GAI-Tronics ILD1000A Local Deskset User and Installation Manual - Manual (Page 20)

Theory of Operation ILD1000A Local Desk Set
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Speaker Volume Control
The actual speaker output level adjustment is done internally within power amp U17. The control circuit
for this provides a precise control current sink connected to pin 5 of U17. The microcontroller examines
the
VOLUME
Up/Down button presses and provides control pulses to electronic pot U16. U16 provides a
variable reference voltage into the divider resistors connected to U11 pin 10.
Speaker muting voltage is also connected to this divider through R105. U11c is connected with Q3 as a
programmable current sink for U17 output gain adjustment. A voltage higher than 60 millivolts dc at Q3
source (and R110) results in muting the speaker. Less than 1 millivolt dc results in full output.
Power Supplies
The power supplies for the low tier desk set operate in the linear series-pass mode. Programmable zener
U21 serves as reference for the 5 V digital and analog power supplies. The reference voltage is 5.00 V
dc, +/−0.1 V dc measured at U21-1. Op-amp U13b controls the series pass element Q21 to provide 5 V
dc at the 5 VD source point at C55, C56. The 5 VA supply is low-current and is fully implemented by
U13a.
The VOS power supply, used to power the op-amp rails, is regulated to be approximately 11.2 volts. The
voltage level of the VOS supply is approximately 1.2 volts above the 10 V (nominal) CMOS switch
power supply. The reference for VOS is the 5 VD output. The divided output sample (through R133,
R134, Q24 and R135) is compared to the reference at Q23, which serves as the control element for the
series pass transistor Q22.