GAI-Tronics IDR1000A DC Remote User and Installation Manual - Manual (Page 24)

Theory of Operation IDR1000A DC Remote Desk Set User and Installation Manual
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Audio Circuits
Audio from the wire line is scaled through amplifier U12a and fed through the line audio adjustment,
POT3. Then, it is high-pass filtered through U12b to remove wire line hum. The resulting signal is gated
to the compressor circuit consisting of U11a, U11b, U11c, Q8, and associated components. Audio at
TPC1 is then gated to amplifier U13b where it is summed with any required tones (such as volume
beeps) from oscillator Ulld. At the output of U13a, audio is fed to speaker amplifier U15. Also, the
signal can be gated to the handset earpiece. Earpiece volume is adjusted through POT 5.
The microcontroller selects the active microphone based on the state of the hookswitch. Microphone
audio is scaled through U12d and gated to the compressor circuit. Microphone audio at TPC1, the
compressor output, is gated to amplifier U12c. POT 4 adjusts the line output level. The output of U12c
is gated to line driver U14 and transmitted over the wire line.
Speaker Volume Control
The actual speaker output level adjustment is done internally within power amp U15. The control circuit
for this provides a precise control voltage connected to pin 5 of U15. The microcontroller examines the
VOLUME
Up and
VOLUME
Down button presses and provides control pulses to electronic pot U9. U9
provides a variable reference voltage into the divider resistors (R156, R157, R158, R159) connected to
U13 pin 12.
Speaker muting voltage is also connected to this divider through R154. If the dc volume control voltage
at U15 pin 5 is below 0.3 V dc, the speaker amplifier switches to the mute mode. A dc volume control
voltage greater than 1.6 V dc results in maximum gain of the amplifier U15.
Power Supplies
The power supplies for the desk set operate in the linear series-pass mode. Programmable zener U7
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 U7 pin 1. Op-amp U8a controls the series pass element Q12 to provide 5 V dc
at the 5 VD source point at C60, C61. The 5 VA supply is low-current and is fully implemented by U8b.
The VOS power supply, used to power the op-amp rails, is regulated to be approximately 11.2 V dc. The
voltage level of the VOS supply is approximately 1.2 V above the 10 V (nominal) CMOS switch power
supply. The reference for VOS is the 5 VD output. The divided output sample (through R178, R179,
and R180) is compared to the reference at Q14 which serves as the control element for the series pass
transistor Q13.