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Re: Reference - "Valve Amplifiers 3" [message #8968 is a reply to message #8965] |
Mon, 16 May 2005 05:54 |
Damir
Messages: 1005 Registered: May 2009
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Illuminati (2nd Degree) |
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See "Valve amplifiers 3" by M. Jones, pages 211-229, especially Fig. 4.4 - various capacitors types, and the quality of them. The air plate caps are the more nearly perfect, with electrolytic Al-foil/bipolar at the "other side". Then citation: "Possible audio uses (of air dielectric, metal plate) include: - ~300pF variable capacitor across the input of a MM RIAA stage to allow cartridges to be optimally loaded by the pre-amplifier - ~50 pF for trimming equalization capacitors to their exact value"And later, author uses just that - 40 pF variable cap for "trimming" the exact value of RIAA EQ cap in his EC8010 RIAA preamp, Fig. 7.37, page 568. And please, calm down your agressive tone, one of the reasons why are all here is to avoid "rubbish" from some other forums...
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"Electrical breakdown is not always associated with high voltage" [message #8970 is a reply to message #8968] |
Mon, 16 May 2005 21:12 |
muman
Messages: 11 Registered: May 2009
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Chancellor |
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Thank you for the reference. I looked and found Morgan Jones book. I don't agree that air is a better dielectric or that it sounds better, but at least this reference is something. At the microvolt level of a moving magnet cartridge, if the air is clean and humidity controlled, it would work. I work in power systems, and we always consider air to be a problem. The dielectric properties of other insulators are much better. Even an air bubble in an insulator can be catastrophic. Advances in materials have made air gap insulators obsolete, IMO. The fact they aren't used in most good radios anymore says something too. Quoting John J. O’Dwyer, "Breakdown voltage is directly related to the dielectric thickness. Its counterpart, dielectric strength, indicates the relationship between thickness and breakdown voltage, and is expressed as a ratio of voltage to thickness. Thermal effects due to local Joule heating can cause thermal breakdown at sufficiently high temperatures. This correlates to electrical conductivity through an exponentially increasing function of the temperature. Other factors that contribute to breakdown include particlates, material contamination and water vapor. Each can influence breakdown; actions must be taken to eliminate their contribution to breakdown voltage measurements. Humidity, for example, reduces the resistance of most dielectrics, thus increasing the return current (the current that opposes a charge build-up). Contamination can contribute to leakage currents and charge mobility across isolation areas." Two plates separated by a 1mm air gap will arc with 3000 volts, and can as low as 300 volts. Air is a fluid mixture and unless controlled, it is a nonspecific gas. Water vapor is a big factor in reducing the dielectric strength of air (or any other dielectric, for that matter) and other contaminations also contribute to modify dielectric properties. It's a crap shoot with air so I don't think an open capacitor is the best way to go.
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Bolivars [message #8978 is a reply to message #8963] |
Fri, 20 May 2005 18:05 |
PakProtector
Messages: 935 Registered: May 2009
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Illuminati (2nd Degree) |
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fron the Island... if your screen is brown, imagine what your lungs must look like, or for that matter how they function. A good friend of mine just got a 4x bypass from a few decades of smoaking. 9 hrs with his chest split open...no thanks. regards, Douglas
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