AUSTRALIAN MONITOR AMIS120 Series, Service Manual, Page: 6 / 26

level set on the master (front panel) control. Connection is via a 2 wire terminal strip on the rear panel of
the amplifier. Shorting this input will mute the amp.
The VCA IC has a Voltage/Gain ratio of approximately -3mV/dB. That is a voltage difference of
approximately 300mV below the reference voltage (pin3, ~7.5V) will provide full mute (-80dB). The need
to have the remote pot on two wire connection means that there is a small amount of attenuation (approx
1dB) when the pot is connected. For the best audio control a log pot should be used. The control circuitry is
a DC attenuator and an inverter. When a pot is connected, the current through R64 (10k) and the pot
resistance generated from the 1/2Vcc bias voltage flows through R76 (330R). This generates a voltage
increase on the output if op amp IC7B (LM1458 or equivalent) above the reference voltage (1/2Vcc). The
second op amp IC7A (LM1458 or equivalent) inverts this voltage to a voltage difference below the
reference voltage. Thus if the pot is turned to S/C the voltage difference below the reference voltage is:
7.5(V) / 10(kohm) * 330(ohm) = 248(mV) = 248(mV) / -3(mV/dB) = -82(dB)
From this you can see that when a 500k pot is attached the initial attenuation is:
7.5(V) / 510(kohm) * 330(ohm) = 4.85(mV) = 4.85(mV) / -3(mV/dB) = -1.62(dB)
These calculations are only approximate and are used to show operation of the circuit.
Tone Generators
Four separate tones are available from the in-built tone generator board. All four tones can be activated
individually via a contact closure connected to the screw terminals on the rear of the amplifier. When any
tone is activated, all inputs (except for inputs 1 and 2) will automatically mute. The level of the tone
generator is controlled by the pot labelled R6 (located behind the Bass adjustment pot). This pot adjusts the
level for all 4 tones.
Tones available on the AMIS60 and AMIS120 include:
- Evacuation Tone
- Alert Tone
- Bell Tone
- Pre-Announce Chime
Theses inputs are pulled up to 5VDC internally through 1k resistors.
The tones are generated through a combination of digital frequency modulation and analogue amplitude
modulation. The output of the microcontroller IC13 (PIC16C54A) on pin 17 is a 0-5V square wave of
varying frequency (depending on the tone selected). This signal is amplitude modulated using a VCA IC8
(M5222P). The envelope is controlled by the charging and discharging of the electrolytic capacitor C47
with the sink and sourcing of current on the microcontroller outputs pins 1, 2 & 13.
Page Chime Enable Button
This allows the user to disable or enable the Pre-Announce Chime. This switch does not activate the chime.
Activation is accomplished via the rear panel barrier strip (normally wired to a paging microphone). If the
pre-announce chime is connected to a paging microphone (meaning that it would be activated by the switch
on the microphone), the Page Chime Enable switch on the front panel of the amplifier allows the user to
disable the chime function on those occasions when it is not require.
VOX Muting
This feature provides automatic muting channels 3 & 4 when signal is applied to either channels 1 or 2. It is
normally used so that a paging microphone can have priority (by muting) over background music. The
muted channels will automatically ramp back up to normal volume when the signal on channels 1 and/or 2
is no longer active. The amplifier ships with the VOX muting function enabled. To disable the VOX