Heat Emissions; Protection Circuits - QSC MX 1000a Owner's Manual

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:
Load 1/3 Power, both channels
MODEL
BTU/hr
MX 1000a 8
4
2
MX 1500a 8
4
2
MX 2000a 8
4
2
MX 3000a 8
4
2

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-
12
kcal/hr
1035 265
1430 365
2140 545
1165 295
1875 475
2770 700
1850 470
3100 785
4730 1195
2980 755
7100 1795
10280 2600
1/8 Power, both channels
BTU/hr kcal/hr
580 150
270 915
305 1035
770 195
1010 255
1415 360
1080 275
1755 445
2760 700
2015 510
3600 910
5040 1275
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
Idle, both channels
BTU/hr kcal/hr
75
75
75
110
110
110
145
145
145
200
200
200
20
20
20
28
28
28
40
40
40
50
50
50