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Old 4th November 2010, 12:26 AM   #1
RobertE is offline RobertE  United States
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Default DC blocking caps ?

Does it matter which direction they're hooked up0?

I know the schematics show the polarization in certain directions, but for small DC does it matter? Any difference between input vs. output caps?

Testing with my multimeter seems to show that it doesn't matter.
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Old 4th November 2010, 12:46 AM   #2
gabdx is offline gabdx  Canada
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Hi Robert, even for small voltages like 5 Volts, reversing the polarity of a capacitor in a circuit can lead to self destruction. Some polar capacitor may have a small endurance to reverse polarity but I would not count on it for long term stability.

If you want to use the capacitor to block DC from the audio signal, a polarised cap should not be used for sake of audio quality, but there is no destruction if you use it for a .5 V audio signal.

Please describe more the DC voltages, the shematic and the type of capacitor you would like to use !
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Old 4th November 2010, 12:58 AM   #3
RobertE is offline RobertE  United States
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These are on audio lines, so the DC is quite low, but almost all of the schematics I've reviewed show polarized DC blocking caps on the audio lines. Some of the DC blocking ones are 10u and I am not even sure I've seen non-polarized in these ??? Many I've seen are .1 or .01 and these I've found in non-polarized ceramic disc.

Why do you not recommend polarized caps for audio quality?
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Old 4th November 2010, 01:35 AM   #4
gabdx is offline gabdx  Canada
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Film capacitors are usually recommended for performance, there is plenty of literature on this all over internet / in books, they are never polarised. Electrolytic capacitors are polarised unless specified otherwise. Paper dielectric is supposed to sound best, followed by metalized paper, metalized polypropylene, propylene, and electrolytic. For some people, or some audio systems, it makes no difference. So take this with a grain of salt.
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Old 4th November 2010, 03:25 AM   #5
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Consider connecting two in series with the positive ends conected together. obviously each of the 2 has to be 2X the size of a single cap for the correct lower cutoff frequency. putting a film cap in parallel will help as well.

I learned (from Jim Williams at Linear Tech in one of his ap-notes) that when you are in the noise measurement biz you want the capacitor with extremely low leakage. A good wet-slug tantalum will set you back about as much as a vintage Bordeaux.

In any event, required reading is the article on capacitor selection on Walt Jung's website.
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Old 4th November 2010, 02:52 PM   #6
DF96 is offline DF96  England
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People can sometimes get too excited about eliminating electrolytic capacitors from audio circuits. Provided they don't carry much audio current, have DC bias and do not form the dominant LF pole, then in most places they don't do too much harm. A good circuit with electrolytics in appropriate places will usually sound better than a poor circuit with more exotic components. Deliberately avoiding electrolytics may force you to use complicated circuits which can bring their own problems.
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Old 4th November 2010, 03:24 PM   #7
RobertE is offline RobertE  United States
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Thanks for all the great info ! This is a car audio device, so in most cases external noise is more a factor than anything else. I am just trying to understand the basic circuit design, and components.

The puzzling thing was the circuit showed polarized dc blocking caps on the audio inputs and I installed them backwards and it didn't seem to affect anything.
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Old 4th November 2010, 03:52 PM   #8
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Pass uses the Elna Silk caps (you can get them at Digikey) -- forgot to mention that yesterday.
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Old 4th November 2010, 03:58 PM   #9
RobertE is offline RobertE  United States
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My enclosure is pretty small - and has to be... And I'm on prototype board...

What are the best "small" cap type to use for DC blocking? It seems people use ceramic but the circuit calls for 10u (and polarized).
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Old 4th November 2010, 04:39 PM   #10
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Elna Silmics (as mentioned above) are reasonably priced and sound good too.

I recently refurbished a B&K ST-140 and used Silmics with good results. There is a 47uF capacitor on the input and the original was a little no-name Taiwan electrolytic. It could have been my imagination, but replacing it with a Silmic resulted in a cleaner, less muffled sound.

http://www.elna-america.com/tech_audio_series.php

Last edited by kstagger; 4th November 2010 at 04:42 PM.
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