| qsc GX3... [message #323380] |
Fri, 16 May 2008 10:20  |
 Jess Bruffett
Messages: 299
Registered: July 2006
Location: Claremore, Oklahoma |
Has No Life |
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its a class b amp??? i thought they stoped using class b amps because they recombining to the upper and lower waves causes major distortion. so why are they using it now? or is it a miss print?
Awaken the Dead Productions
http://www.myspace.com/awakenthedeadproductions
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| Re: qsc GX3... [message #323393 is a reply to message #323380 ] |
Fri, 16 May 2008 10:47  |
Mike {AB} Butler
Messages: 1796
Registered: April 2004
Location: Lynchburg, VA |
Has No Life |
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Did you read Pat Quilter's comment on the GX3? It looks like they were able to make a class B that "just works".. (how and whatever they did)
http://www.qscaudio.com/products/amps/gx/gx_faq.htm#q8
(Reprinted from QSC Website)
"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".
- Pat Quilter "
Looks interesting. I know in my limited experience in doing a few class A and and AB designs from scratch, and even in seting up many a kit amplifier, it's sometimes tricky to get the bias right, and disastrous when done wrong - if thermal runaway occurs. Class B gets rid of that quiescent current issue.. but you have to do that crossover point perfectly.
That's my limit of expertise, anyway..
Regards,
Mike Butler
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