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Really class AB?

Posted by Dan on October 22, 1999 at 08:15:04

In Reply to: Re: class A power posted by mark on October 22, 1999 at 06:41:14:

Hi,
My understanding from some discussions I've had
is that its really not pure class A, but class AB.
Here's a copy of the discussion

Calculations and measurements agree that it
is really class AB, but the truth is class AB is just fine when
there's no output transformer. It's the switching noise at cutoff
of one tube that some people say causes ill effects with class AB
in transformer-coupled amps. That adds some graininess, harshness,
and lowers resolution through leakage inductance and switching noise.
There's no leakage inductance in an OTL of course.

For 60W class A (A1, A2, it does not matter), you have
to put out 2.74 Arms current into 8 ohms.
That would mean you need to put out peak current of 1.414*2.74
= 3.87 Apeak. For class A operation at 60W your amp has to be biased
so that it remains conducting all the way from -3.87 A to +3.87 A.
In the circlotron with opposing power supplies, the case still holds
that you need 3.87 A bias current for class A. When one phase is
fully on (3.87 A) the other will be fully off (0), and vice-versa.
But we bias these tubes to only 0.55 A. That's not class A to 60W.
That is class A to 1.21W. Beyond that and one side is cut off, and
it's really class AB.

I used a scope probe on the output stage's plate resistors to see if
it was actually conducting all the time or cutting off at one swing
and fully conducting on the other. Indeed, it showed early cut-off
(flattened line) at around the 1W level, keeping in mind that one
of these plates gets 1/8 the current load. It shows itself by
measurement to be class AB, proving the math is correct.
I measured the individual plate currents through these resistors
and they add up to 0.55A, as expected, and on average each tube
gets about 70mA bias. Cut-off is only 0.55A away for each output
phase and there's no getting around that.

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