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.

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.

• 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).

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 where channel two's gain control is.

In the off chance your product should need repair, QSC provides a 3 year warranty from the date of purchase. Additional warranty coverage for 6 years is available within the first three years of the original purchase. This extended warranty option converts your 3 year limited warranty into a 6 year full warranty. These warranties are fully transferrable.

We recommend that the gain controls be set between half-way and fully up. The input sensitivity of QSC amplifiers is at about 0dBV or (1Vrms). Amplifier gain controls set at a lower position require input signals to be set to a higher level to obtain suitable power levels. There are other noise and gain alignment considerations. Particularly with unbalanced input lines, the hotter your signal is at the input of an amplifier, the more noise propogation you will have into your amplifiers. Also, the gain structure of your system may become such that you will reach the maximum gain travel of a fader, at your source device, before obtaining expected power within your amplifiers.

Since input circuits respond to the difference between the plus and minus signals, if only an unbalanced (single-ended) signal is available, the unused input terminal need only be grounded for normal operation, without loss of gain. The ability to reject cable induced hum and noise is lost, but this may not be needed in well shielded environments with short distances between audio components.

Normally, an amplifier is expected to reproduce input signals in the same polarity, which is called the non-inverting mode, so that a drum beat, say, pushes the speaker out instead of in. However, if there is any leakage from high-power (speaker) circuits to the amplifier inputs, the amplifier is much more stable in the inverting mode, since the leakage then tends to add to the negative or stabilizing feedback. Using an inverting mode power amp is the opposite of normal practice, but we have observed many cases where "mystery problems" occurring in wide band amplifiers used in the non inverting mode are solved by switching to the inverting mode. Balanced line operation corrects this problem without regard to polarity, but is not always available.

You can always reverse the red to black polarity to all the speakers to restore correct polarity even when using the "more stable" inverting mode. In any case, be sure to use the same polarity for all of the speakers so they work together.

Actually we are describing two different aspects of amplifier output stages.

• IDLE CURRENT: Class AB refers to the amount of idle current flowing in the outputs at zero output. Amplifiers have positive and negative output transistors which handle their respective halves of the output signal. They must "hand off" the output current to each other as the signal passes through zero. A "Class A" output stage begins to transfer current well above its "cutoff point", resulting in much current overlap. This eliminates any chance of "crossover distortion" but generates tremendous waste heat at idle, limiting the possible power of the amp. A "Class B" output stage attempts to make the transfer at exactly zero current, which is impossible to maintain perfectly and leads to "zero crossing distortion" (more commonly called "crossover distortion", a buzzy form of distortion most audible at very low levels). Class AB is the practical compromise--just enough idle current to ensure a smooth transfer between the positive and negative output transistors, without a wastefully high idle current.
• POWER SUPPLY DESIGN. The other major source of waste heat, even in a class AB design, occurs at moderately high output powers. The output transistors drive the speakers by coupling a precise amount of audio voltage from the amplifier's "power supply", which is a steady reservoir of fixed voltage. Most of the time, the output transistors are called on to only deliver a fraction of the power supply voltage to the load, and the unused fraction is consumed as heat in the output devices. We can reduce the losses by providing two or more"tiers" of DC voltage, with "steering circuits" which draw from the lowest possible voltage supply. This way the waste heat in the outputs is reduced. A "Class G" design does this by using two different sets of output transistors, one coupled to the lower voltage and one to the full voltage. The signal transfers from the low to high voltage set as required. A "Class H" design uses additional circuitry to connect a single set of outputs to lower or higher voltage as required. Both approaches are capable of good results; the Class H can be designed for somewhat lower costs, especially in amps with more than two power supply "tiers".

QSC is always looking for ways to make amps smaller, lighter, and more affordable. "Surface Mount Technology" is a new miniaturized method of placing parts on circuit boards without the usual wire leads. SMT parts have small metal bumps which are soldered to pads on the top of the board. This saves 50-75% of the space consumed by "thru-hole" components using wire leads, and eliminates the extra cost of attaching the leads to the parts.

Air filters quickly clog with dust, blocking air flow and causing overheating. Although some dust collects internally without air filters, the spacing of the heat sink fins is much greater than the fibers in the air filter and much of the dust blows on through. The internal air flow pattern is designed to avoid dust buildup in critical places. Occasional cleaning is recommended in very dusty environments, but our experience is that "dust tolerant" designs cause much less trouble for customers than air filters.

In theory, yes, but only by changing certain resistor values, which requires opening the covers and exposure to dangerous voltages. This should be done only by qualified service personnel, or under the guidance of QSC Technical Services.

Yes, certain amplifier models have completely independent muting circuits for each channel. It is normal for the turn-on delay to vary slightly.

The MX3000a uses critical sub-sections of the EX-4000 to deliver similar output power, but we reduce cost by eliminating features of the EX series which are only required by certain users.

During the muting cycle, the "front end" of the amplifier is internally disconnected from the output transistors, ensuring that they are fully turned off in the event of overheating or during routine on/off muting. When disconnected from the feedback loop, the front end circuits may have enough stray signal to flash the Clip LED's. This is perfectly normal, and will stop as soon as the amp enters the normal "run" state.

Almost all QSC amplifiers have 2-speed or variable speed fans that operate at low speed immediately upon power up. Two-speed fan designs reach the highest speed once a certain temperature is reached. Variable speed fans change rotation speed as amplifier operating temperatures change. The exceptions to this are the USA400 / Series One 1200 which are convection cooled amps. EX 2-space fans do not turn until a particular temperature is reached and from that point the fan speed varies with temperature.

The minimum rated impedance for an amplifier in bridge-mono is 8 ohms. A 4 ohm load is possible as this represents a 2 ohm per channel equivalent. Two ohm load precautions still apply, as it would with any amplifier, so supplemental cooling may be found necessary. It will also be important to watch for any impedance transients that arise from the speakers in use. Four ohm bridge-mono loading is the absolute minimum across the entire audio range (20Hz - 20kHz.)

Yes, the LED is driven only when the amplifier output fails to track the input. This condition normally results only from clipping, which occurs when the power amp reaches either its voltage or current limit. The LED begins to become visible at 0.1% distortion, and reaches fairly full brightness at 1-10% distortion, which is clearly audible. Therefore the brightness corresponds to the likeliness of hearing the distortion.

Small amounts of "DC offset" do not indicate a problem, although normally it should be less than 0.05 volts (50mv). At 0.3Vdc, we are dissipating 0.011 watts (11 mw) in an 8 ohm load which is clearly negligible. The 1400/USA 850/USA 900 models may measure several volts on the output if not loaded, but the voltage will promptly settle to zero if a normal load is connected.

The fastest way to tell if the amplifier is the cause of the noise, is to disconnect the input cables from the amp. If the noise is still there, it may be amplifier; if it's gone, it's a source device inducing noise into the amplifier. If after removing the input connectors from the amplifier you find the noise still present, it will then be necessary to determine if the noise is coming from the AC line. This further isolation may be helpful. Try relocating the amplifier using a different AC service, if the same level of noise is present, the amplifier is likely to be the cause. If the noise is lower, the AC service may be the cause.

Yes. If the amplifier was built before 1988, the first three letters of the serial number will tell you it's assembly date. Example: 047 = April 1987. If the amplifier was built after 1987, the first four letters of the serial number will tell you it's assembly date. Example: 0794 = July 1994.