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Master Gain Stage and Output
Limiter
Output Meter
25A1041 (RA)
Shure AMS8100 Automatic Microphone Mixer
The master gain stage is U906B and its associated components.
The master gain stage is an inverting gain stage.
The output stage is U901 and its associated components. The
SSM–2142 chip is an active balanced line driver. The ac output voltage
seen across U901, pin 3 and pin 14, is 6 dB higher than the input voltage
at pin 6. If either the positive (+) or negative (–) pin at the output is
grounded, the output across U901, pins 3 and 14, will stay the same.
C910 and C911 protect the chip from phantom power. C901, C902,
L901, L903, L903, and L904 are for rf filtering.
The limiter circuit prevents the output stage from clipping and keeps
the maximum peak output level below a certain level set by the user.
The limiter is a high quality, full-wave peak limiter with a high limiting
ratio. It has a very fast attack time (about 4 mS) and relatively slow
decay time (100 mS). U911, an optoisolator, performs the actual gain
adjustment, but the limiter circuit provides the dc control voltage neces-
sary for the optoisolator's action.
U907A, U907B, and associated components, form a full-wave peak
rectifier with 6 dB of gain. This rectified signal is fed to U907D and its
surrounding components, which is an inverting gain block with different
gains set by the limiter DIP switch. These different gains manifest them-
selves as different limiter thresholds. When this amplified rectified sig-
nal's level reaches a certain level, current will flow through D923, U911's
LED, and R9186. This current flowing through U911's LED causes
U911's resistor to drop in value, which lowers the gain of U906B.
This current also generates a voltage across R9186. When this
voltage goes below –15 mV, U908C's output will clamp to –15 Vdc. This
turns off U908B, which turns off Q928. This allows current to flow
through the limiter LED segment on LED902. C959, R9169, and U908B
act as a pulse stretcher to extend the "on" time of the limiter LED.
The output meter gives a visual indication to the ac voltage level at
the line output connector. The meter is a full-wave, peak-responding
type with a relatively slow decay.
The rectified output signal as provided by U907A and U907B is fed
into U907C. U907C is a half-wave rectifier/ buffer. When the signal at
U907, pin 12, is higher than the voltage across C948, then U907, pin 14,
swings higher, causing current to flow through D917 and R9173, thus
charging C948. When the voltage at pin 12 is lower the C948, C948 is
discharged through R9173, R9174, R9175, and D917. The output of
U907C sinks this current.
9
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Characteristics
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