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RE: Dynamic impedance explained

Posted by Testing123 on July 1, 2009 at 09:31:18

In Reply to: Dynamic impedance explained posted by JLH on July 1, 2009 at 04:36:38:

My experience with technical discussions like this is that they are kind of like peeling an onion. On one level, a particular argument may seem compelling and correct; but if you really scrutinize it with super precision you can find serious errors. And I'm afraid that's the case with what you're saying here.

First if all, regarding the term "dynamic output impedance," there definitely is some ambiguity here. What you've defined is the Thevenin output impedance of the supply measured at DC. Dynamic in this context usually means "changing with signal level or history" and makes the most sense, for instance, when we talk about a tube's plate resistance, which varies continuously depending on the bias current flowing at the time we take the measurement. Therefore, in a large-signal application, plate resistance is dynamic and constantly changing with instantaneous plate current.

Another example where the term"dynamic" applies is when we are talking about circuit parameters that vary with signal history. A good example of this is so-called "memory distortion" that occurs in solid-state amplifiers due to heating in the input differential pairs. The effect is a function of recent signal history and creates distortions different from what you would measure using steady-state sinewaves.

So, I would say that what we are talking about here is just plain-old power supply impedance . The "dynamic" qualifier sounds impressive but isn't really adding much value for the knowledgeable reader.

The thing about impedance is that it's not just simple resistance. Since the power supply does not look like a plain resistor in series with a battery, it will exhibit storage and resonance effects. But this is all very well described in conventional engineering terms. The tired old complaint that "engineering textbooks" are too simple is both irrelevant and wrong. Most engineering textbooks you might be thinking of are general-purpose affairs intended for teaching basic theory to undergraduates. Even so, there is more than enough material in these books for a studious undergrad with an interest in audio applications to dig into the problem with two or three orders of magnitude better understanding than is typical in discussions here.

About the myth of constant current draw from Class A amplifiers, I haven't said anything about this, so don't make assumptions about what I know. Obviously in a single-ended amplifier the power supply sees the full peak-to-peak variation in plate current. In a push-pull amplifier, the current draw is much more constant. But in both cases, the average current changes from idle to full load due to output stage nonlinearity. Furthermore, the average current really depends on the envelope of the signal current, and this will have infrasonic components even if the lowest signal current is only 20 or 30 Hz. Consider, for instance, the DC current drawn by a push-pull amplifier when reproducing the sound of a tympani being struck four times per second. So, yes, I get this.

The real problem with what you say above is the part about the capacitor competing with the output tube for current. This is just-plain broken logic. For all intents and purposes, the output impedance of the supply is the reactance of the capacitor at the frequency of interest. The capacitor does not compete for current with the output tube; this is ludicrous. The capacitor is the source of current for the output tube. As long as there is voltage across the capacitor, current will flow from the capacitor to the output tube. The lower the impedance of the capacitor, the less a change in current drawn by the output tube will cause a change in the capacitor voltage. This is a good thing.

What you may be trying to say is this. At any given time, the voltage across the capacitor will vary from its nominal (amplifier at idle) voltage. If, say, there is a long run of heavy bass signal, the capacitor voltage will be drawn down, perhaps significantly. Now, a big capacitor, having a low impedance and needing a lot of charge to replenish the lost voltage, will take some time to return to the equilibrium state. It is the job of the upstream components -- the power transformer, rectifier, and filter chokes/capacitors to deliver this current. As with any reactive circuit, there are time constants associated with its behavior. By and large, high storage filters will react more slowly than low storage filters. This is all perfectly well described by standard engineering theory.

Let me say again, the argument that a high-capacitance supply sounds bad because the impedance of the capacitor is low compared to the plate resistance of the tube doesn't hold water. Not at all. It may very well be that high-capacitance supplies sound bad. To some people. Under some circumstances. There may be some good engineering reasons for that. But to date, as far as I'm concerned, there has been very little cogent, coherent discussion of what those reasons may be.

There are other people who swear big power supply filters sound better. A lot better. I've read the reports.

So I beg to differ with you. I think you've just barely started to scratch the surface of this question, if what you're trying to do is understand why the circuit performs subjectively the way it does. For some people, like Jeff, the individual subjective experience is all that counts and no further explanation is necessary. That's fine, but let's not pretend the status quo on this forum really does justice to the subject. And let's not get carried away by half-baked logic that leads us to equally half-baked generalizations.

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