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Dynamic impedance explained

Tre and Testing123,

Your assumption that a Class A amplifier would only undergo 10% change in current is greatly under estimated. Everyone knows, or is told that Class A has a constant current draw. This is only true if you look at the average continuous current draw. However, a simple look at the load line shows us that the instantaneous current change can be more than 100%. Look at the classic load line for the RCA 2A3 data sheet:

http://www.mif.pg.gda.pl/homepages/frank/sheets/049/2/2A3.pdf

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.

Many people are under the assumption that a large capacitor reservoir makes for a low impedance power supply. On the surface it looks that way. For the most part it is impossible to drain a large capacitor reservoir in 50ms (a full wave at 20Hz). The result is a power supply that looks like it has little voltage sag. The thing that most of you fail to understand is not only is the capacitor a low impedance source of current, it is also a low impedance current sink. As soon as there is a slight change in voltage across the capacitor it begins an attempt to sink current to recover its lost charge. Since it is a low impedance device, it competes directly with the output tube for current resources. Considering a 50uF capacitor has an impedance of only 159 ohms at 20Hz, how well do you think a tube that has an Rp of 800 ohms (2A3) is going fair against the capacitor? The answer is not very well. The lower impedance device will always win a larger share of the resources. This is the reason why there are so many people that believe a lot of capacitance sounds bad in a tube amplifier; THAT’S BECAUSE IT’S TRUE!

If you want to avoid having the capacitor reservoir compete with your output tube your choices are limited. The only other viable route is to reduce the series resistance of every component in the power supply and keep the capacitance as low as possible. There is always the even less attractive regulated power supply. I’ve yet to hear a regulated power supply that sounds better than a well done passive low DCR power supply. This is not due to the lack of experience or opportunity.

There is nothing ambiguous about dynamic impedance or low DCR power supplies. It is a less invasive way to achieve a low impedance power supply with the least amount of components. As most have found, the less junk you put in the signal path and power supply, generally the better the amplifier sounds.

I don’t blame anyone who questions some of the low DCR supply design. The main problem is all the electrical engineering books have too simplistic models. They fail to look at a deeper level of understanding. Quite frankly, in the engineering realm, there really isn’t a need for it. So, I don’t blame the text book authors either. However, the case is different with DIY audio. We are looking for a deeper level of understanding and the small things that add up to make our good amplifiers into great amplifiers.


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