Specifications

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is no longer active.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
muting move the jumper (labelled JP2, located just to the left and behind the level pot for channel 1) to the
middle and left pins. (Factory setting is with VOX enabled with the jumper on the middle and right pins).
Channels 3 & 4 are summed through IC1B (LM1458 or equivalent) to the VCA IC4 (THAT2180C) which
does the muting. The VCA is held on (i.e. no attenuation) by R55 (10k) pulled to the reference voltage and
the reverse biasing of diode D3 (BAV21 or equivalent) by R22 and R21. This is to eliminate any
attenuation by the op amp IC1A (LM1458 or equivalent) and its ½ supply which may be different to the
internal reference voltage of the VCA.
The control pin of the VCA is pulled down by op amp IC1A. This op amp runs at very high gain to activate
on even very small signal. The jumper JP3 shorts out the feedback resistor thus removing the sensitivity.
The attack of the muting circuit is controlled by the charging of C16 (100uF) through R16 (100k) and the
release by the discharge if C16 through D1. Note that VCA will mute at 250mV (-80dB) below the
reference voltage so the muting will occur only over the range of approximately (depends on VCA
production batch) 7.2V to 7.5V. The charging of the capacitor occurs from approximately 1V to 8.1V.
VCA THAT2180C pin out
1 – Input
2 – Ec+
3 – Ec-
4 – Gnd
5 – V-
6 – Ground
7 – V+
8 - Output
Fuse Sizes
(240V version)
Mains AMIS60, 230 VAC: 2 Amperes Slow Blow
Mains AMIS120, 230 VAC: 3.15 Amperes Slow Blow
(115V version)
Mains AMIS60, 115 VAC: 3.15 Amperes Slow Blow
Mains AMIS120, 115 VAC: 4 Amperes Slow Blow