QSC GX3 is it a "good" class B?

randombastage

Experienced
The QSC info claims they have the Class B figured out so there is no crossover distortion.

....


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".

Bottom line question, is this a good amp to use for an AxeFx?
 
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Congrats on your new Ultra...

I have read a few posts where some have liked the results of the GX series and some did not. The crossover distortion that QSC is talking about relates to running a satellight/sub and full bandwidth audio. Unless you plan to run that type of setup with the Axe and mixing with backing tracks it doesn't really apply.
 
You are confusing the two uses for the term crossover. QSC is talking about the zero crossing point in a push-pull output stage.
 
You are confusing the two uses for the term crossover. QSC is talking about the zero crossing point in a push-pull output stage.

That is what I as talking about, is their claim to have class B working without having it slip into class C performance characteristic just marketing hype or can we count on it to deliver? I wonder is it going to be like the Class B amps that you guys warn us to avoid? I brought one home and am trying it with a new cab but have no experience with power amps like this and nothing to compare it with. I will say that so far it is really really great sounding but I haven't had a chance to try loud volume yet and haven't tweaked any presets for it I just threw an FXLoop block on my favorites to send a precab signal out to the QSC and have been through them briefly.

I did spend a bit of time with a Shiva clean preset and the AxeFx has never ever sounded that good to me before and I already considered the Axe as fantastic sounding. It was amazingly alive and responsive in a whole new way through this amp and cab.

I'm hoping this is a good amp but don't have much time to take it back and unfortunately nothing to compare it to except my old Atomic 112 18...which the QSC clearly outperforms but then consider the 18 watts to the 300 and it isn't really a great comparison. Is the high end the place where I would notice the problems or anywhere across the spectrum? What do you listen for if you expect the crossover distortion scenario?
 
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I'm not a fan of amps that have a lot of "closed-loop feedback" but I'm interested to hear what people think of this amp. Typically, lots of negative feedback introduces Transient Intermodulation distortion (TIM) although I haven't done any work in this field for a long time so maybe they have some new approaches.

Whether TIM is audible amplifying guitar is debatable.

I'd certainly take this over a cheap Class-D amp. I've yet to hear a good Class-D amp although they certainly are promising. My old band had a Behringer powered mixer that the bass player insisted on using in smaller venues. It was the nastiest, harshest thing I've ever heard.
 
I'm not a fan of amps that have a lot of "closed-loop feedback" but I'm interested to hear what people think of this amp. Typically, lots of negative feedback introduces Transient Intermodulation distortion (TIM) although I haven't done any work in this field for a long time so maybe they have some new approaches.

Whether TIM is audible amplifying guitar is debatable.

I'd certainly take this over a cheap Class-D amp. I've yet to hear a good Class-D amp although they certainly are promising. My old band had a Behringer powered mixer that the bass player insisted on using in smaller venues. It was the nastiest, harshest thing I've ever heard.

Thanks, I'm not sure what to think right now, it sounds fine to me but I have almost no experience with guitar cabs and the Axe. I have the stock speaker from a Fender HotRod Deluxe that I put in an old Atomic 112-18 as my only reference right now. With that as the benchmark the QSC into a pair of G12T75's in a 2x12 closed back pine/birch cab sound really good...obviously those speakers are better suited to some amp models than others but certainly the mid to high gain Marshall stuff sounds really good and the Shiva clean patch I just made sounds fantastic too.

If the QSC was exhibiting the "crossover distortion" symptoms would I hear it only in the upper range of frequencies or anywhere...and if I'm not pushing the amp/wattage to the upper end of the amps power range would it still be noticeable?

Thanks again!
Craig
 
OK, the QSC has been returned. Too many doubts and after trying it with a few speaker types that I'm more familiar with I think I'm hearing an unmusical kind of high end....maybe it's just my imagination after reading so many bad things about it...either way it's now gone and the search resumes.
I'll probably order a Rocktron Velocity 300 next and see where that takes me, at least it has a better reputation around here and at half the power it puts me in a better position to experiment with available speaker cabs too.
thanks guys!
 
