Bose 1800-V Manual page 15

2.5 Power Supply Regulation
The firing angle of the triac controls the voltage on the primary of the transformer, and is determined by
the conduction of the optocoupler U3 on the Regulator Board.
As the conduction of the optocoupler increases, so does the conduction angle of the triac. The photodiac
conduction of the optocoupler is controlled by the current through the LED portion of the optocoupler, the
amount of current through the LED is equal to the amount of current through transistor Q1. When the LED
in U3 is fully ON, the triac conducts earliest in the AC cycle: the power supply is operating at maximum
output. The LED current is supplied by voltage regulator U2 on the Regulator PCB. U5A differential
amplifier senses the secondary supply voltages through R37 and R38. The output voltage at U5A-1
increases at the rate determined by R30 and C8 (slow start-up). The idle secondary voltages are set by
R36 on the Regulator Board .
2.6 Load Regulation
When the amplifier is driven at high power into a load, the high DC supplies (rail voltages) will begin to
"sag". Differential amplifier U5A-1 senses this and increases the LED current to optocoupler U1. This
action increases the phototransistor conduction, which increases the output current of U4-8, increasing
the charging rate on C6. This ultimately increases the triac conduction which increases the primary
voltage, which increases the secondary voltages, thus providing steady, regulated DC supplies for the
amplifier stage.
The -15VDC supplied to U5A-4 is backed up with a voltage divider off the -106VDC supply (R26/R27).
Without this, if the -15VDC supply should fail for some reason, the output of U5A-1 would go high,
drawing maximum current through the LED in U1 and latching the triac into full conduction. To prevent
this, D8 will become forward biased and supply negative DC to U5A-4, keeping it operating normally.
Note that if the +15VDC' supply should fail, the output of U5A-1 would go negative, turning off the triac.
2.7 Secondary Voltages
The secondary winding of the power transformer has two taps that supply the two pairs of DC supply
voltages, ±106V and ±53V, each having its own bridge rectifier and filter capacitors.
The ±15VDC supply is tapped from the ±53VDC supply through R6 and R8, and regulated by Q2, Q3 and
zener diodes D3 and D6. The +15VDC supply powers the op-amps and small-signal transistors.
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