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In Reply to: RE: Neutralization of Miller capacitance posted by Tre' on March 11, 2017 at 16:00:06
searching the forum. I guess what you meant was the post by Lynn Olson (linked below).
Lynn's objection was that neutralization would only work under condition of perfect balance. Any imbalance (e.g. transition into Class B) will cause positive feedback exceeding negative feedback and causing bursts of oscillation.
This is a real problem indeed, but there are at least two ways to mitigate it. First, a differential stage can be forced into complete symmetry (pure Class A) by a CCS or a choke. Second, neutralizing capacitors can be made smaller than Cpg, which will cause partial rather than complete neutralization, but safeguard against excessive positive feedback.
Yet another source of problems could be imperfections in neutralizing capacitors introducing distortion at the critical signal juncture. However, since capacitances in question are relatively low, very high quality capacitors can be used. Vacuum caps come to mind.
Whether or not Miller capacitance neutralization would cause instability is an open question. With high transconductance tubes like 2A3, RF oscillation is inherent to the circuit. Accordingly, neutralizing capacitors may actually block oscillation rather than promote it. That is the opinion of Mr. Marshall, the author of the Audio Engineering article.
I like the way you're thinking.
I suspect (zero evidence) that vacuum capacitors (1) would sound better than anything else (no dissipation or "memory") and (2) would be microphonic (maybe treat them with clay or loctite on the outside?).
You also couldn't use them in much because of their size/unit energy (capacitance*voltage).
I have no doubt that vacuum capacitors are closest to ideal. One of the reason why tubes sound better than transistors is that their interelectrode capacitances are of highest quality, whereas those in semiconductors are of the worst.
What I meant weren't purposeful vacuum capacitors, like those that HAMs use, which are indeed too bulky to be useful in this role. The simplest neutralizing capacitor would be a dud tube of the same type, however, with tubes like 2A3 this is also clumsy. But there are smaller tubes with relatively high inter-electrode capacitances that won't take too much space, like 7-pin 6 volt full wave rectifiers or sub-miniature pentodes.
Microphony problem in tubes used as vacuum capacitors is the same as it is in tubes used as amplifiers. A 6X4 or a sub-miniature pentode would be virtually free of microphony if used for neutralization in a 2A3 power stage.
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