Maybe, i just tell again. I dont care about the efficience of 5w tube amps.
I'm going with 'yes' then. But no matter- you and I are on the same page.
Err, academic response not to the question.
Class A operation has superior sonics to the others, for headphones they make the best choice , class D is not for sonics but for high power @ reduced cost, size , etc ..
Same for class H , AB , B , these are all compromises to sonics based-on application where high biased class A is prohibitive ..
Class A is not best for high powered application , well not all ..
Regards
Sy, you can take this post as a Citation
Class A used to be used to place the output devices in the most linear part of their range of operation. This is traditionally done because in the old days it was hard to put enough feedback on the circuit to prevent non-linearities raising their ugly heads. So every means possible to reduce the innate distortion of the circuit was used to allow feedback to better do its job; class A operation is one tool used for that.
These days class D allows the designer to use a considerably higher amount of feedback than is often practical in a class A or AB design as in those designs doing so without
very careful design of the feedback network likely will result in stability problems.
In all amps you have something called 'Phase Margin' which describes the limits of bandwidth and feedback, which if exceeded, results in oscillation (due to phase shift at higher frequencies, usually ultrasonic). You are familiar with this problem as it has appeared a number of times in class A or AB amps over the years. Usually its something to do with the load such that frequencies which exceed the phase margin of the circuit pass through the feedback network (a well-designed feedback network can prevent this so is more than just a simple resistor). That is why I said 'without
very careful design' earlier.
Examples are the Futterman OTL (in particular the New York Audio Labs versions) which could oscillate if the load was not ideal and especially if the amp was overloaded. Other examples showed up in the 1980s when a number of solid state amps would oscillate when used with a particular Polk Audio speaker cable. There are a good number of other examples. In the tube era, one of the lowest distortion amps made was the
Khronhite UF101, which ran up to 80dB of feedback at some frequencies. But if you look at that design, two things are obvious- the first being that the designer was very good at feedback network design and the second that he recognized both the limits of tubes to have enough Gain Bandwidth Product and also that phase margin must not be exceeded. So the feedback varies in that design depending on the frequency.
Class D can solve the linearity problem and the oscillation problem with one stone, using a technique called 'self oscillation'. In this case, so much feedback is applied to the amp that it goes into oscillation as soon as the amp is powered up. The feedback network is carefully designed so that the amp can only find one frequency at which to oscillate, and that frequency is used as its switching frequency. This allows for a considerably higher amount of feedback than was previously possible in many cases, although there are some class AB amps available now that do have more sophisticated feedback network designs which allow for more feedback with them too.
The point of this is when you get that much feedback in an amp design (in excess of 30dB at all audio frequencies, not just in the bass or the like), the feedback allows the amp to reject a great deal more of that which is not the signal, such as distortion- so can correct for phase shift due to limited bandwidth or the effects of AC power line sag. This also means that class A no longer has a particular distortion advantage it might have had in the past. I should point out in case you are not aware that any class D amp which uses a choke in the output filter (which is all class D amps used in audio) is incapable of any kind of crossover or notch distortion associated with the zero crossing of the audio signal. For more information on this topic see
Bruno Putzeys' paper on negative feedback.There you will see how many class A and AB designs had limited feedback owing to limited Gain Bandwidth Product, and he presents it all with the math, diagrams and engaging text if the math isn't your thing.
