GX SeriesProfessional Power Amplifiers
Frequently Asked Questions
Over the forty years that QSC has been in business, the most common question end-users ask is "which amp is right for my speakers?" We wanted to make the GX as easy to choose as it is to use. Most loudspeaker manufacturers recommend an amplifier with power output equal to the speaker's "Program (Music) Power" rating or two times (2 x) the "Continuous (RMS) Power" rating. We're just cutting to the chase and giving you the result of this calculation. That said, an amplifier's true measured power output at the onset of clipping is important. This data can be found in the GX Series Specifications , but to match the right amp to speakers requires a little math.
Why the emphasis on loudspeaker power rating instead of amplifier power rating with the GX?
Use our amplifier selector tool to match the right amplifier to your loudspeakers.
Can GX Series amplifiers power 2 ohm loads?The GX Series amplifiers were designed to power 8 or 4 ohm loudspeakers. QSC does not recommend driving loads lower than 4 ohms. That capability is present in the RMX Series and PLX2 Series professional power amplifiers and will be more suitable for applications that require 2 ohm operation.
Isn't 2 ohm capability important?
Two ohm capability is important if you are going to drive two ohm loads. It's like four wheel drive on an SUV. If you need it, it's indispensable. If you don't, it's just something that adds weight, complexity, inefficiency and expense while providing no added benefit.
How can the GX Series amplifiers cost so much less than the RMX models that have lower power output?QSC's designers, headed by Pat Quilter have applied 40 years of amplifier design expertise to the GX Series to optimize output and appearance while reducing raw material cost and weight. While less skilled designers will attempt to solve problems by adding circuitry and complexity, the QSC engineering team has developed numerous techniques that solve these same problems by actually reducing complexity. The result is an ‘elegant’ design that does more with less. Also, by removing bridge mono mode and 2 ohm operation - features that are never used by the majority of amplifier users, the price and weight can be lowered further while still allowing the amp to operate effortlessly into 8 and 4 ohm loads with ample headroom.
What was behind QSC's decision to design the GX Series?During the design process, we took surveys of entertainers, musicians and DJs to find out how they used their gear. What we found was that most needed to power 8 or 4 ohms so we designed the GX series to work best with those loudspeakers and also included features that would be most useful to this group. The GX Series may not fit all applications, but for the majority of entertainers and DJs, the GX Series will fit perfectly into their sound reinforcement system and budget and deliver QSC quality and reliability.
What is GuardRail? Is it just a fancy name for a clip limiter?GuardRail is a combination of two protective systems, clip limiting and thermal limiting. QSC recognizes that many of our users want to squeeze the last drop of dynamic range from their amplifiers, and the reality is that this means the amps will sometimes be pushed to clipping. Just as a drag race wouldn't be the same without the roaring engines and screeching tires, high energy music isn't the same without some evidence that the gear is giving its maximum peak performance. Therefore, QSC's Guardrail is not intended to prevent all clipping, just as you wouldn't want a race car to stop short of full power. However, we don't want our users "spinning out" or damaging their gear, so some kind of backstop is desirable - a guardrail, if you please. The GX measures the onset of clipping, using the activity of the Clip LED, which is driven whenever the global feedback loop detects that the output is no longer able to track the input. This method automatically takes into account all variables of load impedance, AC voltage, and power supply dynamics. Mild triggering of the Clip LED does not trigger clip limiting. Prolonged, heavy clipping causes a "slow" limiter to gradually reduce volume until the percent of clipping is reasonable. This method does not change the beat-to-beat dynamics, and allows for momentary clipping, but intervenes as gently as possible if gross overload is detected.
Similarly, if the amplifier overheats, the gain will be gradually reduced to prevent the need for thermal muting. The combination of these two features keeps the show "on the road" without limiting the dynamics of the performance any more than absolutely necessary. Users are encouraged to monitor the Clip LED to determine if they are overdriving the amp, but we recognize that during a show, your attention is most likely to be focused on giving the best possible performance, and the amplifier should basically take care of itself.
The GX3 is a class B amplifier, how does this differ from the more common class AB?The GX3 is a Class B design, as distinct from Class AB. True Class B requires that the positive output stage turn off exactly when the negative stage turns on, and vice versa. If the positive and negative stages overlap, you have Class AB operation, which ensures against any discontinuities, but results in an idle current which creates losses and needs careful bias tracking circuitry. If you have a gap between positive and negative actuation, you actually have "Class C" operation, which is indeed unsuitable for audio due to crossover distortion. Many actual attempts at Class B design end up with Class C results, due to poor understanding of on-off delays, feedback stabilization, and lack of loop gain.
In effect, Class B is the exact dividing line between Class C (no idle current, but distorted), and Class AB (idle current assures continuity, at the expense of extra heat). To optimize efficiency and power in a compact chassis, and eliminate problems of bias tracking, the GX series is set up for zero idle current. Careful design techniques and high closed-loop feedback ensure that there is no appreciable crossover gap, qualifying us to claim "true Class B" as an indicator that we combine maximum efficiency with no perceptible distortion. To put this in practical terms, the GX Class B design achieves less crossover distortion than many imitators of our Class AB designs.
Bottom line - the GX is "Class B done right".
The GX5 and GX7 are specified as a Class H amplifiers, what does this mean?In a Class H system, the output devices are normally powered from a low power rail but shift to a high power rail in less than a millionth of a second when the program material demands higher output. This generates less loss and allows the design to output more power in a smaller and lighter chassis. The GX5 and GX7 are 2-tier Class H designs where the output devices operate as Class B within each tier.
With the rear panel crossover switch enabled, a full-range signal on the channel 1 input will send 20 Hz - 100 Hz to the subwoofer connected to the channel 1 output and 100 Hz - 20 kHz to the full-range satellite speaker connected to the channel 2 output. The front panel gain controls balance the sub and top box, while GuardRail optimizes peak power into the sub and top individually, for greater musical impact and dynamic range.
For stereo operation you would of course need two GX amplifiers, but this configuration has some distinct advantages over using one amp for tops and one for subs. Input connections are simplified as you only need to send one cable to each amp from your board. And by driving only one sub per amplifier, when a heavy low frequency section hits, the amplifier's power supply can use the excess headroom from the relatively lighter loaded full range channel.
What are all the input and output connection choices for the GX Series?
On the input side, the GX Series amplifiers feature balanced XLR and 1/4" TRS in addition to RCA phono connectors to cover just about any professional or consumer audio source. On the output side, Speakon® combo connectors accept 1/4" TS plugs or Speakon 2-pole and 4-pole (however only 2 poles are used) plugs. There are also binding posts to cover all other speaker wiring configurations.