has a single output line named PROTECT which, when pulled
down from Vcc to ground, instructs the system μP IC151 to shut down the whole amplier. This occurs
when any of the following events happen: