What are the difference between CLASS A and PURE

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Extra Virgin Olive oil usually means "First pressing" and it is better ..
Class A = LOTS OF HEAT! but me likey!!!

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.

It's like Madonna vs Mary Poppins.

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...