Crown PSA-2 - Page 96

Diodes D117 and DI18 are part of an instantaneous
current limiter bias arrangement which limits the peak
output current to approximately 7A per device. The
remaining part of this current limiter is provided by
diodes D114 and D116.
When sufficient current flows in the output devices,
diodes D114 and D116 will become forward biased by the
drive voltage to the output stages. When this occurs the
current will be effectively limited. Note that the diodes
are connected to the opposing output current sensing line
rather than to ground. This is a simple means to limit
common-mode current spiking in the output stage to
values less than 7A per device. The spiking current can
only reach a value which is 7A minus the per device load
current. Since the worst spiking occurs when the load
current is large, the reduction in spiking current is large.
The differential amplifiers are rather precisely balanced
by the precision resistor packs RNIOI and RN102.
RN102 contains the bias control which is used to tweak
the balanced to result in the desired bias. The standard
bias at quiescent temperature is 2.5mV as measured
across the current sensing resistors R409-412.
The input to the balanced gain stage can be conveniently
probed at pin 7 of RN1O1. This point constitutes a virtual
ground in the amplifier receiving input signals through
the network from pin 6 of U100 and feedback from the
output signal via R114. The signal propogates from this
virtual ground to the bases of Q102 and Q103 (pin 2 and
pin 6 respectively). These devices act as emitter-followers
and drive the remaining half of each differential pair
through resistors R112, 111 and R115, 116 respectively.
Acting in the grounded-base mode of operation these
remaining devices provide a high quality current
generator to drive the output stages.
R113 and C110 are used to compensate the closed-loop
amplifier formed by the balanced gain stage and the
output stages. It has a closed loop voltage gain which is
about -8, as measured from the output of U100 to the
output terminal. When debugging the amplifier this is a
useful relationship to observe if the output voltage is
unusual. In normal operation the diodes D104 and D105
should not be in conduction. They are provided to limit
the charge which can be stored on C110 upon overload.
This charge if allowed to become large would produce an
overload recovery delay. By limiting the voltage at pin 7
of RN1OI in this manner, the voltages necessary to limit
the drive of the balanced gain stage are confined. This is
in regard to the voltages that would result in conduction
of D111 and D112. These voltages are produced by the
protection circuits analog computer junction
temperature simulator UI0IC and U10IB. Conduction
of these diodes limit the drive of the balanced gain stage
to the respective overheating output stage.
7-29
The resistors in RN102 which parallel these diodes are
used to provide the bias servo temperature feedback
signal as is provided by the heatsink attachment of the
bias servo transistor in other Crown amplifiers. The
heating of the output devices is used to reduce the bias
current control signal.
In the original design the diodes D106-110 were used to
limit the voltage on pin 7 of RNIOI. R113 was chosen
small enough that the effects of these diodes become
negligible and as such these diodes are omitted from later
units.
C107 and C111 are used to optimize the high-frequency
behavior of the grounded-base side of the differential
pairs.
C. Input Amplifier - PSA-2/SA2
At the far left of the full schematic is the stage which
receives the input signal and the overall feedback loop
with the resulting highly amplified error signal being used
to drive the amplifier composed of the balanced gain
stage and the output stages.
A monolithic dual JFET, Q10I is used as a differential
amplifier to drive the Bi-FET op-amp U100. The source
leads of Q1IOI are current degenerated by resistors R105
and R106. The drain circuit loads of the differential
amplifier are provided by RN100.
Compensation is provided by C102, R109, C105, and
C103. The diodes D102 and D103 are used to limit the
stored charge that could accumulate on C102 upon
overload.
The main feedback loop is composed of R102, RIO1,and
C100 which controls the closed loop frequency response
as in all other Crown amplifiers. Note that the channel
two equivalent of R102, R202 is a larger value. This is
because in the stereo mode of operation the “mono-ing”
resistor R200 is- grounded, parallelling R202. R200
results in a gain of -I at its input.
The DC offset voltage of QIOI can be compensated by
the adjustment provided by R104 in conjunction with
R103. Input overdrive protection is provided by R108.
Diodes D100 and D101 are used to prevent overdriving
of the common-mode input range of the input stage upon
overload. R107 is used to provide a ground reference for
the input signal should the signal line become open. Since
the bias current of QIOI is negligible, no bias current
adjustment is needed.
R110 is used to provide an output to the 1OC or
compressor circuits. This output will be at approximately
13 volts if any form of overload should occur in the
amplifier. Should no overload be present the signal will
be a scale inverted replica of the output signal. This fact
will allow the PSA-2 compressor to respond to levels
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