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Old 6th August 2013, 08:02 AM   #11
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Quote:
Originally Posted by PhrazeMaster View Post
We have 3 caps: a .01uF ; a .1uF , and a 1uF cap:... a 10K square wave at 10V into a 100R resistor.

...I'm no EE (my dad is), and perhaps I don't know what the f I'm doing. But can anyone explain why in cat's heaven
Ask your Dad about rise time of an RC circuit. Or visit wikipedia.

Hint -you've got 100:1 difference in capacitance and time constants that neatly bracket a 10kHz square wave.
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Old 6th August 2013, 08:03 AM   #12
John8 is offline John8  United Kingdom
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I assume that you are measuring the voltage across the capacitor in your test.
If so then the 0.01uF is as expected but the two larger ones are exhibiting high impedance effects due to series inductance within the capacitor. The 1uF one should show a triangular (ish) waveform of reduced amplitude if it is presenting a low impedance to the higher harmonics of the square-wave. Wound foil types with few connections to the foils do tend to exhibit large inductance, the better performing ones have all the turns connected at each end.
With the higher impedances involved in a valve amplifier then the effect may not be as pronounced as you are seeing. Try the same with a 100K resistor and you may get a more "text book" waveform.
I tried a cheap and cheerful 1uF 1000V capacitor from RS and it did produce the expected triangular waveform.
I agree capacitors do not need burning in, they will not improve no matter how long you keep then with a signal applied.
Risking being unpopular by saying:- if expensive capacitors sound better maybe they are just producing a more acceptable form of distortion?
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Old 6th August 2013, 08:29 AM   #13
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Thanks Enzo - point taken, I was a bit gruff in the opening. Thanks Elvee, Arno, thoglette, John8: thank-you; excellent points!
...sq225917, haha! I know they're not electrolytics.

Enzo, I was measuring them wrong - I was measuring across both R and C. After much googling I see how to do it across the cap only. I also had the R attached to the (-) to ground, instead of having the cap leg to ground.

I did not mean to come across as crass, so apologies, and thanks for the support.

I am concerned there may be something wrong with my 'scope, but as I continue to upgrade my understanding it may be me all along. I get that since the cap values are the same, the waveforms will be totally different. In my initial haste I just didn't stop to think.

Thanks all for the thoughts.

Mike

Last edited by PhrazeMaster; 6th August 2013 at 08:34 AM.
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Old 6th August 2013, 08:33 AM   #14
AJT is offline AJT  Philippines
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what did you expected to find?
did you measure tolerance, or actual cap value against the label?
how about esr and DA, did you measure them as well?
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Old 6th August 2013, 08:42 AM   #15
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By the way, who says you can't hear an expensive capacitor? I dropped one today. Boy I heard it!
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Old 6th August 2013, 08:43 AM   #16
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AJT no I did not measure esr and DA - just (wrongly) took the scope across the cap and resistor. I'm learning.
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Old 6th August 2013, 08:50 AM   #17
Arno Pf is offline Arno Pf  Netherlands
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Quote:
Originally Posted by PhrazeMaster View Post
By the way, who says you can't hear an expensive capacitor? I dropped one today. Boy I heard it!
Try blind-testing
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Old 6th August 2013, 09:17 AM   #18
DF96 is online now DF96  England
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I should not really be posting in a thread where the OP says from the beginning that I am not welcome, but I suggest you relabel the 'scope pictures with cap value rather than brand - as that will highlight the only relevant factor to the entirely predictable results you see. You could then ask why do different cap values behave differently, but then we would have to talk about circuit theory and time domain CR response rather than brands.

But thanks, anyway, for making me smile.
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Old 6th August 2013, 09:25 AM   #19
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Sorry DF96 - I was grumpy. Apologies. Your opinions are welcome.
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Old 6th August 2013, 09:46 AM   #20
DF96 is online now DF96  England
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Thanks.

There may be minor differences in response between different caps with the same value but different constructions, but you would need much more complex instruments to see this. You would then find that the cap dielectric and construction method was of greater importance than brand, except for those cases where the brand is an indicator of poor construction (despite high cost) or even fakery. Try Googling for Cyril Bateman's cap tests.
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