DC-blocking caps - orientation

[YoSynthi]

If you look at a random sample of circuits where electrolytic DC-blocking capacitors are used, it seems that most input caps have their positive side nearest the input socket, and most output caps have their negative side nearest the output socket. But this isn't *always* the case.

So, it got me thinking. How important is orientation in this instance? In fact, if you're building, say, a stompbox, you may want to connect it, at different times, in various different configurations with other devices, so you can't be sure of the relative voltage differences between the inputs and outputs of the circuits you're connecting to. And if you don't know that, then how can you be sure you've got the orientation of your electrolytic DC-blocking capacitors right?

( By the way, I know about back-to-back electrolytics, to make 'em 'non-polarised', but that's missing the point - i.e. most designs just don't seem to do that ).

Sorry if I've just exposed a huge amount of ignorance, but you're generally a forgiving bunch...

[Inventor]

I have thought about this topic a lot, and my conclusion is that for our audio purposes it doesn't really matter.

Although I'm no expert, in practice I have found that an electrolytic cap will take a small reverse bias current without affecting it's performance noticeably. Also I have found that 0.1uF ceramic caps work just fine as a replacement, but maybe others have different opinions. I dunno.

Les

[Uncle Krunkus]

I suppose it really comes down to the fact that a certain impedance across a certain cap will produce a certain amount of roll off at lower frequencies. I would probably go for Bi-polar electrolytic caps wherever a polarised one would do. If the impedance is low(?) enough shift down to MKTs or ceramics(?)
The reason most comercial equipment uses polarised electrolytics in this case would be simply a matter of cost. There are heaps of pieces of old musical equipment out there which could have gained considerably with the use of better quality caps, metal film resistors etc. etc. etc. but it would have added $20 to the overall price at that time. Unfortunately, the bottom line is the bottom line.

[YoSynthi]

Thanks for the quick replies, fellas.

Inventor: it's a big relief that someone with your experience still has to think a lot about this. It suggests that the answer isn't so straightforward. And yes, of course, the answer I'm really wanting is "it doesn't really matter". So, in my very biased way, thanks.

Uncle K: yes, I definitely understand what you're saying about cost. On the other hand, DIY developers might not to begrudge the extra few pence/cents for extra/different caps, but their actual designs don't seem to reflect this. They still seem to specify single, polarised electrolytics. (I realise you may do things differently).

Bear with me, and let me put the question a different way. If the orientation of DC-blocking caps *does* matter, then what about the commercial and DIY circuits that use them? Statistically, wouldn't half the caps fail over time? None of the manufacturers can be sure of the characteristics of the devices that are going to be attached, so how could they be sure that they've configured the caps the 'right way round'?

[johans121]

Someone correct me if I am mistaken, however, I don't think that the polarity matters in an AUDIO path. Polarized electro's are not the ideal type of blocking cap to use for this application, however they are generally the only cost/size effective option available to provide DC blocking capabilities while ensuring the cutoff value of the filter that it forms is outside of the audio range (due to the normally large capacitance that is used for the application)

[Inventor]

You know what just occurs to me? If we were to set a standard practice where the input capacitors are polarized with the - to the input terminal and the output capacitors are also polarized with the - to the output terminal, then the caps would naturally form a back-to-back nonpolarized capacitor when inputs are connected to outputs. Maybe that's the kosher solution?

Les

[Uncle Krunkus]

??? !!!