Cap polarity, non-polarity, DC blocking, amp circuitry.

[R.G.]

See, that's a good example of what I was getting at. It's not always so clear about what the polarity of the cap should be.

Actually, it is clear. If there is not enough voltage to have there be an unequivocal answer to "which polarity?", then it should be non/bi polar.

So why don't manufacturers use bipolar caps instead in these scenarios? Let me guess, it's probably because they cost a few cents more?

That, plus the fact that there is not immediate fallout. These low-voltage reverse biased setups fail slowly in many cases, and some cap brands are more resistant (although my suspicion is that they're not immune) to small reversals.

The good design practice is clear: if you don't have a clear way of predicting that the cap will be polarized by the circuit, use non-polar.

[Paul Marossy]

See, that's a good example of what I was getting at. It's not always so clear about what the polarity of the cap should be.

Actually, it is clear. If there is not enough voltage to have there be an unequivocal answer to "which polarity?", then it should be non/bi polar.

I can agree with that. I think that is a good rule to live by.

[Jdansti]

Just a quick non-scientific poll of schematics on the web:

Most of the "Fuzz Faces" I just looked at in an image search show the positive side of an polar cap toward the input. A few show a non-polar cap, and this one shows the polar cap with the positive pole toward the transistor.

http://4.bp.blogspot.com/-M4RTvP2htXU/T_WokOm6uzI/AAAAAAAABpM/Aq-6LguDHrU/s1600/DUNLOP+JH-F1+JIMI+HENDRIX+GOOD+SOUNDING+SILICON+FUZZ+FACE+SCHEMATIC%2521%2521+CORRECTED+Dunlop_JH-F1.gif

The preponderance of "backwards" polar caps in this circuit could be more of a copy-cat phenomena than each person deciding which way the cap should be oriented.

A quick review of schematics resulting from an image search on "guitar pedal schematics" shows that most pedals in general use a non-polar input cap, with fewer using a polar one with the negative side toward the input jack, and very few with the positive side toward the input jack.

Now I'm going to have to review my builds and check the input caps!   

[GGBB]

Most of the "Fuzz Faces" I just looked at in an image search show the positive side of an polar cap toward the input. A few show a non-polar cap, and this one shows the polar cap with the positive pole toward the transistor.

Were they all NPN or PNP fuzzes?

[duck_arse]

Electro caps survive best/longest if they are constantly held at some large fraction of their rated voltage all the time.

manufacturers don't help us here, what with so many of the small values being 100V or more rated. is this why small value electros cause so much trouble?

whenever I come up against a mystery cap, I stick something in, and check either end with the cro. mostly I just avoid electros in signal paths.

[R.G.]

Electro caps survive best/longest if they are constantly held at some large fraction of their rated voltage all the time.

manufacturers don't help us here, what with so many of the small values being 100V or more rated. is this why small value electros cause so much trouble?

The un-availability of 1uF-10uF caps in low voltages is one facet of MBA disease. Manufacturers' pet MBAs found that it is cheaper to manufacture all 1uFs (for instance) as 50V and higher than to have multiple manufacturing lines for making the lower-voltage caps, which take less time and materials to make.

Note that higher voltage electros do un-form over time. The decay process for electros involves the oxide dissolving into the electrolyte, becoming thinner due to the lack of a polarizing voltage forcing the oxide into being. The thinner spots become lower voltage withstanding spots, but because they're thinner, they have higher capacitance per unit area. So a higher voltage electro cap will tend to become lower voltage but higher capacitance as it ages. In effect, the manufacturers are giving you a more variable part, but possibly a longer-lived one **when used as intended**. You're getting a higher hill to coast down.

But the cap would last as long or longer when used at, say, 80% of rating. Electrolytic caps tend to self-repair when running on a polarizing voltage near their rated voltage. As the oxide thins, leakage on that spot goes up, and the leakage current helps re-form the oxide in the thinner area as long as the chemical makeup of the electrolyte near the spot can support re-forming and the current flow is not big enough to be self-heating.

When used at zero bias or backwards, they also last longer than a similar lower voltage part would under similar circumstances.

whenever I come up against a mystery cap, I stick something in, and check either end with the cro.

Good idea.

mostly I just avoid electros in signal paths.

That's not a necessary practice. Electros work well when properly applied. However, "proper" may have ramifications that are not widely understood. And after all, electros are there to form a portable and affordable alternative to paper and film caps which may be extremely expensive and weigh more than the whole unit you're designing. Using film and ceramic up to about 1-2uF is good. Above that, small, cheap parts may not be available and electros are your only good choice short of redesign from the start.

[Jdansti]

Most of the "Fuzz Faces" I just looked at in an image search show the positive side of an polar cap toward the input. A few show a non-polar cap, and this one shows the polar cap with the positive pole toward the transistor.

Were they all NPN or PNP fuzzes?

Hmmm... A quick check shows that most are PNP with the input cap oriented with positive toward the input. If I understand, this is the correct orientation for PNP.  However, it's still hit and miss with both NPN and PNP varieties. Here's and example of two PNP schematics with opposite input cap polarity:

http://www.diystompboxes.com/smfforum/index.php?topic=58106.msg452211#msg452211

http://juansolo.demon.co.uk/stompage/schematics/Fulltone/Fulltone%2069%20Schematic.gif

BUT... I just noticed the battery polarity relative to the input is also not the same in these two schematics. Perhaps that is why they are different.

Bottom line: to us relative noobs, cap polarity orientation can be confusing!   

[Thecomedian]

In the first one, positive voltage might be larger for the input cap to the base of transistor than to the input.

second one, positive for feedback cap is still large, but the negative might be much larger between input cap and base.