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Anybody try air variable caps for RIAA networks?

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Posted on October 31, 2001 at 08:20:53
DrM
Hi,
I'm working on another phono pre, which you know is a difficult thing to do. I would rate doing a good phono stage as the hardest project, followed by power amps, followed by line stages. The problem is that you have to get the &#$%#@ RIAA frequency response to less than a dB. Practically, this is tough because it varies based on the actual plate impedence of the tubes involved, among other factors. Most of us don't have spectrum analyzers to help us measure this.

Anyhow, another problem is getting quality caps at those tiny values. I need a 750 pF value, which by golly is available as an air variable type used in radio receivers. Air is certainly an excellant dielectric, and the cap has the bonus that it can be accurately tuned. How does it sound? Has anybody tried this? The only downside I can se a-priori, is that they tend to have thick plates, which in my experience probably would not sound too good.

Dan

 

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Re: Anybody try air variable caps for RIAA networks?, posted on October 31, 2001 at 09:24:45
dave slagle
Manufacturer

Posts: 5429
Location: NYC
Joined: April 27, 2001
just a note here... as capacitors go...depending how you look at it air can be considered a very poor dielectric in that its DC is 1.. replace that air with say teflon with a dc (dielectric constant) of 2 and you get double the capacitence for a given plate size and spacing...

the whole reason we want our trannies to have low DC materials is to reduce capacitences, but if its a capacitor we are trying to build...

i am looking at it from the capacitence vs size issues and not linearity......so maybe bigass air caps are the best??? what could be better.. well maybe vacuum caps... but like air their values are so tiny...

so i guess it goes back to that whole quality vs. quantity compromisees, but i suspect the air caps should be quite good anybody have a reason why they should suck??? i see no reason why not to give it a try, when you get it all dialed in.. then try swapping it out for matched values of different constructions and report back on the sound.

looking at the tables... i have always wanted to do a water capacitor... its DC is up around 70 so you could get some pretty impressive values by just sinking a few metal plates into a vat of water...

i almost went and bought one of those developing tanks for 4X5 film and got some 4X5 sheets of copper cut and filled the whole thing with distilled water to just see what values i got... of course the problem is water is very instable and impractical in this situation, but... it would be a goof to try.

someday :-)

dave

 

Re: Anybody try air variable caps for RIAA networks?, posted on October 31, 2001 at 17:14:43
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
Dave,

Pure water is a bad conductor of electricity. The problems start because of dissolved minerals. Use either high quality distilled or demineralized water.

Eli D.

 

Re: Anybody try air variable caps for RIAA networks?, posted on October 31, 2001 at 18:30:08
dave slagle


 
dammit eli... don't make me try this... the whole thing was a joke... the copper plates would tarnish and contaminate the water too... maybe silver plates :-) possibly gold plated :-) i'm not worried about the breakdown... just want to get a clue if the whole thing is actually possible... and maybe get my ears on it for a day or two if it actually works... think of the threads... evian is smooth... pelligrino has a bubbly edge... and if you hav the $$$ cristal is the way to go... fwiw alcool has a lesser DC than water, but it is more stable wrt impurities... this is the diy fourum... so why not diy caps??? some sort of sealed system would be needed for anything liquid... hmmm batteries use plates and liquids... how different are they than caps... maybe they could be adapted...

dave

dave

dave

 

Re: Anybody try air variable caps for RIAA networks?, posted on November 1, 2001 at 00:11:49
Paul Barker


 
Go on try it Dave, I've been bored to death lately we need a filip!!

 

Re: Anybody try air variable caps for RIAA networks?, posted on November 1, 2001 at 07:20:52
dave slagle
Manufacturer

Posts: 5429
Location: NYC
Joined: April 27, 2001
well dan M could dunk that variable air capacitor in a glass of water and see what happens to the values... leseee assuming air = 1 and water = 70..... 750pf * 70 gives us a .5mfd varaible cap :-)

just looked at the dielectric constant table... water is closer to 80... but get this... hydrogen peroxide is a whopping 121, anybody know how stable this is, and how it might respond to voltage potentials??? and for those of you whio had to know, cocaine is a mere 3.1, so save it for insulating your transformers.

dave

 

I would be kinda worried about arc-over, myself. (nt), posted on November 1, 2001 at 07:45:29
J Epstein
Reviewer

Posts: 1041
Joined: April 6, 2000
/

 

Dave you Knut!, posted on November 1, 2001 at 08:15:14
DrM
I ain't gonna stick no capacitor in no stinnkin water! :) I've got enough project brewing without some wacky thing like that. I just want to know if these sound at all good before I plunk down some $$

Dan

 

Re: I would be kinda worried about arc-over, myself. (nt), posted on November 1, 2001 at 09:37:44
dave slagle
Manufacturer

Posts: 5429
Location: NYC
Joined: April 27, 2001
sissy... use them for cathode bypasses or for filament supplies... don't go tossing the plates out with the bathwater...

dave

 

Re: I would be kinda worried about arc-over, myself. (nt), posted on November 1, 2001 at 10:28:23
Paul Barker


 
Looking down your list dave I suppose bath water comes somewhere between urea and h2o, about 2 then!

 

Just don't let Zelda drink out of them (nt), posted on November 2, 2001 at 07:54:55
J Epstein
Reviewer

Posts: 1041
Joined: April 6, 2000
/

 

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