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In Reply to: Opinions sought on vacuum capacitors posted by lipmanl on December 18, 2006 at 11:09:23:
There has to be an air-tight bottle to preserve the vacuum, and the bottle will ring unless properly damped. The internal plates will also ring, and damping materials that do not out-gas are difficult to imagine. If you want to use a vacuum pump, then that opens a whole range of problems.The size will imply a lot of parasitic inductance, so the self-resonance frequency will be low.
The size also allows for a lot of noise induction. Any shielding to prevent this will also have acoustic and electrical ringing issues.
Follow Ups:
Vacuum capacitors, at least the one's I recall, should be excellent for audio, they certainly worked well at RF. Since the interdigitated plates are concentric cylinders and each one is connected at one end to a common plane, inductance is very low. And they certainly don't use vacuum pumps, even the variable ones use a bellow's system to seal out the air. The glass tends to be rather thick so I doubt it will ring. And naturally you can minimize noise pickup by connecting the outside plate to the lowest impedance. And they should have very low dielectric absorption as almost none of the field penetrates the glass.Since the vacuum is only used to increase their voltage rating, even if they have leaked they should be just fine for audio. The problem with the low values is that you have to use high-impedance circuitry, tubes would be good...
Rick
Al,You bring up some interesting issues. Perhaps the least of these issues is outgassing. In a vacuum tube, gas leads to short tube life, among other things. I think in a vauum capacitor it would only lead to a lower voltage rating and maybe a little df. So I would guess that a little outgassing wouldn't be disastrous.
On a second note, it just occured to me that there is the possibility of making a vacuum state integrated circuit. For instance, the grid could have two leads. One for biasing (or grid leaking) and one coupled through an internal vacuum capacitor for signal input.
Thanks for your response.
Google for the Loewe 3NF tube. They where used to avoid the per socket tax in the 20's.
World's first op-amps, the Philbrick K2-W.I used this picture from Philbrickarchive.com without asking but I'm sure they won't mind.
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