Bose 1600-VI Service Manual download pdf (Page 12)

THEORY OF 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 1600VI and 1800VI amplifiers use 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 33.3 dB for the 1600VI and

36.0 dB for the 1800VI. Removing jumper JP100 disconnects R111 from the circuit and sets

the closed-loop gain at 27.6 dB for the 1600VI and 30.3 dB for the 1800VI. The factory default

settings are 33.3 dB for the 1600VI gain and 36.0 dB for the 1800VI 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 +90V/1600VI and +106V/1800VI power supply rails. Q104

and Q122 are connected as an NPN-PNP conjugate pair and used as a V

multiplier for bias

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.

Note: Output transistors Q112, 118, 212 and 218, along with the corresponding channel 2

output transistors Q113, 119, 213, 219, are not used in the 1600VI amplifier.

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 for more information). From the

Amplifier Board, the signal passes to the Output Board via E100 (E200 for CH 2). R100, L100,

R110 and C100 on the Output PCB comprise the output pole circuit for amplifier stabilization.

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Bose 1600-VI Service Manual Page 12