Protection Circuitry - Bose 1800-V Manual

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 established 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 establish 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. 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. Base current is also provided 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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