QSC Audio MX 1000a - Manuel (Page 12)

12
HEAT EMISSIONS
Essentially, a power amplifier draws electrical energy
from the AC mains, converts it to DC, and then converts
it again into an analog of the input signal to send out to the
loudspeakers. Any AC power that enters the amplifier
through the power cord and does not exit through the
speaker outputs turns into heat, which the amplifier must
rid itself of by exhausting it to the outside. In indoor use
this may present a sizeable challenge to a building’s air
conditioning system. Use Table 3 to predict the heat that
will be emitted by your amplifier.
Table 3: Heat emissions of MXa amplifiers under various conditions:
MX 1000a
MX 1500a
MX 2000a
MX 3000a
8
Ω
4
Ω
2
Ω
1035
1430
2140
265
365
545
580
270
305
150
915
1035
8
Ω
4
Ω
2
Ω
1165
1875
2770
295
475
700
770
1010
1415
195
255
360
8
Ω
4
Ω
2
Ω
1850
3100
4730
470
785
1195
1080
1755
2760
275
445
700
8
Ω
4
Ω
2
Ω
2980
7100
10280
755
1795
2600
2015
3600
5040
510
910
1275
75
75
75
20
20
20
110
110
110
28
28
28
145
145
145
40
40
40
200
200
200
50
50
50
Load 1/3 Power, both channels
BTU/hr kcal/hr BTU/hr kcal/hr BTU/hr kcal/hr
1/8 Power, both channels Idle, both channels
MODEL
PROTECTION CIRCUITS
The design goal in high-efficiency, lighter-weight ampli-
fiers such as the MXa Series is to control more power with
fewer or smaller load-bearing components, e.g., resis-
tors, transistors, transformers, etc. However, the higher
power flow through these components makes effective,
responsive protection circuitry absolutely vital. To this
end, the design of the MXa series takes a comprehensive
approach to protection.
As in all QSC amplifiers, the inputs are resistively buff-
ered for overload and RF protection. Chassis bypass
capacitors at inputs and outputs further improve RF
rejection.
MXa amplifiers use the proven Output Averaging™ short
circuit protection system. This circuit permits full output
current even into resistive or reactive 2-ohm loads, but
reduces the current safely by about 75% if the output is
shorted.
Turn-on/turn-off muting keeps transients—both from the
amplifier itself and from upstream equipment—from reach-
ing the speakers when the amplifier is turned on or off.
The turn-on delay is approximately three seconds to
allow the power supplies and circuitry to stabilize. Turn-
off muting occurs almost immediately after power is shut
off. Muting occurs whether power is turned on and off
using the front panel power switch or externally at the AC
source.
An NTC (negative temperature coefficient) thermistor in
series with the power switch and transformer primary
limits inrush current. The thermistor initially has a high
resistance, which then diminishes rapidly as it warms, to
avoid power loss. Typically, the inrush current of an MXa
Series amplifier thus is equal to that of another amplifier
of about 1/3 to 1/2 its power rating.
When the DC fault protection circuitry senses a DC
voltage on the amplifier output, it activates a relay which
shorts the output and load to ground. If the DC is due to
a fault in the amplifier channel’s output circuitry, it will
probably produce enough current flow to blow the fault
fuse in series with the output. This will disable the channel