Block Diagram Circuit Theory - Crown PSA-2 Service Manual

crown
7.12 Block Diagram Circuit Theory
General
PSA-2 Input Explanation Only
An input signal is fed to the initial stages via the standard
unbalanced input or the balanced input. Both cannot be
used simultaneously due to the "interrupt" function of
the unbalanced input jacks.
The balanced input jacks are located on a separate, rear
panel plug-in module board which also contains many of
the professional features unique to the PSA-2.
A Variable Gain stage, next in line on the Balanced Input
Module, adds an adjustable voltage gain (0-10) ahead of
the main amplifier.
Connected to this stage, are Hi and Lo pass filters,
factory set at S0Hz and 1SKHz respectively.
The resultant of the above mentioned stage, along with a
switch-controlled wide-bandwidth Test Tone Generator
signal, is fed to the Compressor-Limiter circuitry. At its
output point, an unbalanced signal may enter if so
desired via 4" phone jacks.
PSA-2/SA2 Explanation
'The input amplifier receives the signal next and sends any
necessary error-correcting information to the
Compressor Control circuitry as well as sending the main
signal on to the Balanced Stage. Essentially, this
feedback path (from the output of the input amp through
the Compressor Control circuitry) adjusts the amount of
compression needed at that particular instant to provide
distortion-free output.
In order to drive the NPN and PNP Output Stages, a
Balanced Stage is necessary. Should a situation be
encountered where protection of the Output Stages is
needed, the Protection Circuitry will automatically
reduce the drive available to the Balanced Stage and thus
remove the stress on the output devices.
Both the NPN and PNP Output Stages consist of four
SOA analyzed and VBE matched output transistors plus a
predriver/driver combination that also aid in carrying
the quiescent power load. Together they help form the
quasicomplementary, Class AB method of operation
used in the PSA-2/SA2.
Feeding positive current to the NPN OUTPUT STAGE,
and negative current to the PNP OUTPUT STAGE, are
the POS and NEG Vcc (high Voltage) Supplies. The
common point between the two Output Stages is ground.
A departure from previous smaller Crown amps, this
method allows sophisticated information to be fed to the
protection Circuitry from the Output Stages with
reference to ground. Both channel's High Voltage
supplies work independently of gne another.
The point Common to the Neg and Pos Vcc supplies is
the "hot" signal of the output terminal which also feed the
front panel Display, the Mono switch (for selectable
stereo-mono output) and several of the main feedback
paths.
The Control Logic is responsible for the action of the Lo
Freq Protect, Delay, Standby and thermal protection of
the unit. When signaled by the Lo Freq Protect, Standby
and/or Delay feature, the Control Logic will remove the
power from the Vcc supplies. In the case of Low Freq
Protect, when the output has subsided it will place the
high voltage supplies back into operation from
STANDBY or cycle through the same procedure again
depending upon the existence of the problem. Thermal
protection may involve the same procedure as mentioned
above but only in extreme cases. A thermal switch
imbedded in the high voltage transformer's windings will
activate the Control Logic when potentially damaging
current demands are being placed on it.
The Low Voltage supply drives all low-power signal path
circuitry including the Control Logic, Display and Fan
speed logic. At an internal temperature of 47°C, the unit
will automatically shift to "high" fan speed operation for
additional cooling.
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