Quote:
Originally Posted by dave-G
..class-A: one amplifier does the entire signal; both swings of the waveform (+ and -) .. AB means one amp does the positive side, and another does the -
Dave, this is sorta correct, but lets not confuse Class A with "push pull" circuits. A push pull circuit can also be Class A. Class A is best defined as a circuit where the elements are never turned off. In a normal linear power amplifier, you have a push pull output stage where one device handles the negative swing and another the positive (Class AB). The point at which one device takes over from another is set by the "bias". If the bias is raised to such a point that neither device turns off, then it's Class A.
The only real disadvantage with Class AB is there is a glitch as one device takes over from another, called crossover distortion.
Neve 1073 is single ended Class A, ie, as you say, one device handles the whole signal. In our Class A stuff, we use push pull circuits (sometimes referred to as "differential") which are biased as described above.
Most opamps are Class A up until the output stage which is biased Class AB. But - and this is very important regarding Class AB circuits - the output stage will remain in Class A if the load it is driving draws less than half the bias current. This means, if the opamp has a high load impedance, it will be still run in Class A.
Bearing this in mind in opamp design, you can achieve very good results with these devices, so just because it's an opamp does not mean it's going to sound bad - it's only going to sound bad if the designer has not used the device in a sensible manner.
Sorry for the long post, hope it helps.
Tim.