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1.4 Series description
QSC's RMX Series amplifiers are entry-level professional audio
products, designed for good, basic performance and reliability at
low price. The series comprises three models: the RMX 850, RMX
1450, and RMX 2450. Each one has two audio channels and is two
rack spaces tall. See page 2 for complete specifications.
The RMX 850 and RMX 1450 have single-sided printed circuit
boards. The RMX 2450 uses double-sided boards.
1.5 Technical descriptions and
theory of operation
Note: Some of these descriptions concern circuitry that is duplicated
in the amplifier's two channels. For the sake of simplicity, the
descriptions are of Channel 1 only. Components in Channel 1 have a
3-digit designation with "1" as
the first digit; their equivalents in
Channel 2 have a "2" as the first
digit, followed by the same two
numerals. For example, R122 and
R222 have identical functions in
their respective channels.
Power supplies
Unlike other recent QSC amplifiers,
the RMX line uses strictly conventional power supplies, with large
transformers that operate at the 50 or 60 Hz frequency of the AC
line. The electrical current in the secondary circuitry is converted to
DC through a full-wave bridge rectifier. The resulting 100 or 120 Hz
ripple is filtered out by large capacitors that also serve as current
reservoirs for short-term, transient demands.
The supply provides a bipolar set of supply rails for each channel, with
equal quiescent positive and negative voltages, as shown in Figure 1.7.
Note that unlike many bipolar supplies for complementary transistor
arrangements, the secondary windings are not connected to ground at
the center. This is because the output transistors are directly mounted
to the heat sink, metal-to-metal, to maximize heat transfer; this grounds
the collectors, requiring somewhat different output and power supply
arrangements. The grounded-collector concept is described later in
this chapter.
In the RMX 2450, the
secondaries are tapped to
provide an intermediate set
of bipolar rails for the Class H
output circuitry. Figure 1.8
shows one channel. Class H
operation is described later
in this chapter.
The 24-volt cooling fan is
driven by a separate DC
supply that is powered by a
6
+Vcc
-Vcc
+Vcc
-Vcc
Figure 1.7
+110V
-110V
+55V
-55V
Ch. 1 Center Tap
0.047 µF
Ω
12 5W
×2
To Channel 2 Center Tap
Figure 1.8
20-volt tap on the transformer primary. To minimize fan noise, the
fan speed is controlled by varying its actual DC voltage in response
to the amplifier's heat sink temperatures. An optocoupler isolates
the fan control circuitry from the thermal sensors.
Audio circuitry
The audio inputs are balanced to offer a reasonably high amount of
common-mode noise rejection. The input balancing is done using a
single op amp (one half of an NE5532 dual op amp) arranged as a
differential amplifier. The degree of common-mode rejection is
dependent on a close match between the input resistors (R100 and
R101 in Figure 1.9) and between the feedback resistor and the shunt
resistor (R105 and R106). The circuitry uses 1% precision resistors
to ensure at least 40 dB of common-mode rejection.
Channel 1
R100
+IN_A
10.0K
^R_0805
Channel 2
R101
-IN_A
10.0K
^R_0805
To LM13600 operational
transconductance amp
Figure 1.9
The feedback and shunt capacitors, C101 and C103, add a first-order
high-frequency roll-off, down 3 dB at 88.4 kHz (over two octaves
above the high end of the audio spectrum). This makes the amplifier
less susceptible to RF interference, high-frequency oscillations, etc.
Also in this stage, the feedback loop contains one half of a 13600 dual
operational transconductance amplifier (Figure 1.10). The OTA is part
of the clip limiter circuitry; when the clip limiter is activated, a control
voltage increases the transconductance of the OTA, which essen-
tially decreases the impedance of the feedback loop and reduces
the gain of the stage in order to reduce the amount of clipping.
The gain control uses a linear potentiometer, but the impedances
loading the wiper to ground make the pot approximate an audio
Channel 1
taper over most of its rotation. After the wiper, RC networks roll off
the low end, if the LF filter is set for that channel, at either 30 or
50 Hz, depending on the DIP switch setting on the rear panel.
The next active device is another 5532 op amp, U101:2. Its output
drives the driver transistors, which in turn drive the output transistors.
The output section has a Class AB+B configuration; the drivers (a
complementary pair, Q105 and Q106, comprising an NPN MJE15032
and a PNP MJE 15033) are class AB. A series network of two diodes
and a 100-ohm trimpot provide the small amount of forward bias on
C101
180p-5%
^C_0805
R105
10.0K
A1
^R_0805
NE5532
U101:1
R102
C106
+
3
1K
1
^R_0805
10-50NP
-
2
R106
10.0K
^R_0805
C103
180p-5%
QSC Audio Products, Inc.
R113
W
R112
270
10K
^R_0805
A1
10K LINEAR 3B
RIGHT ANGLE POT
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