Crown PSA-2 - Page 97

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below the overload threshold when the threshold is
appropriately selected.
The supplies to the input differential amplifier stages are
filtered by R7, C3, R8, and C4 to remove the noise
induced by the 15 volt regulators.
D. Protection Circuitry
The protection circuitry of the SA2 amplifiers is perhaps
the most unusual of all its new features. It is the result of
an indepth study of the heating behavior of
semiconductor junctions and the design and construction
of a considerable amount of special hardware. It was
necessary to implement the following equipment:
A. SOA UI Transistor Analyzer
B. 12 bit A/D Convertor Data Aquisition Unit
C. Microcomputer (Altair 8800)
D. Instrument Amplifier
E. Dummy Heatsink with devices slaved to SOA
Once it was known what the nature of the sought after
information would take, it was readily realized that this
information would only be available by direct
observation, since it was not available from the
semiconductor manufacturers.
What was needed was a direct observation of the
heating/cooling characteristics of a large sample of
devices of the desired type. Data must be gathered over a
large time interval (tens of seconds) with a maximum of
data being gathered indicative of the shortest of time
intervals. This constitutes a data gathering strategy which
is not compatible with means such as_ storage
oscilloscopes, strip-chart recorders, etc.
To meet this requirement it was necessary to program a
computer to gather the data at the precise time intervals
desired. The Altair was programmed to gather the data
and then to transmit the data to the Crown Engineering
Wang Computer for storage on disk and do the very
elaborate number crunching which is needed to uncover
the equivalent thermal circuit of the physical devices
Without the use of the computers the proper
development of this circuitry would have been nearly
impossible. The circuitry acts to simulate via an electrical
signal the junction temperature inside the worst device
that is likely to be mounted in the output stage. The
circuitry does this without any direct probing of the
output chip.
The knowledge we required was the time behavior of the
junction temperature for an arbitrary power input signal.
This was deduced by watching the cooldown phase of
power transistors which had been heated in an
environment identical to the heatsinking used in the
amplifier.
In the amplifier we must also know what power has been
applied to the output devices. This information is
provided by the multiplier circuits; i.e. Q104 with U101D
and QI05 with UIOIA. Assuming that we limit our
attention to the protection circuit that protects the NPN
output stage of channel one; Q104 is used to multiply the
Vce of the output stage as sensed through R120 with the
collector current as sensed by R304 via RN104 pins 8 to 7
and R119. Q104 is what is commonly referred to as a two-
quadrant transconductance multiplier. Its operation is
based on the logarithmic nature of the base-emitter
voltage as a function of collector current.
Since the output of Q104 is balanced, the currents at its
collector must be converted to provide an unbalanced
signal. U101D forms an op-amp current mirror. The
current of the collector of Q104 pin 7 is mirrored by the
action of the output of UI0ID pin 7 RN104. The current
in the feedback resistor pins | to 2 is equal and opposite to
the current in the collector of Q104 pin 7. Since the
resistor from pins 2 to 3 is of equal value to the first, the
current will be identical. This current is joined with the
current from the other collector output of Q104, pin |.
This node constitutes the output of the multiplier.
Summed with these currents is a current proportional to
the heatsink temperature upon which the output devices
are mounted.
The current from the IC sensor on the output stage is
bypassed with a capacitor C117 to minimize audio signals
capacitively coupled to the sensor wiring. Since the
sensor is intimate to the heatsink which is electrically hot,
such coupling is to be expected. To allow for monitoring
of the temperature by the fan speed control circuits and
troubleshooting, the current is input to a precision sense
resistor (10K) in RN104. This converts the sensor signal
to a voltage which is proportional to absolute
temperature with a scale factor of -l10mVDC/degree
Kelvin. In other words, the voltage for room temperature
(25 degrees centigrade) would be -2.93VDC at +Ts. This
is a convenient point to probe should either the sink
temperature or the sensor be suspect.
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