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transient) the output stage is allowed to drive even higher amounts of current. This is
accomplished by AC dividers consisting of R26, R76, C16 and R75, R87, C21. C17 and C20
provide for high frequency stability of the limiter, and D22 and D25 isolate this stage from the rest
of the amp, when not being used.
It’s important that the output stage never saturates. When an output stage saturates, it
slows down, resulting in higher distortion (bad) and common mode conduction (real bad
because this can blow-up a power amp). Common mode conduction is where both the
positive and negative half of the darlington conduct at the same time, not a good thing! To
prevent saturation, a Baker clamp is employed. For positive going signals, as the amplifier clips,
Q52 and D1 conduct and hold the Voltage at the collector of Q6, 2 diode drops above the
+64V supply. D6, D5, D23, the drop across Q5-BE, and Q4-BE, result in the voltage on the base of
the output parts being three diode drops (1.8V) below the +64V rail. Since the base of the
outputs can never be any greater than 1.8V below the collector, the output stage can not
saturate. The negative Baker clamp functions the same way.
The second stage voltage gain is set via R104, R105, R80, R79, R81, and R82. The gain is
somewhat less than the simple ratio of these parts due to the limited transconductance of Q42
and Q43. On the positive half, Q41 acts as a unity gain buffer and it’s output drives Q42. The
output of Q42 feeds common base amp Q6 forming a cascode, with D7, C10 R2 and R50 being
biasing and decoupling elements. The negative half, made up of Q44, Q43 and Q8, is a mirror of
the positive half. The topology of this stage has intrinsically lower distortion, better high
frequency response, and substantially better defined voltage gain, than many other commonly
used topologies. The quiescent current in both of these stages is determined by the voltage drop
across R100 and R109. D41 -D44 allow the second stage quiescent current to remain constant, as
temperature changes.
The first stage is fully complimentary and differential in design. Overall input to the system,
and all feedback for the amplifier occurs in this stage. Differential pairs Q56, Q57, Q58 and Q59
are cascode coupled to the second stage via Q40 and Q45. The gain of the first stage is
determined by the transconductance of the differential pairs, R121, R122, R126, R127, R100 and
R109. Current to this stage is provided by constant current sources Q66 and Q65. The current
sources can be switched on and off in tandem via Q67, which allows the amp to be turned on
or muted silently (no pop presented to speaker outputs). D65 and D64 linearize these current
sources with temperature. Again this first stage of the amplifier is quite linear, has good
frequency response, has well defined gain, and has excellent common mode and power supply
rejection. All these things are quite important, as negative feedback can only provide optimum
correction if it is summed in a very linear stage.
Open loop pole number one is determined by C18, C19, and R79-82. The second open loop
pole is determined by C32, R102, R100, C33, R107, and R109. The open loop zero is determined by
C32, R102, C33, and R107. The final loop compensation is the feedback zero composed of C49
and R145. Closed loop gain of the amp is 27 and is set by R123 and R145. C48 and R123 roll-off
the amplifier’s gain at low frequencies. At DC, the amp has a closed loop gain of 1.
The amplifier has well defined open loop gain. The loop compensation is dominated by the
above mentioned parts, and is not sensitive to circuit parasitics. The voltage amp has
intrinsically low distortion. The high current gain of the output stage effectively isolates the load
from the voltage amplifier. Consequently Mackie can get nice low distortion numbers, with less
negative feedback than the competition. Less feedback, and good frequency response, results
in a very stable amplifier (it won’t oscillate).
circuit theory continued.