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Miller capacitance
Miller capacitance
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Old 27th September 2001, 03:36 PM   #11
GRollins is offline GRollins  United States
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I finally stole enough time to read the article you mentioned. Unlike many on this site, I've long been a fan of tubes as they do things that solid state simply can't--for whatever reason. I've heard a lot of reasons postulated for the differences between tubes and solid state over the years, but this is the first time I'd heard anyone bring up the difference in quality of the Miller capacitance. However, as soon as you mentioned it, I could see the direction Olsen was going and it made perfect sense. Now, whether it actually is an audible effect, I don't know, but it certainly is an attractive concept.
C Simpson's points I find unconvincing:
--Compensation caps. One, tube circuits frequently don't have them, so strike one up for simplicity. Two, it's not a question of the quality of the compensation cap, it's a question of the quality (or lack thereof) of the capacitance in the gain device; bypassing an electrolytic with a film cap still isn't as good as a bank of pure film capacitance.
--Feedback as a cure. The use of feedback is rather a Faustian bargain. Sure you get some good things up front, but you pay in the end. The more feedback you use, the more dry and lifeless the music becomes. This has been known for years in contexts ranging from subjective assessment of the sound quality to the TIM/slew rate thing.
--MOSFET capacitance. Sure MOSFETs have capacitance. So do bipolars and tubes. Again, the question Olsen raises isn't the quantity--it's the quality.
Hoffmeyer's point about cascodes may or may not be applicable, as it's based on where the signal enters the gain device, i.e. base/gate/grid, or emmiter/source/cathode and how the circuit's used. It's not that the capacitance goes away, it's that you're trying to render it irrelevant. I'll have to think about this after I've gotten more sleep. In any event, not all circuits use cascodes or are able to. I don't know that I could say that I've ever noticed a 'cascode sound' that was identifiable as consistently better. For all that the topology has theoretical benefits, it doesn't seem to sound any better in practice. For what it's worth, the differential circuit also accomplishes the same end, but only if you take the output from the second (uninverted) side.
In summary, I think Olsen's point has possible merit. I'll roll it around in the back of my mind over the next few days.

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Old 28th September 2001, 07:01 AM   #12
hifiZen is online now hifiZen  Canada
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I guess my point was that with careful design, the miller capacitance effect can at least be reduced or circumvented in some cases, and that IMHO there are greater gremlins to be dealt with in the solid-state world. I am certainly not contesting the obviously superior characteristics of tubes in this regard, nor that there may be an audible effect. Indeed Lynn's hypothesis brings the quality of capacitance to the fore, and both MOSFETs and BJTs have horrid capacitive characteristics.

As far as feedback goes, I used to believe that high feedback = bad sound too. But, my latest experiments just havn't supported this belief, and i've been forced to re-evaluate my opinions. I think it's all too easy to assign bad sound to the presence of feedback without taking the time to wrestle with the mechanisms behind it. So far, I havn't heard many (any?) arguments WHY feedback can lead to poor sound... it's always stated as self-evident, then dismissed as some mysterious effect we can't explain, and given no further consideration...

I postulate that there is some specific mechanism(s) behind this effect, and that this mechanism interacts with or manifests itself through feedback, leading to a subjective decrease in sound quality. I further hypothesize that if we can identify and eliminate this mechanism, then feedback will no longer be the sonic culprit people make it out to be...

The error correction concept seems perfectly valid to me, so we need to investigate why it doesn't always work the way we want it to and attack the problems if we're ever to come to some understanding and advance the art and science of solid-state amplification. So far, the open-loop output impedence explaination seems the most plausible to me, because it introduces the potential for error signals which are not consistent across frequency, amplitude and time, very probably leading to a constantly changing distortion spectra, which is then percieved by the human ear as artificially reproduced sound. Just my theory... criticisms and further analysis welcome...
- Chad.
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