Complementary Zen- class A or AB?

[needtubes]

HI-

I have seen designs such as this that are labled Class AB (example, the SDS headphone amp) and then the Zen that is labled class A. Which class is this amp? Is it just a matter of bias current? What I mean is if you bias the amp at 2.2A per channel, it will run class A up to 2.2A out, then become AB for everything above. Is this remotely correct? Could this circuit be adapted for lower power (around 1W instead of 20 or so)?

Thanks

 

[Nelson Pass]

It's just the bias current.

If the amp is a push-pull type, the output current peak will
be twice the bias current, so if you bias a 1 amp, it will leave
Class A at 2 amps. Into 8 ohms, this would be I^2 * R which
is 4 * 8 = 32 watts peak, which for a sine wave is 16 watts
continuous. It will usually operate AB above these levels.

If the amp is a single-ended Class A biased by a constant current
source or an inductor, then the peak output current is equal to
the bias current, and will not deliver more.

If a push-pull Class A amp has a varying bias circuit, and there
seem to be plenty of them around, then you can set the ratios
anywhere you like, but the performance tends to suffer at
lower bias figures.

If a single-ended Class A amp has a varying current source
like an Aleph, then you can set the figures where you like, but
they are only pretty within a limited range, usually not higher
than peaks of twice the bias.

 

[needtubes]

Thanks! That pretty much cleared everything up for me.