It's the same. Just marketing.
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Depending on the design details, an amp labeled "Class A" may under some extreme circumstances move to Class AB operation. These conditions usually involve load and power demand.
One way to look at it is that in a push-pull power stage the higher the bias (within limits of course) the longer the stage stays in Class A operation before moving into Class AB. Class amps run with very high bias to stay in Class A under most operating conditions.
Pure Class A is the limiting case of this situation. The stage is designed and biased so that it will not move out of Class A operation under any normal operating conditions.
Note that be definition all single ended amp stages are Pure Class A. If the stage move out of Class A it clips.
In practice the difference is mostly marketing.
I am with Ghostrider...I suspect "Pure Class A" means it does not flip to Class B when driven at higher output - so a "Pure Class A" will clip more easily, which limits dynamic range (usually a bass problem as bass needs oodles of power).
I assume people understand that the whole point of Class A is no crossover - so no risk of non-linear distortion as the amp goes outside its linear range and a precise transition is not needed between a complementary pair.
Class A is most important/beneficial in tube designs as tubes age more than transistors and are therefore harder to maintain biased precisely but it can also be beneficial in transistor designs too. All Class A designs have a drawback in that they may not last as long as they always work so much harder and run hotter, furthermore, many types of transitors conduct more as they heat up and therefore the situation can worsen and cause failure once they begin to get too hot on a hot summer day, for example (although designers often include temperature shut down circuitry to try and save the amp).
Real world example:
My active speakers are Class A until two thirds power and after which they are supplemented by Class B (it may help to think of the simplest configuration being a complementary pair of transistors at the ouput end whilst remaining pure Class A in prior stages). This gives the best of both worlds ....Class A for 99% of the time and Class B assistance only occasionally and at the final stage (for example, the 1 % of the time that you want to ensure that the whole street is up and out of bed!) Unfortunately, 280 Watts of Class A per speaker channel does get a little hot in my case. And as would be expected, at idle the system draws about 3 amps from AC mains (as the amp is always conducting 50% power, even at idle = lots of waste power). So Class AB power amps that operate mainly in Class A are designs that you really don't want to leave on all week (unless you want to heat the home with a very expensive form of heater)! Alternatively at full power (playing music loud), there is remarkably very little difference in current drawn from mains (typically an extra 0.1 to 0.2 amps).
I hope this little non-technical explanation and real life example helps...
Dear Paulo: Till today I never " see " amplifiers ( SS, tube or hibryd. ) that through its operation manuals or through advertasing states: class A that switch to class B.
All the ones that I know that states class A or pure class A are the same: 100% class A operation.
Regards and enjoy the music.
Dear Aldavis: Could you name some amplifiers ( SS ) that in its advertasing/operation manual speak about class A operation that are not really class A ( that switch to class AB?
The amplifiers that switch to class AB are clearly named: class AB, I never read that the manufacturer state class A when they really are class AB.
Now if a manufacturer states class A when in reality is a class AB then he is " playing "/marketing with words and obviously is telling a lie.
Regards and enjoy the music.
Thank you to all of you.
Its been a wild ride for my stereo, my wallet and me in the last 3 months.
Everything start with the born of my new baby boy when my wife told me Paulo you need to sale some stereos and speakers that you are not using we need the room. So I did decide that was a great opportunity to reduce quantity but up-grade in quality. So here we go in my 3 month roller coster.
I did change everything except for my speakers Martin Logan Aerius I and my CD player a Marantz 67SE.
I had a Sony Preamplifier a Sony MD and a Parasound HCA-1000A power amplifier, then two HCA-1000A running my Speakers bi-amp.
Then I bought an Audio Research SP-9 preamplifier and an Audio Research D-90 power amplifier to replace my sony and my two parasounds and bought a turntable.
However the D-90 power amp works but on the shipping was bang by UPS, so the insurance from UPS is paying me back. Same thing with the first turntable I bought.
So I bought a New Turntable two weeks ego and yesterday I bought a B&K 4420 power amplifier and a Audio Research D-200 and Im planning to keep them till I decide which one I will keep. (Probably will keep the Audio Research because should sound better and second I love the looks on Audio Research). Neither one arrives home yet. Probably in a week from now I will be able to compare them.
Think of Pure Class A and Class B as defining the endpoints of a spectrum. The zone between them is Class AB, and AB1 and AB2 are simply defined points on the spectrum.
Consider amplification of a sine wave. The entire output stage on a Pure Class A amp conducts on all 360 degrees of the waveform. The topology may be single ended or push-pull.
A pure Class B amp must be push-pull. Each half of the output stage conducts for exactly 180 degrees of the waveform before the other half takes over for the remaining 180 degrees.
A Class AB amp must be push-pull. Each half of the output stage conducts for more than 180 degrees of the waveform. The limiting case of this is that both halves of the output stage conduct for 360 degrees, or pure Class A operation.
Depending on design parameters, an amp's class of operation may vary with signal level, so that the output stage operates in Class A for relatively small signals but smoothly moves into AB operation at higher signal levels. The stronger the signal and corresponding power output, the further into Class AB the amp operates.
I have some Yamaha amplifiers (M-80 and M-40) that have "Class A" switches on them.
Normally, the amps operate in class AB mode with output of up to 250wpc (for the M-80). But, if you engage the "Class A" switch, the amp will operate in Class A mode to approximately 30wpc (for the M-80), then automatically go to AB mode for >30wpc.
The downside is, of course, the amps run much hotter with the "Class A" switch engaged.