From the QSC Site
POWER AMPLIFIERS
Is it ok to run only one channel on my amplifier and can I mismatch loads?
You may do both. Your amplifier can be loaded with a very wide range of speaker impedances. The minimum impedance is 2 ohms for stereo operation while it is 4 ohms for bridge mono operation. All QSC amplifiers are designed to operate safely into infinite load impedances (no speakers). Loading arrangements from one channel to the next can vary in any way you wish. In fact, you can load one channel with a distributed voltage load, using an output transformer, and use the other channel to drive a direct low impedance load.
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What is the difference between parallel and bridged mono modes?
The "Parallel" setting duplicates the effect of cross-patching the two inputs, using an internal switch to save the hassle of an extra cable. The two input jacks are connected together, so that a signal on either jack now appears in BOTH channels (therefore only a single signal should be connected). Each channel's Gain control regulates its volume as usual, and separate speakers are connected to each channel as usual. When using this mode, DO NOT combine the output of both channels into a single speaker; this is likely to damage the amp. The Parallel mode is useful when you want to drive several speakers with the same signal, but with two separate volume controls. Bridged Mono mode reverses the polarity of the signal going to Channel B, and matches its gain to Channel A. This makes the voltage between the two red speaker outputs DOUBLE the usual value, which allows several times the normal power to be delivered to a single speaker. This also increases the stress on the amp; this is not "something for nothing" but is a way to combine the 4-ohm ratings of both channels into a single 8-ohm load, or the combined 2-ohm ratings into a 4-ohm load. The Bridged mono setting is useful when you want to deliver the entire power of the amp to a single 8 or 4 ohm speaker.
The proper position of the Gain control varies by model:
* Series One and USA series: Control the gain with Ch A, set Ch B to zero (the setting doesn't matter if there is no stray input to Ch B).
* First-generation MX products: match gains of both channels.
* EX and MX-a products: Control the gain with Ch A, set Ch B Gain on FULL.
* PowerLight and CX products: all control is automatically handled by Ch A's Gain, and Ch B's Gain is
disconnected. (In case of doubt it can be matched to A, but if moving it has no effect on the sound it can be set to zero).
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How do I set my amplifier into the Bridge-Mono mode?
This depends on the amplifier you have. Generally, you will first turn the power off, set the bridge mono switch to the bridge position and attach your speaker wires onto the two red binding posts of each channel. It is then necessary to apply your input signal to channel one on your amplifier and adjust the gain as needed. For some amplifiers, the gain of channel 2 must be turned all the way down while others are required to be all the way up. Furthermore, with other QSC amplifiers, it does not matter where channel two's gain control is.
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