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Air Dialectric caps [message #8940]

Sun, 08 May 2005 09:04

Manualblock
Messages: 4973
Registered: May 2009

Illuminati (13th Degree)

So whats the scoop with these things. Are they good for RIAA circuits since they can be fine-tuned?

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Re: Air Dialectric caps [message #8941 is a reply to message #8940]

Sun, 08 May 2005 14:48

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

Variable capacitors are used in the tuners of radios. They have small capacitance values, in the picofarad range, because they are physically small. Capacitors are simple devices, and using a moving mesh of plates to provide adjustable capacitance is perfect for for a tuner.

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Dielectric Constant [message #8942 is a reply to message #8940]

Sun, 08 May 2005 20:07

muman
Messages: 11
Registered: May 2009

Chancellor

That has been universally dissed. Dielectric properties are poor and they shift which is why old radios lose their station. Crystals were used for accurate tuning because of that. Imagine how changes in humity and particulates change the dielectric of that kind of capacitor. The dielectric constant of a vacuum is 1.0, air is ~1.0001 and plastics are 2-3 (depending on which one). Paper is 2-6 (depending on what it is impregnated with) and mica is 6.8. All of these are better and more consistent dielectrics than air.

Capacitors and Dielectrics

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Re: Dielectric Constant [message #8945 is a reply to message #8942]

Wed, 11 May 2005 12:58

Manualblock
Messages: 4973
Registered: May 2009

Illuminati (13th Degree)

I know but they set the value then cement the vanes in place. Someone did a study and found that a 30% change in humidity translates to a less than 1% change in regulation. So if that is correct it would make them attractive for use as adjustable RIAA caps.

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Universally dissed? [message #8946 is a reply to message #8942]

Thu, 12 May 2005 14:29

Triode_Kingdom
Messages: 13
Registered: May 2009

Chancellor

I don't think so. Just try to find a forty-year-old air cap that leaks like paper. Air and vacuum exhibit less loss, no hysteresis, no aging, a neutral sonic signature, and best of all, they are adjustable. If you can find suitable values in a reasonable size, go for it!

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Re: Universally dissed [message #8947 is a reply to message #8946]

Thu, 12 May 2005 16:49

muman
Messages: 11
Registered: May 2009

Chancellor

That's right --- universally dissed. No one but crackpots use capacitors like those for audio. Even radio receivers have abandoned them, except the cheap ones, of course. Air is one of the lowest dielectrics there is. It isn't uniform either and the plates are free to vibrate too. Neutral sonic signature my arse.

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Here's a thought [message #8948 is a reply to message #8947]

Fri, 13 May 2005 00:06

Wayne Parham
Messages: 18786
Registered: January 2001

Illuminati (33rd Degree)

Some of the better grade capacitors use pure copper and even silver film plates. Caps the size we're talking about here are generally less than 400pF and are no larger than about 102 inches total plate area, with ~1/32nd inch air gaps between plates. For low voltage circuits, the gap can be smaller, making higher capacitance or smaller plate areas possible. Capacitors like these could easily be made with aluminum, copper or silver film sheets, trimmed to size.

One way to do it is to make several 1" square layers separated with wax paper or whatever other insulator is desired. If you want air as the dielectric, just use thicker plates so they don't flex and touch. Separate them with thin nylon washer spacers and hold them together with nylon screws. That way you'd get the flexability of tuner caps while using better quality materials. Just a thought.

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Re: Here's a thought [message #8949 is a reply to message #8948]

Fri, 13 May 2005 08:21

Manualblock
Messages: 4973
Registered: May 2009

Illuminati (13th Degree)

What would you do then; dip them in something like orange drops?

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Sorry, you're wrong [message #8950 is a reply to message #8947]

Fri, 13 May 2005 12:50

Triode_Kingdom
Messages: 13
Registered: May 2009

Chancellor

>Dielectric properties are poor and they shift which is why old radios lose their station.

Not true. Old radios drift primarily due to the inductors and positive coefficient ceramic capacitors. That's why NPO capacitors were developed.

>All of these are better and more consistent dielectrics than air.

You obviously don't know much about the electrical properties of these materials. Paper is used only where the advantage of its higher dielectric constant outweighs its otherwise poor qualities. It cannot be used in frequency-sensitive circuitry. Its dielectric constant changes over time, as does its leakage and loss, and it is extremely sensitive to changes in temperature.

Please stand in the corner until you get this right.

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