Bose 1600-VI Service Manual download pdf (Page 16)

THEORY OF OPERATION

The output signal is sensed at the speaker output via the I/O Board (J2-3 Output Board to J6-3

I/O Board to J3-6 I/O Board to J1-6 Display Board). D22 half-wave rectifies the signal and

provides a DC voltage proportional to the amplifier’s output to drive the signal display circuit.

C2 and R19 determine the response characteristics of the display.

The signal driver circuit comprised of U1-U4 is basically a ladder comparator driving LEDs,

with a twist. Assume that the signal at U2A-3 is zero volts (ignore R24 and D23 for now). R13

and R14 are a voltage divider that establishes a reference voltage for the comparators (four

per channel). The comparators compare this reference voltage against the voltages estab-

lished by the tapped voltage divider made up of R22, R20, R15 and R25. The CH1 LEDs are in

the following sequence (lowest to highest): D105 (red/ green), D13 (amber), D15 (amber), D14

(amber), D12 (amber), D11 (amber), and D104 (red).

With the input at zero volts, all of the comparator outputs are at -12V, except for U2B-7 which

is high. None of the signal LEDs have any voltage across them, all are extinguished. As the

input signal rises, it crosses in sequence at the thresholds established at each of the four

comparators. First U2A-1 fires; its output goes high and D13 illuminates. Next U1B-7 fires, its

output goes high, D13 extinguishes (no net voltage across it) and D15 illuminates. Finally U1A-

1 fires, D15 extinguishes, and (this is the twist) D23/R24 supply current to the bottom of the

R15, R20, R22 and R25 voltage divider, which inverts the relationship of the comparators to

each other.

When U1A-1 fires, the current through R24 reverses the sequence of the voltages that estab-

lish the thresholds for the three comparators. This allows the same comparators to perform

double-duty. The new thresholds leave U1A-1 high, U2B-7 low, U2A-1 and U1B-7 low and D14

on. D11 and D12 are off. As the input signal rises further, U1B-7 fires, extinguishing D14 and

illuminating D12. Next U2A-1 fires, extinguishing D12 and illuminating D11. Finally U2B-7 fires,

extinguishing D11. The last LED is the clipping indicator, D104.

3.0 Protection Circuitry

Protection functions are provided that will deactivate the output relays. Protection is provided

for the following fault conditions:

3.1 Over-Current Protection

The amplifiers are protected from short-term excess current through the output stage by

electronic current limiters. When the current through the output transistors becomes excessive,

the voltage drop across the emitter resistors R148 and R149 bias the current limiter transistors

Q105 and Q106 on, which shunt the drive current via D106 and D107. R139, D102, R140 and

D103 determine the V-I limits.

When the current-limiters turn on, the voltage at voltage divider R127/R128 becomes less

positive, providing base current for Q1 on the I/O Board through R38. When Q1 turns on two

things happen; current flows through U3 (LED/LDR module) via D31 which attenuates the

input signal, removing the high current condition as well as providing base current to Q5

through D1 which turns off Q2 and Q3, causing the relay to disengage. C13 provides a time

delay to prevent the relay from disengaging during momentary program peaks. When the relay

disengages, it causes the red LED in D105 (READY LED) to illuminate and also turns on Q100

and D104 (CLIP/PROTECT LED).

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Bose 1600-VI Service Manual Page 16

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