Series Description; Technical Descriptions And Theory Of Operation - QSC RMX Series Technical & Service Manual

1.6 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 six two-channel models; this
manual covers the four two-rack-space models: the RMX850,
RMX1450, RMX1850HD and RMX2450. See page 2 for complete

1.7 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 1.9. Note that unlike many
bipolar supplies for complemen-
tary 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 RMX1850HD and
RMX2450, the secondaries
are tapped to provide an
8
+Vcc
-Vcc
+Vcc
-Vcc
Figure 1.9
+110V
-110V
+55V
-55V
Ch. 1 Center Tap
0.047 µF
Ω
12 5W
×2
To Channel 2 Center Tap
Figure 1.10
specifications. The three-rack-space models—the RMX4050HD
and the RMX5050—are covered by another service manual.
The RMX850 and RMX1450 have single-sided printed circuit
boards. The RMX1850HD and RMX2450 use double-sided boards.
intermediate set of bipolar rails for the Class H output circuitry.
Figure 1.10 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 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 1.11) and between the feedback
Channel 1
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 2
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 an NE5517
(equivalent to the more familiar LM13600) dual operational
transconductance amplifier (Figure 1.12). 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 essentially
Channel 1
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
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.
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