Class-A in a nutshell?
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bcarso
Well-known member
Stefan, it is less a matter of topology, and more a matter of bias ("quiescent") current relative to signal current. I could take a class B stage and turn it into a class A simply by increasing the current flowing through it at zero signal. The criterion for how much current: such that the current in any active device never go to zero during any part of the anticipated signal.
See as well the recent thread of Samuel Groner's, particularly PRR's remarks.
See as well the recent thread of Samuel Groner's, particularly PRR's remarks.
> take a class B stage and turn it into a class A simply by increasing the current flowing through it at zero signal.
And most push-pull "Class A" stages (if there really are any) will go Class B if you increase the signal and/or decrease the load impedance.
A versus B/AB is mostly advertising bull-plop. Most stages in most amps are simple Class A. When the power output needed gets "big", you want to use Class AB: it is much more efficient, and changes some of the problems of hard-worked Class A stages. Then you cast around for some silly "breakthrough" so you can say "Class A" in the ads.
> can somebody please explain briefly the differences in class-A topology
No.
Not being rude: it can't be explained abstractly or briefly.
And most push-pull "Class A" stages (if there really are any) will go Class B if you increase the signal and/or decrease the load impedance.
A versus B/AB is mostly advertising bull-plop. Most stages in most amps are simple Class A. When the power output needed gets "big", you want to use Class AB: it is much more efficient, and changes some of the problems of hard-worked Class A stages. Then you cast around for some silly "breakthrough" so you can say "Class A" in the ads.
> can somebody please explain briefly the differences in class-A topology
No.
Not being rude: it can't be explained abstractly or briefly.
pstamler
Well-known member
One bit of bumf that needs to be cleared up: recently manufacturers have been using "class-A" as the sole adjective to describe their discrete-transistor products. As in, "We make both tube and Class-A microphones!" Well, the fact is that you can make Class-A amplifying circuits from tubes, bipolar transistors, FETs or any combination of the above; as long as the thing is biased so that the output device(s) is(are) always conducting, at any level the product is likely to be asked to supply, it's class-A. I haven't seen a tubed preamp that *wasn't* run class-A, although I suppose they're out there someplace. And I haven't seen any microphone, anyplace, with an amplifier that was anything but class-A.
The reason manufacturers are fostering this kind of confusion is that they don't want to say "solid-state", and they really don't want to say "transistor". Pfui.
Peace,
Paul
The reason manufacturers are fostering this kind of confusion is that they don't want to say "solid-state", and they really don't want to say "transistor". Pfui.
Peace,
Paul
Thanks a lot for all your input, very appreciated.
I guess I had basically been wrong assuming there are obvious differences in topology between A,B,AB. So according to that looking at a schematic there is no simple way to tell in which class an amp will work (can we say that?) under normal conditions, right?
Stefan
I guess I had basically been wrong assuming there are obvious differences in topology between A,B,AB. So according to that looking at a schematic there is no simple way to tell in which class an amp will work (can we say that?) under normal conditions, right?
Stefan
To add my .001c Re: guitar amps........from the couple I've built....
True class A in the power section (as opposed to claimed) can give varying results depending on the brand of valve even when the same type is used. I believe Jim Marshall once asked players whether they preferred tone or reliability. They asked for reliability. So the power tubes were biased cool.
The change may be desirable or not depending on the the goal. The ears are the best guide....and opinions will vary.
True class A in the power section (as opposed to claimed) can give varying results depending on the brand of valve even when the same type is used. I believe Jim Marshall once asked players whether they preferred tone or reliability. They asked for reliability. So the power tubes were biased cool.
The change may be desirable or not depending on the the goal. The ears are the best guide....and opinions will vary.
[quote author="StefanM"]So according to that looking at a schematic there is no simple way to tell in which class an amp will work (can we say that?) under normal conditions, right? [/quote]
Right. The same amp can be run in both class A and A/B or B. But remember that all single-ended stages (like most discrete preamp stages) are class A.
Best regards,
Mikkel C. Simonsen
Right. The same amp can be run in both class A and A/B or B. But remember that all single-ended stages (like most discrete preamp stages) are class A.
Best regards,
Mikkel C. Simonsen
[quote author="PRR"] [...] and changes some of the problems of hard-worked Class A stages. [/quote]
Are there any other problems than power / heat issues with class A?
Or, putting it the other way round, is there any advantage of class AB over class A, other than wasting less power?
JH.
Are there any other problems than power / heat issues with class A?
Or, putting it the other way round, is there any advantage of class AB over class A, other than wasting less power?
JH.
> I haven't seen any microphone, anyplace, with an amplifier that was anything but class-A.
AKG 414 will enter class B if the signal is high and the load is low.
It has to: it can put close to 20mA into a load, while eating 2mA and using a 1:1 transformer (no transformer in the TL model).
AKG 414 will enter class B if the signal is high and the load is low.
It has to: it can put close to 20mA into a load, while eating 2mA and using a 1:1 transformer (no transformer in the TL model).
