Crown PSA-2 - Page 100

The gain stage is constructed with UIB whose output is
routed to the low-pass filter using U3C. The low pass
filter is standard as a 3-pole Butterworth type of ISK Hz.
It also may be readily reconfigured. The input and output
of the LP filter are routed to SW101 for selection of the
desired signal. The output of SW101 is then input to the
limiter-compressor.
The limiter -compressor is constructed with deliberately
restricted compression range. Compressors with large
gain compression capability often exacerbate feedback in
systems where the gain is increased only to be reduced by
the compressor action. Should the input be reduced the
gain will suddenly appear as the compressor recovers and
feedback will result.
The compressor is nota part of the circuit function untila
control signal biases on U4. U4 is what is known as an
operational transconductance amplifier or OTA. Such a
device converts a small input voltage into an output
current which is proportional to the control current
injected into pin 5. When Q100 is off, R117 acts to assure
that no leakage current will enable any amount of
compression. When Q100 is on, the current in R118 turns
on U4 which becomes an additional feedback loop in
parallel with R112 and decreases the net stage gain
proportional to the current in R112. The input signal to
U4 is provided by the divider R114 and R113. Since the
OTA has an offset voltage like any other op-amp, this
must be offset by a small DC voltage from R115 and
R116, the offset adjustment. Failure to null this error
would result in thumping of the compressor as it changed
gain. C111 is used to decouple any DC from the module.
The output is then routed to the jack switch in J21. Ifan
input is applied to J21, the balanced input module is not
part of the signal path.
The control signal for the OTA is derived from a
threshold detector (full-wave) composed of comparators
U6A and U6B. Should the main input amplifier signal to
the balanced gain stage exceed the thresholds established
by the wiper of R7 or the output of U2D, the comparators
will signal Q100 via R120 to turn on. C112 acts to filter
the control signal such that the turn-on of Q100 is
governed by CI12 and R120 while the turn-off is
governed by C112 and R119. This makes the attack time
of the compressor much faster than the decay time, as is
desired for minimum distortion operation of the system.
The output of U2D is the inverted replica of the DC
threshold reference on the wiper of R7, the threshold
control. When R7 is set to maximum the only signals
which have sufficient amplitude to reach the threshold
are feedback error signals caused by overload of the PSA.
All overloads will then result in compression of signals
processed by the balanced input module. If R7 is
decreased the thresold will pass below the overload
values and into the signal range (remember that the signal
here detected is a -'4th scale replica of the output). This
7-33
will allow the compressor to restrain the output power of
the unit to protect fragile drivers, etc.
Should stereo tracking of the compressors be desired the
test points TPS and TP6 may be shorted together. This
will cause the compressors to compress equally despite
which channel may have initiated the gain reduction.
If the action of the compressors is undesirable in an
application, the OTA’s U4 and US or comparators U6
may be removed from their sockets to prevent all
compression.
When making a hurried hook-up of a sound system it is
often handy to know if the amplifier is attached to the
loudspeakers that it is to drive, i.e., no open or shorted
speaker cables. To make this test easier, the PSA-2 has a
built in tone generator which provides inpulses at a SOHz
rate. As such the spectrum will excite tweeters or woofers
with a signal that has very little power yet is quite
distinctive and audible.
The generator is constructed with UID which excites
both signal channels with a pulse shaped by Cl in
conjunction with RII and R211. UID is wired as an
astable which has sufficient regeneration to oscillate only
when SW1 is closed.
1, PSA-2/ PSA-2X Display
The display of the PSA-2 is a combined set of indicators
to show the e of the output stage supplies, power
applied to the control supply, signal on the outputs and
outputs overloaded (IOC).
Amber LED D1, powered by RI and the -24VDC
unregulated supply is used to indicate power applied.
Yellow LED's D112 and D212 are used to indicate the
standby condition of their respective channels.
The following discussion will center around the channel |
circuitry which uses a quad-comparator, UI00. Channel
2 is of course identical.
If a sufficient output signal is present to forward bias
D107 and D108 with the current through R106 and
overcome the bias of resistors R107-110 to ULOOC, then
the monostable U100D will fire and turn on QUOI. This
will in turn light LED D111 (green) to indicate that a
signal is present on the output of channel |. Should the
signal cease, the current in RII1 will act to charge C101
and reset U100D. D109 is used to assure proper resetting
of the charge on C101 when the monostable resets. The
use of such a monostable driven signal indicator makes
even short transients highly visable.
The operation of the IOC is very similar to the signal
detector with the difference that U100B senses the output
signal of the main input amplifier for excursions beyond
approximately twelve peak volts, Diodes D100-104 in
conjunction with zener D102 are used to sense such
om