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RE: Class A to B switching nonsense

However, when the amp is leaving Class A it is possible that it will be heard depending on when it moves to AB (actually B because at that point it will cutoff at zero crossing). If the speaker is pretty insensitive and the bias only a few watts then probably quite a bit of listening is done in B or at least at low enough volume levels that the added distortion might be audible.

If you read Boyk and Sussman's simulation paper you will see that MOSFET Class A PP output stages are actually theoretically perfect...real world says otherwise but they get close. When they go into Class B (when the bias runs out so to speak) all hell breaks loose and they distort pretty badly. Funny enough bipolar transistors are less Schizophrenic... they just have quite a bit of distortion all the time unless you use a lot of negative feedback.

A Class A MOSFET single eneded stage is quite interesting because it has only very simple low order harmonics and CANNOT leave Class A (it is single ended afterall).

So, while Nelson is right that it is not so abrupt as to have a "klunk" it might be audible if the amount of Class A is low and the sensitivity of the speaker is also low.

Obviously, an amp that runs 20 watts in Class A will be in Class A most of the time with most speakers.


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