QSC RMX Series Technical & Service Manual page 12

DC protection
The RMX1850HD and RMX2450 have a crowbar circuit, based on a
triac and two silicon controlled rectifiers, on the output to protect
against DC faults. If an amp channel puts out a DC voltage, which
could be the result of a component or circuit failure, it will first
trigger either D119 or D120, depending on the polarity of the
voltage. The triggered SCR will in turn trigger triac Q113, shorting
the output to ground through fuse F100. The fuse will blow,
safeguarding the speaker load from the DC fault.
The output sections of the RMX850 and RMX1450 are AC coupled.
Class H
The RMX1850HD and RMX2450 utilize a two-step
Class H output section. It is essentially a Class AB+B
circuit but with two sets of bipolar supply rails. On
both the positive and the negative sets of rails, a
comparator circuit, called a "step driver," compares
the audio signal to the lower rail voltage. When
necessary to fully reproduce the signal's voltage
swing—just before the instantaneous signal voltage
reaches the lower rail voltage—the step driver turns
on a TMOS power FET to pull the output transistors'
supply rail up from the lower voltage to the higher
one, and then back down again when the signal
allows. By keeping the transistors' supply rails low
whenever possible, the devices dissipate less unused
power and generate less waste heat, making the
amplifier more efficient than a straight class AB
amplifier with the same power points (Figure 1.13).
The comparators are 311-type ICs: U170 on the
positive step and U171 on the negative. Each one
drives a high-gain complementary transistor pair
(2N3904 + 2N3906), which drive the gate of their
respective MOSFET.
Bridged mono operation and protection
When the amplifier is operated in bridged mono, its two channels
work in tandem to produce up to twice the voltage swing that a
single channel is capable of. To do this, Channel 2 produces a
signal identical to Channel 1's, but opposite in polarity—in other
words, a mirror image.
Channel 2's signal feed (bus BR_MONO_FEED) is an attenuated
version of the signal on Channel 1's speaker bus. Closing DIP
switch #6 (set to "BRIDGE MONO ON"), connects the
BR_MONO_FEED bus on Channel 1 to the BR_RET bus on Channel
2. The BR_RET bus drives the non-inverting input of op amp
U201:2 directly.
With two channels operating as one, but each having its own
feedback and protection circuitry, it is vital to keep both running as
mirror images. A protection circuit monitors the balance between
Channel 1's and Channel 2's signals. Resistors R22 and R23 (R22A,
R22B, R23A, and R23B on the RMX2450) are equal in value and
10
form a voltage divider between the two channel outputs. If the
output signals are mirror images, the voltage at the junction of the
resistors (bus BR_BAL) will be zero. If the signals are not mirror
images—for example, one channel is defunct, distorting, or
reduced in gain—a voltage will appear on BR_BAL. Through DIP
switch 7, the BR_BAL bus becomes bus BR_CUT and feeds the
bases of transistors Q8 and Q6, which are part of a 4-transistor
circuit across the +15V and -15V rails that supply the op amps and
the input circuitry. If the voltage on BR_CUT goes positive enough
to forward-bias Q8, the transistor's collector will collapse the +15V
rail. At the same time, the emitter current from Q8 will flow
+V
HI
+V
LO
Power saved Power saved
0
-V
LO
-V
HI
Figure 1.13. A two-tier class H amplifier circuit operates on the lower voltage
rails until the signal amplitude requires the higher voltage.
through R25 and into the emitter of Q7, forward-biasing it, too.
The collector of Q7 will then collapse the -15V rail.
Similarly, if BR_BAL goes sufficiently negative, it will forward-bias
Q6, in turn forward-biasing Q9, and these will collapse the ±15V
rails.
With the rails collapsed, the op amp and the input circuitry will not
function, which will mute the audio.
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