Service manual

AMIS120XL
Circuit Description
The ACM120XL is a 120 watt mixer amplifier designed for commercial installations. It features 8
microphone/line input channels and can be used for either low impedance (4 or 8 ohm) or [70v/100v]
(240V version)/[25v/70v] (115V version) line speaker systems. A host of unique features including
multiple levels of muting, a VOX relay output, 24 VDC operation and VCA master level control make the
AMIS120XL a very flexible amplifier. The unit can be mounted in a standard 19” equipment rack (rack
ears supplied) or it can be located on a shelf or table. [The 115V version also has a transformer balanced,
600 ohm telephone input] (115V version).
Power Switch
This switch controls the switching of AC power to the amplifier. A blue ‘On’ LED will indicate whether
the amplifier is switched on or off. This switch will not switch DC power on or off in DC operation. In DC
operation mode, the amplifier is always on and the blue power LED will always be illuminated.
Signal LED
The green “Signal” LED indicates that the AMIS120XL is passing audio. It is connected to the output of
the power amp/primary side output transformer and begins to eliminate dimly at approximately -20dB (ref
100V).
AC Power Inlet
The operating voltage is [230/240 VAC @ 50 Hz] (240V version)/[115 VAC @ 60Hz] (115V version). The
3 pin IEC power inlet is located on the bottom left of the rear panel and accepts a standard mains power
lead fitted with an IEC connector. Before plugging in a power lead, please check the rear panel of the
amplifier to ensure that the voltage switch is set correctly for your part of the world.
The inlet is equipped with an in-built AC fuse holder fitted with a [3.15 amp] (240V version)/[4 amp]
(115V version) fuse plus a spare within the holder. Power consumption is 250 VA.
24 Volt DC Power Inlet
The AMIS250 features optional 24VDC power to run off a battery supply if required. This is connected via
the rear terminal strip. The front panel Power Switch will not switch DC power ‘on’ or ‘off’ in DC
operation. In this mode the amplifier is always ‘on’.
[230V/240V Slide Switch] (240V version)
The operating voltage of the amplifier is user selectable between 230V and 240V via a slide switch located
on the right of the AC inlet. This switch should be set to match the AC voltage of your country. The mains
transformer is wound with a 230V winding plus a 10V winding internally connected.
Power Amp
The power amp is a push pull single supply amplifier driving a centre tapped transformer. The amplifier
has an overall gain of approximately x10 and the transformer has a turns ratio of approximately [x7] (240V
version)/[x5] (115V version). The sensitivity of the amp is approximately 2.7V.
A particularly good aspect of this amplifier is the current limiting circuit. The sensing circuit is a standard
rail load line limiting circuit but it is the drive circuit that is important. As transistors V12 & V14 (BC640)
turn on transistor V11 (BC639), it pulls bias current away from the amp through diodes D1 & D4 (BAV21)
and pulls drive away from the op amp IC1 (LM1458) through the diode/resistor pair D2/R12 (BAV21/2k2)
& D3/R23. Individually each topology acts to limit the current in the amp but it is the combination of the
two and the fine tuning of there interaction that produces the characteristic soft limiting without the harsh
crossover distortion. It is not until the amp is in hard clip does the amp produce the high freq crossover