What are the advantages to a Class A amp & what are the trade offs?


I've never had a class a amp but am considering one now. So what am I getting myself into?
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The Jazz Bakery isn't an ideal sounding venue.  I've frequented it half a dozen times.  Bob Wilber played there a decade ago and his sax and clarinet sounded peaky and bright, actually aggressive sounding, so unlike his playing style.  Locally, many churches and concert halls have great acoustics.  The absolute worst with a 5 second reverb time is the Queen of Angels Cathedral.  I've sung there four times and I couldn't hear myself in the choir with all that reverb.  My wife said it sounded like mush.
Class A amps are the least efficient kind. They are bigger heavier and draw more power (per watt output) than the rest so total cost of ownership is high. To make up for that you tend to want speakers that are more efficient. Quality full range hifi speakers that are also more efficient also tend to be larger, heavier and often more expensive as well.

Class A amps from reputable vendors are usually very highly regarded in terms of inherent sound quality.   


@mapman  I think you have given  a good explanation of what i was trying to find out. I have the efficient speakers which somewhat piqued my interest in class A

Class A is a tool in the designer's box which allows for lower distortion.
With all amplifiers, distortion is essentially what causes amps to sound different from each other, moreso than actual frequency response.

The reason for this is that the ear converts distortion into tonality. For example the 2nd harmonic often associated with tubes adds to richness; the 7th harmonic creates a metallic quality. We've know this fact about harmonic distortion since the 1930s.


Now it is the higher ordered harmonics that contribute to harshness and brightness. This in a nutshell is why solid state amps tend to sound harsher and brighter than tubes (keeping in mind that the tube amp may well have bandwidth equal to that of the solid state amp).

Class A in a push-pull amplifier allows for even ordered harmonic cancellation throughout the entire waveform right up to full power. This means it will be lower distortion- and in part due to the fact that the output devices never turn off (so no crossover artifact either). Now its often said that its that first watt that counts the most in most audio situations and this is true. When you operate class A, your distortion at low power levels is lower, and this translates to greater low level detail; distortion is not obscuring that information.

Now using loop negative feedback is good for reducing distortion also, but IMO/IME its a poor means of reducing feedback. This is because feedback, while suppressing distortion, actually introduces some of its own and its almost entirely higher ordered harmonics (plus IMD introduced at the feedback node). So any amplifier employing feedback will be brighter and harsher than real life. This is one reason why class A is important, because it is a tool that can allow the designer to make a low distortion amplifier that does not employ feedback.
@atmasphere  Thanks Ralph. Fwiw you have a way of making the complex easy to understand. Are/Were you also a teacher?