I tried the QSC GX3, Rocktron Velocity 300, and a couple of different studio monitors at the same time. The Rocktron KILLED the QSC in sound quality. I ultimately ended up going with monitors (better suited my needs), however if I ever need a power amp, the Velocity 300 will be on my short list of amps to test out. I think you will be happy with your decision to return the GX3 for the Rocktron.
 
I tried the QSC GX3, Rocktron Velocity 300, and a couple of different studio monitors at the same time. The Rocktron KILLED the QSC in sound quality. I ultimately ended up going with monitors (better suited my needs), however if I ever need a power amp, the Velocity 300 will be on my short list of amps to test out. I think you will be happy with your decision to return the GX3 for the Rocktron.

I have tried a bunch of power amps so far. Mesa 50/50 ( loud and punchy, but made all the amps sound like a mesa ) QSC GX3 ( sounded horrible ), Art SLA2 ( no my cup o' tea .. though it may sound better now since I tried it on version 7 ), Atomic MB50 ( great sounding power amp and my main at this point ). Just recently I bought a Velocity 300 and wow ... I am VERY impressed. It is loud and punchy and the reactance and definition controls really warm it up and give it some nice high end. Not sharp and piercing like the other solid state amps I have tried, it is very "tubeish". Next test will be to try it with the band. I am certain it may end up being my main power amp, especially with such a nice 1U form factor.
 
I'm not a fan of amps that have a lot of "closed-loop feedback" but I'm interested to hear what people think of this amp.
If they've really implemented a full-on Class B output that is free of crossover distortion - and I'm extremely skeptical of this claim - they could not possibly have gotten there with nothing but high negative feedback. If that were possible, the market would have been flooded with Class B designs for the past 30+ years.

Typically, lots of negative feedback introduces Transient Intermodulation distortion (TIM)
It's the combination of negative feedback with inadequate open-loop slew rate that increases TIM. The problem I see with the QSC claims is that, even with 100% negative feedback (unity gain, which would render the amp useless), there will still be crossover distortion, unless there are additional undisclosed design elements present.

FWIW, the Crown D75 and D150 used a Class B + AB topology that biased the output drivers with zero quiescent current and relied on the driver stage, which was biased Class AB, to drive the load for the first few millivolts of voltage swing. It kept quiescent current low - driver bias only - but did not suffer from any crossover distortion.

I've yet to hear a good Class-D amp although they certainly are promising.
There are several amplifiers out there that use B&O's "Icemode" output stage, and they compare favorably with the best class AB designs. They're not cheap, however. With power amps, cost correlates more strongly with performance than does the specific circuit topology.
 
Just a quick update for anyone in the future who may be thinking about this same situation. I got the Rocktrom Velocity 300 and it sounds better than the QSC GX3 and that was before I added about 30% of the "Reactance" adjustment on the Rocktron..once I dialed in some of that magic mojo it became a major improvement over the GX3!

For a few days until the Rocktron arrived I had been using the power amp from an old Atomic 112-18, it is EL34's (and an AX7 I believe) and the Rocktron is better sounding than that as well by a fair amount.

Anyway I'm happy with the Rocktron Velocity 300, it will be staying. I may try to get a hold of the Randall Rt2 50 but truthfully at over $1000 it no longer is a priority now that the Rocktron ($369) is in the house.
Thanks to all who steered me in this direction!
 
I used to have one of those GX3 amps - we used it for our rehearsal PA. When we switched to using our live power amp (QSC PLX1804) - just that switch was an immediate improvement in sound quality. It's been a while since we made the switch so I'd have a hard time being too specific. The things that were most noticeable is that the high end was a lot easier to take (less ear fatigue - the GX3 was a bit harsh sometimes) and the PLX is quieter.
 
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