Rick: The sonic difference between Class A and Class AB amplifiers happens because transistors are not "perfect." It turns out that if you have an input voltage of less than about 0.6 volts, a transistor won't work at all; the output is nil even though there is an input. In a Class A amplifier, one ggets around this by "adding" a large constant "bias" to the input signal (about half of the maximum input voltage), so that the transistor will always be in the "active" region, where it amplifies the way one expects it to. In a Class AB amplifier, one uses a pair of transistors (or pairs, or more) to drive the output, one for the plus part of the signal and one for the minus part, as Michiel said. This way, there isn't a need for a huge constant bias, which wastes lots of power as heat when there is no signal. Instead, one just needs to add a little bias to get the transistors above the little part near zero when the transistors don't amplify. Unfortunately, it's near impossible to eliminate the little distortion just by biasing, and it will manifest especially in quiet, delicate passages. With the little Class B amplifier I built, this makes percussive elements sound like crumpling wax paper and things like flutes sound like kazoos. Certainly not acceptable, but fortunately it isn't anywhere near that bad in commercial amplifiers (or so I hope).
Class A amps
I've read a few articles on class A amps but still don't have a good understanding of how it affects sound quality. I've even seen amps which can be swithced into class A mode or AB. Does anyone have experience with this type of unit? Does class A have a true audible advantage or can it be detected only by test instruments? If it is audible, in what resepects? Thanks, Rick M.
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- 43 posts total
- 43 posts total