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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 4th February 2014, 09:39 PM   #48001
Steve Eddy is offline Steve Eddy  United States
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Originally Posted by Kindhornman View Post
SE,
Not so sure about whether the paper phenolic would be more hygroscopic than glass epoxy, but I will give you that the glass epoxy should be stronger in a shear application. Phenol resins are used in aerospace applications I believe due to the high heat distortion properties and resistance to burn, I think they char and somewhat self extinguish but it has been years since I talked to my friend who sold those chemicals. If I remember correctly the glass fibers in the trimmed edges of a glass/epoxy laminate are hygroscopic in nature. One of the failure modes of that material under certain conditions if again I am remembering
Yeah, but realistically, just how strong does it really need to be? Unless you're loading it with heavy transformers or big-*** caps and it needs to survive the rigors of shipping, it's a perfectly good material for audio. Also, FR1 and FR2, which are paper/phenolic based are fire resistant, though they're not the beautiful dark brown like the old XP and XXP materials.

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Old 4th February 2014, 10:12 PM   #48002
Kindhornman is offline Kindhornman  United States
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John Curl's Blowtorch preamplifier part II
SE,
I have a sheet of some of that in 1/4" thickness that I just so happened to use to mount some crossovers on, a bit of overkill I would have to say. Looks much more like standard pcb board color though with a slight greenish tint.

Back to audio for now and trouble shooting a car problem no mechanic can find. Of course it is an electrical problem, intermittent at that, heat related under the hood.
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Old 4th February 2014, 11:16 PM   #48003
fitzfish is offline fitzfish  United States
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Yeah, I use those materials. Also Solvay and Arkema.
Some are so soft they can be stripped from the wire with a fingernail if not too thick...

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Nice link, thanks.

Their velocity equations are valid only for coaxial cables as well as for striplines as long as the aspect ratio is large, IE very thin insulation with conductors at least 10 to 100 times wider. Once we go to parallel runs where the magnetic and electric fields are no longer constrained, we have to modify the equation a tad..v = 1/sqr(LC), or 1/sqr(EDC), or 1/sqr(epsilon*mu).


jn
My pleasure. How about calculating an ~9awg star quad (12awg per conductor) that is moderately constrained (0.030 between conductors, Teflon insualted) and 5-6 twists per foot?

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Originally Posted by jneutron View Post
Yes. The problem is in the level of inductance. Cables run the range from 200 nH per foot down to under 10nH per foot, depending on design. Many vendors do not auto-zero their instrument or realize it is needed, some will use Lp/Rp instead of Ls/Rs, some will try to measure 100 nH using 100hz or 120 hz test frequencies, and many do not rigidly control the setup to prevent variation of parasitic inductance.

Capacitance turns out to be significantly easier to get valid or near valid values close to reality.

jn
Using a GR 1689M LCR meter down near the bottom end of the Ls range is challenging. To get repeatable numbers I use a 4 wire - to - 2 wire fixture and have to re-zero at each new frequency.

By "parasitic inductance" are you referring to proximity effects from things like metal benchs, loops made in the cable under test, etc.?

As an aside, do cable AC parameters change appreciably at higher currents other than IR losses? Instead of testing cables at 1mA or 10mA for example, what happens with the increased fields at 1A? 10A? higher?

Thanks,
Dave
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Old 5th February 2014, 05:03 AM   #48004
Kindhornman is offline Kindhornman  United States
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John Curl's Blowtorch preamplifier part II
John,
I would still like to know why you made the statement about the 1/3 conditioning test earlier? Is there something inherently incorrect about this type of test?
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Old 5th February 2014, 06:26 AM   #48005
john curl is offline john curl  United States
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It is not a typical application, it is 'worst case'.
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Old 5th February 2014, 06:31 AM   #48006
PMA is offline PMA  Europe
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John Curl's Blowtorch preamplifier part II
It is unrealistic.
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Old 5th February 2014, 06:50 AM   #48007
Kindhornman is offline Kindhornman  United States
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John Curl's Blowtorch preamplifier part II
Pavel,
Is that because of the load on the output or the source not being a realistic substitute for an actual music signal? I'll have to go back and see what it was that John had posted earlier.
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Old 5th February 2014, 06:55 AM   #48008
PMA is offline PMA  Europe
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John Curl's Blowtorch preamplifier part II
Steve, it is because of the constant level signal during the test. No real music will stress amplifier at constant 1/3 of rated power during 30 minutes.
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Old 5th February 2014, 07:00 AM   #48009
Kindhornman is offline Kindhornman  United States
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John Curl's Blowtorch preamplifier part II
Thank you Pavel.I thought it was either that or an unrealistic load stressing the amplifier. Now that is one more reason to discount some of the Stereophile testing that often seems very suspect. The subjective listening tests I have ignored for a long time as they can be so silly.
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Old 5th February 2014, 07:37 AM   #48010
john curl is offline john curl  United States
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However, it is a test STANDARD, and many amps pass the test. Please Kindhornman let the designers decide these things.
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