More of the power you need, less of the stuff you don't
By Jon Chappell
The QSC GX7a power amp is the latest and most powerful of a line that includes the 300W GX3 and 500W GX5. Unlike these two previous models, however, the GX7a is based on QSC's advanced-technology PL amp platform, as found in their PowerLite, and Cinema lines. The technology at the core of the GX7a is QSC's high-frequency switching operation, versus the heavy "copper donut" of the toroidal transformer in the other two models. Consequently, even though the GX7 is more than twice as powerful as the GX3, it's twelve pounds lighter. Beyond its light weight, the GX7a also sports a refreshingly familiar and straightforward front panel: a heavy-duty rocker On/Off switch, two substantial rotary knobs for channel attenuation, and three different-colored LEDs (blue, green, and red) to indicate signal status.
Around back are the input and output jacks for virtually any cable you have in your bag: RCA phono, balanced/unbalanced 1/4" and XLR inputs, outputs with XLR/1/4" Speakon combo connectors, and binding posts that accept both banana as well as bare speaker wire. A single two-position switch activates the 100Hz crossover for subwoofer/satellite operation. Engaging the crossover routes the low frequencies to Channel 1 and the highs to Channel 2, each with separate control on the front panel.
The less the better
To help understand the appeal of the GX7a, it might be helpful to consider QSC's tagline for it: "It's more, it's less." The "more" part is easy: more power—725 watts of it. The "less" is the compelling part. Here, the term refers to the efficient and economical choice and implementation of its features. The GX7a is full of state-of-the-art circuitry and QSC has made best-case choices for the most popularly used and logical configurations. For example, let's take the crossover: The GX7a active crossover allows the amp to be used: in full-range mode or in crossover mode, which splits the signal at 100Hz to power midrange/HF top boxes from one side of the amp and a low-end subwoofer from the other. But instead of offering a bewildering array of crossover-frequency choices—and the accompanying knobs, menus, or selectors that you will have to fumble with—the QSC GX7a opts for a single frequency: the overwhelmingly popular 100Hz. Moving the switch to engage the crossover also takes care of the signal routing for you.
Not your grandfather's clip limiter
The GX7a has a unique system for managing signals that threaten to overdrive the system. GuardRail was developed by QSC as an innovative way to manage signals before they can audibly distort at the output. More than just a simple clip limiter, GuardRail adjusts the signal whenever the output is no longer able to track the input. This doesn't merely flattop the signal (like a typical clip limiter), but takes into account all variables of load impedance, AC voltage, and power-supply dynamics. In typical QSC fashion, occasional lighting of the red LED does not mean waveform clipping. However, if there is prolonged overdrive, a slow-acting limiter gradually reduces volume until the percent of clipping is reasonable, without changing the beat-to-beat dynamics of the music.
GuardRail also acts as a thermal limiter. If you're driving your amp to where the heat is reaching unsafe levels, GuardRail steps in and reduces the gain to protect the amp's components, rather than resorting to the disruptive "thermal muting" of other systems. GuardRail keeps your music coming uninterrupted while protecting both the audio's integrity and the amp's circuitry itself.
Once I had the amp hooked up, it was fast and easy to use. With the crossover set to full-range mode (I wasn't using subwoofers in my tests), everything I needed to know I got from the front-panel controls. The knobs have "notched" (detented) steps, ensuring quick balancing between right and left channels. The blue LED told me the unit was receiving AC power while the green LED indicated signal presence. If the red LED was flickering, it meant only that Guardrail was doing its thing (again, not audio-waveform clipping). Since the red LED glows solid red only when there's significant clipping, I managed to eke out the maximum power through judicious triggering of the red LED. I figured if I wasn't seeing an occasional flash of red, I still had plenty of headroom! Time and again that proved to be true.
I ran the gamut of signals through the GX7a, from my high-res audio (24-bit/96kHz) recordings of full-range orchestral music (Beethoven's Symphony No. 9 and Howard Shore's Lord of the Rings scores) to techno-pop on my Denon DN-S1000 DJ station to a Mackie VLZ-1402 mixer routing a full-band mix 15" two-way cabs. The sound was clear and punchy in all bands of the frequency spectrum, and I had a very, very hard time getting the red LED to flicker even a little bit. The amp faithfully reproduced whatever I threw at it—from modeled guitar sounds (preserving the warmth and character of my carefully dialed in tube saturation) to my best stereo grand piano virtual instrument from my DAW. Imaging was preserved and transient response was excellent. The GX7a took it all in stride, and never once blinked. The only indication I even had of the GX7a's presence was its variable-speed fan operation (it moves a lot of air!).
QSC has redefined power amps by taking them back to the role that nature intended: workhorses whose core business is powering your speakers. Features? There are plenty of them, but they're like trusted audio servants—working behind the scenes, anticipating your needs, and making good decisions. And like the rest of the GX line, the GX7a is backed by QSC's free six-year extended warranty (with product registration). For musicians and DJs who want lightweight power and straightforward operation, the GX7a provides the solution for all your mobile-music needs.
- 8 ohms / 1 kHz (both channels driven) 725 W
- 8 ohms / 1 kHz (single channel driven) 800 W
- 4 ohms / 1 kHz (both channels driven) 1000 W
- 4 ohms / 1 kHz (single channel driven) 1200 W
- Distortion (1 dB below rated power, 1kHz) 8 ohms: less than 0.02%
- 4 ohms: less than 0.05%
- Signal to Noise (20 Hz - 20 kHz): 100 dB
- Input Sensitivity: 1.2 Vrms
- Voltage Gain (8 ohms): 36.1 dB
- Output Circuitry: 2-tier Class H
- Power Requirements (1/8 power, pink noise at 4 ohms, 120V): 10.1A
- Frequency Response 20 Hz - 20 kHz: +0, -1 dB
- Dynamic Headroom (4 ohms): 2 dB
- Input Impedance (ohms): Greater than 20k (balanced)
- Maximum Input Level: +24 dB (16 Vrms)
- Input Connectors (each channel): 3-pin XLR and 1/4" TRS, balanced, parallel
- Phono, unbalanced
- Output Connectors (each channel): Speakon, 1/4", Binding Posts
- Amplifier and Load Protection: Short circuit, open circuit, thermal, RF protection
- Load protected against DC faults
- Front Panel Controls and Indicators: Gain controls, 21 detents
- Red Clip LEDs, proportional, 0.1% THD threshold
- Green Signal LEDs, threshold -35 dB
- Blue Power LED, AC on
- Rear Panel Controls Full Range / Crossover switch
- 100 Hz, 3rd order LP (sub), 2nd order HP (top)
- Height: 3.5" 2RU
- Width: 19"
- Depth: 10.1"
- Weight Net: 15.5 lb.
- Agency Approvals UL, CE, RoHS/WEEE Compliant
To learn more about the QSC GX5 power amp, check out our Hands-On Review here.
To learn more about power amps and live sound gear in general, check out our How to Choose the Right PA System.