Bose 1800-V Manual page 13

The amplifier can be operated in four different output modes: normal (two independent channels) dual
mono (one input channel, two separate channels), bridged mono, or parallel mono. These modes are
selected via SW1 and control the operation of op-amp/switching ICs U6 and U5. Parallel mono operation
requires internal modifications to the amplifier, contact the local Bose ® Pro Product dealer for information
on parallel mono operation.
When SW1 is set to the NORMAL mode, CH1 and CH2 operate independently. In this mode a logic high
is applied to the control pins of U6 and U5. This routes the outputs of U4 and U3 to their respective
channels in the amplifier via J1-20 and J1-16.
In DUAL MONO mode both amplifier channels are driven by the signal applied to the CH2 input. U6-1 is
driven high (+15V) and U5-1 is driven low (+7.5V), selecting CH2 to drive both channels of the amplifier.
In BRIDGED MONO operation the CH2 signal is routed directly from the equalizer switch U3 to the
amplifier inputs. U5 control pin 1 is driven low (+7.5V) selecting the inverting input of the op-amp
switching IC signal. This inverting signal is then sent to the B input of U6 whose control signal at pin 1 is
driven low selecting the B input. This routes the inverted signal to the CH1 input of the amplifier.
For information on sequence operation see Section 2.8, Sequence Send/Receive.
2.2 Power Amplifier Circuitry
The 1800-V uses a conventional class AB push-pull power amplifier circuit, with a commutated two-stage
(dual-rail, class H) power supply. U100A-1 is the input stage, providing differential inputs for input and
feedback connections as well as most of the open-loop voltage gain of the circuit. Local and global
negative feedback from the output stage via R109, R113, R111/ R112 and R129 sets the closed-loop
gain at 36.7 dB. Removing jumper JP100 disconnects R111 from the circuit and sets the closed-loop gain
at 30.7 dB. The factory default setting is 36.7 dB gain. The output of U100A-1 drives Q100 and Q101,
operating as common emitters that level-shift the drive signal and couple it to the pre-driver amplifiers
Q102 and Q103. Q102 and Q103 provide additional voltage gain, and when combined with the voltage
gain of the input op-amp is sufficient to swing the input signal between the +106V power supply rails.
Q104 and Q122 are connected as an NPN-PNP conjugate pair and used as a V multiplier for bias
BE
control. Q104 is thermally connected to the output transistors and together with Q122 provides bias
stabilization over a wide temperature range. R124 allows the bias current to be adjusted to its optimum
value.
The predrivers (Q102 and Q103) provide the base current to the drivers Q108 and Q109. These drive the
output transistors; Q110, Q112, Q114, Q116, Q118 and Q120 for the positive half-cycle, and Q111,
Q113, Q115, Q117, Q119 and Q121 for the negative half-cycle.
Q105 operates as a V-I limiter, sensing the voltage drop across emitter resistor R148 (Q106 across R149
for the negative side), and reducing the drive signal to the output stage under overload conditions (see
Section 3.1 Over-Current Protection further on).
From the Amplifier Board, the signal passes to the Output Board via E100. R100, L100, R110 and C100
comprise the output pole circuit for amplifier stabilization. Relay K100 is used to connect the output signal
to the Speaker Output Binding Posts.
13