Crown PSA-2 - Page 99

crown
Focusing our attention upon just the channel | relay
control circuitry (channel 2 is identical) we find the
following. Upon power-up the capacitor C105 is
discharged and starts to charge with the current supplied
by resistor R107. When the potential on UIB pin II
exceeds the approximately 7.5 volts on pin 10 the output
on pin 13 will proceed positive allowing Q100 to turn on.
The result of Q100 turning onis the production of a small
hysteresis voltage on pin 10 as a result of RIO9. This
insures the decisive switching of UIB and Q100 for
proper relay action. If C103 were switched in parallel
with C105 (which is the function of the DELAY switch),
the time needed to charge this circuit would result ina 4-5
second delay in the turn-on of the unit.
R105 is wired in series with a thermal switch wound in the
windings of the output stage power supply transformer
TI. Should T1 overheat the thermal switch will open
causing UIB via D107 to turn off Q100. This method of
protecting Tl allows for much larger outputs from the
amplifier without fuse blowing. The use of conventional
fuses or circuit breakers is not an optimum means of
protecting a power transformer because the time
constants of these devices are many times shorter than the
thermal time constant of a large transformer whose time
constant may be a matter of hours.
Low frequency protection of loads is made available by
switching the output signal to drive R100 and C104 which
act to low-pass the output signal. Should the output be
too long in excess of 26VDC the diode network
composed of DIOI-104 and zener D105 will conduct
removing the bias-off voltage from the input of the
comparator UIA as provided by resistors R101-104. The
output of will act to discharge C105 and C103 via R106.
This will remove power from the output stage and cycle
the supply back on as soon as the DC input dissipates
which caused the shut-down. This approach is more
reliable than the use of a relay in the load circuit since
such a relay may not be able to break a DC circuit due to
prolific arcing caused by load inductance.
D106 is used to prevent back biasing of C105 and C103.
Were this not necessary R106 would be unnecessary
G. Output Power Supplies
The output supplies are two thermally protected
transformers Tl and T2 with bridge rectifiers driving
filter capacitors C101, C102, C201 and C202. Capacitors
C100 and €200 are used for RFI suppression.
TI
and
T2
each
have
are
suppression
across
their
relay
contacts
(C2,
RS
and
C1,
R4)
to
improve
relay
life
and
to
reduce
radio
frequency
interference.
These
transformers
have
universal
primaries.
Fuses
Fl
and
F2
are
the
aforementioned
fault
protection
fuses.
Only
shorted
supplies
should
be
able
to
dislodge
these
fuses
in
normal
circumstances.
Switch S| is arc suppression protected by C3 and R3 and
is the main front panel power switch, Should F3 be
blown, indicating a failure in the control supplies,
nothing will function in the amplifier. It would be as if the
power were turned off.
The fan motor operates in low speed when the power is
applied through resistor RI. To operate in highspeed RI
is paralleled by R2. R2 is used to prevent destruction to
the fan speed relay when engaging and charging C4.
All of the relays have damping diodes across their coils to
protect their drivers.
H. PSA-2 Balanced Input Module
The balanced input module is used only on the PSA-
2(D). It provides the following functions that are
particularly useful to professional sound reinforcement
users:
A. Balanced inputs with XLR connectors.
B. Gain adjustments for normalizing line levels.
C. Filters for use as crossovers, etc.
D. Variable threshold compressor limiter.
E. Impulse tone generator.
The channels are identical in design and the discussion
will focus on channel I. The balanced input amplifier is
provided by UIA which receives its signal from J24. R101
is used as a low frequency common-mode rejection
adjustment while C102 is provided for optimizing the
high frequency common-mode rejection.
The output signal from UIA is high-pass filtered by U3D
and is routed to SW100 where the user may select either
the HP filtered or unfiltered signal for input to the gain
stage. The filter constructed by U3D is a 3-pole
Butterworth type of SOHz, as supplied with the standard
unit. It may be changed by swapping the filter module
board or changing the component values on the filter
board. This allows a sound installer to have an inventory
of his preferred crossover frequencies available for use.
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