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RE: Dynamic impedance explained

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On page three it is easy to see that at full power the tube swings from 118mA down to 12.5mA. That is a change of 105.5mA with the quiescent current of 60mA. That is a current change of 176%. The average current change from the quiescent point is 52.75mA. In PSUD I set the current draw to 60mA and then do a current step of 52.75mA. I then divide the resulting voltage drop by the step current to get the dynamic impedance of the power supply. An additional point of detail is I let the power supply simulate several seconds after the current step so an over sized capacitor reservoir doesn’t hide the true impedance of the power supply.
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John,

I agree that Class A amps do NOT always have constant current draw but
I don't quite follow your reasoning here. My take on it is as follows:

The quiescent current is 60mA. When "full signal" is applied, the output
current into a resistive load swings to 118mA on the positive crest and
down to 12.5mA on the negative crest. In other words, the current swings
to +58mA above quiescent and -47.5mA below quiescent

In other words, the current swing is +58 ma from quiescent on positive
half-cycles and -47.5 mA on negative half-cycles. The average current,
under full signal conditions, over the entire cycle is thus
[(118 - 12.5)/2 + 12.5] or 65.25 mA (OK so far?)

So we have idle current of 60mA and full signal current of 65.25mA for
a 5.25 mA increase in average current. This is the DC current change from
zero signal to full signal. If you applied a full signal rectangular tone
burst to the amp, you would observe a square wave current on the B+ of
5.25 mA peak amplitude. I believe this is the correct value to plug into
PSUD for the current step in this case.

The AC swing from 12.5 mA to 118mA must also be sourced by the B+ supply,
but even at 20 Hz the final filter cap has a pretty low impedance (50 uF
~= 160 ohms) compared to the Ri of a 2A3.

It's debatable by engineering types whether a 10% current step in a class
A amplifier power supply should be audible, but all you need to do is AC
couple a similar signal onto the B+ of an amp and listen for yourself.

Now I do believe that the time response to the current step is important,
and tuning the power supply to have a fast ( <20mS), near critically
damped voltage response to the signal current step might make the amp
sound more dynamic and "fast", or at least prevent the B+ from making
it sound "slow".

Cheers,

Michael

PS another way to say this is that the second harmonic distortion inherent
in large signal swing operation of a class A SE amp increases current draw
from the B+ in proportion to the amount of 2F distortion added.

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  Kimber Kable  

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