Bose AmPlus 50 Supplement Manual page 5

Introduction
This card accepts a pair of mic or line level sources at its Page 1 and Page 2 inputs at
J1. These signals are amplified in three variable gain stages, passed through a muting
circuit, and presented to a line out source selector. The page signals are also mixed with
the signals fed to the Page option card from the AmPlus™ 100 and fed back into the
AmPlus 100.
Sheet 1 of 2
Refer to the Opti-Voice Page Option Card schematic diagram, sheet 1 of 2 for the
following information.
Note: The designators inside the brackets "[ ]" are the schematic grid coordinates which
are provided in order to make it easier to locate components on the schematic sheet
indicated in the description.
The Page 1 and Page 2 inputs at J1 [B/C8] accept any source from mic to line level
sources. U100A [D7], and U100B [A7] are differential input amplifiers, with inputs well
protected against RF and ESD, and with gain selectable between 0 and +30dB by
switches S1 [C7], and S2 [A7]. The amplified page signals pass through U101B [D7],
and U102B [A6], stages with gain controllable between 0 and +20dB by R1 [D6], and R2
[A6]. After this stage, S3 [D5], and S4 [B5] select whether the page signals pass through
another adjustable gain stage (U101A, U102A) or through a limiter. The adjustable gain
stage can range between -20 and +10dB, controlled by R1, 2. If the page signal is routed
through the limiter, the limiter can automatically control the gain, from -20 to +10dB, in
response to the signal level coming in. The compressor is implemented with U106 [C6],
U103C [C6], and U104C [B6]. U106 has a current in/current out gain cell providing
negative feedback around the op amps. The gain cell is controlled by an internal level
detector circuit which monitors the signal applied to the limiter. Once the input signal has
risen beyond the threshold, the detector reduces the stage gain with a slope of nearly
1dB/1dB, holding the output level nearly constant over a 30dB variation in input level.
Op amps U103A, B [C5] and U104A, B [B5] watch the highest of the signal levels out
of the first two gains stages and the compressor itself. They also watch the voltage on
the compressor release capacitors. Ultimately, they drive a bi-color LED, D104[C4], and
D105 [B5]. When the compressor is active, the LED glows green and brightens as the
signal level and compressor action rise. If the level out of the compressor or one of the
gain stages approaches its linear limits, the LED changes to red to warn the installer that
the source gain has been set too high.
After the limiter, S3, 4 feed the signal into a page mute circuit. Connected to the junction
of two input resistors of the inverting gain stages U105A [D5], is a transistor pair, Q100,
101 [C5]. The same circuit is used for the other channel with U105B [A4] with transistors
Q102, 103 [A4]. When these transistors are turned on, they attenuate the page signal,
essentially turning it off. The transistors are controlled by the voltages PG MUTE1, 2,
which are developed by the page trigger circuits. The page trigger circuits begin with a
trigger level control, R3 [B3], and R4 [B2] which feeds the page signal through an amplifier
and half wave rectifier and comparator (U202C, D, [B2/3] and U202A, B [B2]). Signals
greater than about 0.8 volts trigger the comparator, turning on the Page Trigger Indicator
LED D104 [B1] and D105 [B1], setting VPG1, 2 [B1] high, and raising PG MUTE1 [A1],
and PG MUTE2 [B1]. Raising PG MUTE1, 2 disables the page mute transistor, allowing
the page signal to pass.
THEORY OF OPERATION
®
Opti-Voice Page Option Card
5