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darlingtons (causing the transconductance stage not to saturate, which reduces the possibility of
saturation in the gain stage).
The drivers and output devices are mounted on the large extruded aluminum heat sink.
The drivers have extremely linear gain with changes in current. They also have an F t of 50mHz
keeping the gain linear with frequency. The output devices are 16 amp, 250 volt, 200 watt
devices. These are very strong devices. The output stage is configured in a common collector,
or emitter follower configuration. The drivers are connected again in a discrete complimentary
darlington configuration. This allows for a class A bias and protection scheme that offers very
close bias tracking and simple current limiting. Also there are 1.2
the output devices. This helps linearize the gain with frequency and reduce high current
parasitics. When the output stage is biased properly, the DC voltage drop across the emitter
resistors should by 2-3mv cold and may rise to as much as 6-10mv when hot. Finally the output
of the amplifier is de coupled from the load with a traditional termination network. This network
isolates the amplifier and feedback loop from loads at high frequencies, especially capacitive
ones. This is how unconditional loop stability is achieved. Under bench test conditions it is
recommended that the amplifier not be driven to full power at 20kHz and above for periods
longer than a few minutes as this will cause R53 to over heat. Under music conditions there is
never enough energy to have this be a problem.
1.40 Output Device Protection
Output device protection is accomplished with a relatively simple circuit. The protection
circuit is broken up into Q17 that protects the NPN output devices and Q18 that protects the
PNP output devices. These devices are complimentary as are the output devices but they also
have similar V be N to P. This allows selection of a point of protection that will be about equal for
each half of the output stage. R55 and R56 sense the voltage at the emitter of two of the output
devices and sum them. The emitter of Q17 is connected to the output of the amplifier. When a
voltage across the emitter resistors of the output devices reaches about 0.7 volts, Q17 will
conduct if there is no voltage across R54. This condition exists for a short circuit or something
very close. As the amplifier impresses a voltage across a load it also has that same voltage
across R54. As the voltage across R54 gets larger more current must flow through the emitter
resistors of the output devices to cause enough voltage to be developed at the base of the
protection device to turn it on. This is what is known as the load line. As the voltage across the
output devices gets lower they can deliver more current in keeping with a constant power. When
the protection device is conducting the current from the transconductance stage is essentially
being diverted around the output stage to the load. Everything described for the positive half
cycle is the same for the negative half. C18 and C19 slow down the protection device and keep
it from any possible oscillation condition. CR3 and CR4 are used to protect possible reverse V be
conduction of the protection devices and CR4 and CR5 prevent conduction of the protection
devices while in the opposite half cycle.
1.50 Bias Circuit
The bias circuit is more that just a single device V be multiplier seen in many audio
amplifiers. The circuit consists of an active shunt regulator. The reason for this is two fold. The
shunt regulator has a much lower impedance than the single device regulator. This helps control
the bias voltage better when there is a change in the quiescent operating current of the
transconductance stage. Since the operating, or quiescent current of the transconductance
stage is supply dependent, the need for a bias circuit that doesn't change voltage with current is
imperative. Also by reducing the current through the bias sense transistor a larger change in
V be with temperature can be realized. This tracks the needs of the output stage better. The 5k
pot in the bias circuit adjusts the potential of the regulator. A 1.5k resistor (R55) sets the current
Alesis A4/A8 Amplifiers Service Manual------
resistors in the bases of all
3
03/07/03
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