John Curl's Blowtorch preamplifier part II

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Low technology talk.
Check where the outer foil of a cap is to be connected in low noise circuits.
Check by
George

The way I tell which is the outside foil comes from cap mfr when I asked them --- is ---

If you can measure capacitance...... place the tubular film cap inside a metal tube. The C from one lead to the metal tube will be highest with the lead which is attached to the out side foil.

Done. Simple and works without a lot of circuitry (not counting the cap tester).


THx-RNMarsh
 
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Not ideal, but 2xINA110 will do nice for audio.

Too noisy especially at low gains. 5nv at a gain of 5 should be achievable with a 389 pair and the THAT 1580, it does require some DIY work though.

It does remove the output common mode which is a separate issue. I have a solution for that too, I think PMA posted a version it awhile back (a simple FET in-amp with only a FET pair and a dual op-amp). Connect two of those like that late figure on the BB datasheet (I wonder where that came from).

http://www.waynekirkwood.com/images..._Noise_Wurcer_Kitchin_EDN_October_13_1982.pdf
 
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I used my AP and built a shaker table out of a JBL pro woofer. Used an accelerometer to verify shake, rattle and roll.

Stacked plate capacitors could be oriented so the microphonics were minimized. Electrolytics had minimal problems. The physically larger version of many capacitors of the same manufacturer and type also were better.

No real surprises.

Now for the microphone preamps used outside small rooms, a transformer input is my first choice. But the center tapped transformer does not work with Earthworks or other differential pair style phantom power microphones as the current draw is not balanced.

I did some tests a few weeks ago and posted the results up here somewhere. Smacked the hell out of the caps and the 110k bias resistors. I think high value R's might be a bigger problem than electrolytic caps - still to complete the tests to verify though.

Nice practical test set up BTW.
 
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The way I tell which is the outside foil comes from cap mfr when I asked them --- is ---

If you can measure capacitance...... place the tubular film cap inside a metal tube. The C from one lead to the metal tube will be highest with the lead which is attached to the out side foil.

Done. Simple and works without a lot of circuitry (not counting the cap tester).


THx-RNMarsh

an example... to find the ground side or outside foil-lead -----get some tin/alum-foil from the kitchen and wrap around body of cap/DUT:

I'll let the pictures do the talking.


DSC02906.JPG

DSC02907.JPG


THx-RNMarsh
 
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QUOTE=gpapag;5046427]Thank you Richard

George[/QUOTE]

:up:


If one is careful to zero out the c between leads and not move the leads more than a fraction of an inch during tests, there is a distinct different. Notice, my meter has the zero feature. And don't have the foil go over/past the cap ends.


-Richard
 
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8.0 uF 200V WONDER-CAP purchased from J. Peter Moncrieff / IAR

183 pF: tinfoil to cap left lead

191 pF: tinfoil to cap right lead

photo attached

If you are careful in c measurement, it would be the right lead which is the outer foil.


In a few circuit applications, one could also use copper foil or tin and solder a wire to it and ground it... for a shield.



-Richard
 
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Dug out the Agilent LCR meter with zeroing feature (they call it "Null" , it's the keypad at bottom right) and trimmed the tin foil. Same IAR WONDER-CAP 8.0 uF 200V capacitor.

260.1 pF foil to cap right
264.2 pF foil to cap left

images below. BTW waving the leads around a few inches, changed the displayed capacitance by 1-4 pF.
 

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