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Old 13th March 2006, 07:28 PM   #1
Nordic is offline Nordic  South Africa
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Default Input cap - outer foil end?

If I have polypropelyne caps that have marked outer foils, do this side go towards the input or the amp, or should I just try it both ways?
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Old 13th March 2006, 08:06 PM   #2
poobah is offline poobah  United States
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Hey Nord,

You want the outer foil going to the terminal of lowest impedance... this would generally be the output. In this way any induced voltage from electrostatic fields is shunted into the lowest impedance for the least amount of noise.

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Old 13th March 2006, 08:30 PM   #3
Nordic is offline Nordic  South Africa
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thanks poobah, now if you had to gues which whould have the lowest impendance, my ecc88 buffer or the 100k input impendance amp I'm driving?

Lol at your avatar
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Old 13th March 2006, 08:42 PM   #4
poobah is offline poobah  United States
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Hi Nord,

Just in case you're serious... the ECC88 probably has the lowest impedance!

Oh and thank you... others have noticed my new hair style... sorry about the cigarette.

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Old 13th March 2006, 08:43 PM   #5
Nordic is offline Nordic  South Africa
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Dead serious, most appreciated thanks...
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Old 15th March 2006, 03:33 AM   #6
lndm is offline lndm  Australia
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Can you tell me how significant this is? Is it only necessary with low level signals?
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Old 15th March 2006, 01:38 PM   #7
poobah is offline poobah  United States
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It is a fine point... a very fine fanatical point. But unlike somethings here, it is legitimate.

It is all about preventing the coupling of electrostatic fields (not magnetic) into the higher input impedance of the next stage. One could argue that, with the cap "reversed", the same noise would just couple through the cap anyway... but that coupling WOULD be more direct... even if just a bit.

And yes, low level high impedance circuits are where you would concern yourself with this.
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Old 15th March 2006, 02:08 PM   #8
anatech is offline anatech  Canada
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Poobah is right, but this is generally more important where one end is connected to ground. You want the outside foil to be grounded to shield the structure. Next is, as stated, the lower impedance side.

Anything else is open to lively debate. Something to think about too. Suppose you stick the foil side to the input tube on the amp (lower impedance, say 25K) and then replace the high impedance preamp with a low impedance one? It's a silly argument since the circuit layout in that area should have been shielded from noise to begin with.

The above example was extreme, but I have seen it in practice. I guess what I am trying to point out is that circuit layout is more important than foil orientation in low level coupling applications.

-Chris

Poobah, love the 'do too, but are you insured for fire? You don't use hairspray, do you?
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Old 15th March 2006, 02:42 PM   #9
poobah is offline poobah  United States
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Excellent point Chris,

The grounded caps is where this all makes the most sense... from a practical standpoint.

No... no hairspray dude... it actually looks so nice because I washed it... thanks for noticing though.

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Old 15th March 2006, 02:49 PM   #10
anatech is offline anatech  Canada
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Hi poobah,
I am starting to think many of use get distracted by some details and lose the big picture.

Sure, some things may make a small difference, but not as large a difference in the grand scheme of things. This is where a good engineer stands out.

Now the hair. I'll bet it's like a veil was lifted from the music! A little sultry and smoky though, but that's your taste.

-Chris
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